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CHAPTER 1 (#ulink_ea10b0f9-eda4-5f4c-adb9-301d8875215a)
ANTS AS INSECTS (#ulink_ea10b0f9-eda4-5f4c-adb9-301d8875215a)
ALL ants are social insects – but only in the female sex; males exist briefly to generate sperm for sexual reproduction. Females are of two distinct sorts. One is relatively large and designed to fly and seek out new places in which to live; she lays fertilized eggs and influences the manner of brood-rearing in the colony. The other is smaller and wingless with a much simpler form altogether; she devotes most of her time to nest making and defence, brood hygiene and feeding and, later in life, to collecting food outside the nest. As ant societies last a long time and make easily defended nests that are climatically equable it is possible for the young to be legless grubs, but whether they have evolved from a normal six-legged state or whether they were already legless in a pre-social evolutionary stage, perhaps as parasites, is uncertain. They can do little more than arch their bodies, suck and digest the food that the adults give them, and urinate. They are, however, well designed for growth and when, after several moults to allow for expansion, they have stored enough raw materials, all their food residues are ejected at one go and the buds of the rudimentary adult inside grow and join up. After one more moult a further metamorphosing stage is produced (the pupa) from which the final adult later emerges.
In the tropics ants have evolved enormous societies with nearly every conceivable way of living in the terrestrial habitat. Many thousands of species are already known and more doubtless remain to be discovered. Here in north-west Europe ants are by no means so spectacular but their social organization is impressive. They live nearly everywhere and play an important part in natural processes.
DEVELOPMENT OF THE SOCIETY
Ant societies maintain themselves and reproduce during the summer but stay quiet and inactive during most of the winter. Batches of large reproductive females and males (collectively known as sexuals) are produced which fly from the nest, congregate and copulate. Enough sperm is transferred to a sac inside the female to last her lifetime, which may be more than ten years. After flying around searching for a good place the females settle, break off their wings and dig a cell. Wings are a definite disadvantage for soil-living insects; they are difficult to keep clean and easily stick on to damp soil. The wing muscles are then of course also superfluous; they degenerate and their substance is transferred to eggs growing in the ovaries and to food-producing glands in the head. A cell is dug in the soil or soft wood and eggs are laid. These are sticky and form a cluster which the queens, as they may now be called, guard. No food is fetched from outside but some of the eggs are eaten by the first grubs which thus grow solely on the products of the queen’s body. After the usual number of moults and metamorphoses, small, relatively simple, wingless females mature. These relieve the queen of the care and defence of the brood, enlarge and open up the nest and bring in food from outside; they are called workers.
At first all the workers are small but as time goes on bigger and bigger ones appear. After this come males and the society can be considered fully mature when finally winged females are formed. Normally societies live as long as the original founding queens but if re-queening can take place then, at least in theory, indefinite survival is possible.
A typical situation has just been described; in fact there are many variations. Thus in primitive societies the queens may forage to begin with. Again, not all queens can fly and some can but do not (they prefer to stay in the parental nest). Often new colonies are founded as buds of existing ones, complete with all types and stages from the start. Quite a lot of species are socially parasitic, either temporarily during colony initiation or permanently, and in some of these workers no longer appear and males are wingless. Although in all ants the workers have no wings, and in most they lack sperm sacs, in many species they do in fact lay eggs. These are of course unfertilized but they may develop into males or, rarely, females but they can also serve as a food store.
EVOLUTION OF SOCIAL LIFE
By the Eocene period (70 million years ago) a great many varieties of ants had evolved, many of them much the same in size, shape and structure as those that exist today; presumably they were social, too, but perhaps to a lesser degree. Even from the Upper Cretaceous period (100 million years ago) a primitive worker (Sphecomyrma) is known. It shows characteristics in common with certain families of solitary wasps. The first few steps towards social life are obscure. A likely speculation suggests that it all started with a wasp-like creature that collected soil animals, put them in cells, laid eggs on them and then went off. On hatching the larvae burrowed into the prey and ate it; if too many eggs were laid the smaller, weaker sisters were eaten, too. If the female remained with the brood and protected it from enemies an incipient society would exist. Social evolution might have progressed from this stage if the parent continued to collect prey and lay eggs after the first ones had hatched. This would provide the larvae with additional food which could have been taken directly while the parent herself was eating. An important stage in evolution occurred, it is suggested, when the female lived long enough to control the behaviour of the first young adults, prevent their escaping, finding males and founding new groups. Later perhaps wingless progeny which were able to work immediately, by-passing resting stages, dispersal flights, copulation and other phases of the earlier life cycle would be produced. Caste size differences would follow slowly. Direct feeding on prey gave way in most ants (some of the time but not entirely) to egg-eating, sucking up regurgitated digests and, ultimately, to taking milk made in one-time digestive glands. If several equal-aged females associated to start a society, it is likely that the work would be shared in some way: those with bigger and better-developed ovaries would lay eggs whilst others foraged (perhaps with an interchange of functions occasionally). Today the most primitive of surviving ants are found in Australia. On one of these, Amblyopone australis, the first cell to be made is left open and the queens forage for prey in the soil, even before the eggs hatch. These eggs are formed into a mass and may be eaten and, as it were, re-laid fresh at any time. The first larvae are fed on solid insect food; they eventually mature into workers which are as big as the queens and very similar structurally (except for the absence of wings). They can get out of their cocoons without help. These workers forage for prey and feed the brood. In spite of this help the queens go on working and more may join the group from time to time. Myrmecia is another primitive type; it constructs an open cell and forages regularly for nectar, only collecting insects when its eggs hatch. If deprived of prey it can make some growth so presumably it has a body source of food, perhaps its thoracic wing muscles. On a similar level is the European genus Manica of the myrmicine ants which excavates a small nest, leaves the entrance open and goes out to collect prey for its larvae. The workers the queen produces are considerably smaller than herself.
Finally, a stage of evolution is reached which has been called claustral; in this the queen seals herself in a hole and produces the first brood from her own body reserves entirely. Such a limited food supply would favour the evolution through natural selection of small workers which can be produced both more quickly and more economically. However, a big caste divergence necessitates the establishment of effective methods of intercommunication, co-operation and, above all, control. Thus the general theme of ant evolution is from small groups of structurally similar, very versatile females to large groups of structurally dissimilar, highly specialized, interdependent females.
CHAPTER 2 (#ulink_26264a70-60ff-5daa-8ef4-1c62f926a237)
ANT STRUCTURE (#ulink_26264a70-60ff-5daa-8ef4-1c62f926a237)
THIS is no more than a brief account of the main structural features of females (queens and workers) and their larvae. Males and pupae are only treated superficially. Myrmica rubra will be used as a type; it is a fairly unspecialized species whose structure was meticulously studied by Charles Janet at the end of the nineteenth century. Some of his diagrams have been adapted and reproduced here.
EXTERNAL FEATURES
Like all insects, ants have a hard external covering. This cuticle is many-layered and chemically complex; apart from protecting the body mechanically and providing a strong basis for muscle attachment it also screens out dangerous solar rays and reduces the amount of water vapour that escapes. Water loss can be substantial even in quite humid conditions and this last function is very important to terrestrial animals. Flexibility is achieved by joining rigid segments by a supple connecting membrane folded and tucked inside for protection. Although ants are by no means as hairy as bees they do have a great many simple and usually rather short setae scattered over most parts of the body. Spines also occur in some groups.
As in all insects the ant’s body is divided into three parts – head, thorax and abdomen. The unusual feature in ants (and most other members of this insect order, the Hymenoptera) is the evolution of a petiole towards the front of the abdomen. This is formed by reducing one or two segments to narrow tubes and articulating them. This separates a middle body (called the mesosoma) composed of the thorax and one abdominal segment from a hind body (the gaster) made up of the last seven segments (not all of these are visible). The number of petiole segments is two in the sub-family Myrmicinae (fig. 1 (#litres_trial_promo)) and one in other ants; formicine ants usually have a scale on their petiole (fig. 7 (#litres_trial_promo)). Most of the hinging motion occurs between the mesosoma and the first petiole segment and lies in a vertical plane; the gaster can be brought round under the mesosoma until its tip reaches up to near the jaws. In this way an enemy or prey can be located with the antennae, grasped with the jaws and immediately stung or sprayed with toxin. Damage caused by the bite probably helps the venom to penetrate. In a similar way eggs in the genital aperture can be brought round to the mouth and picked up in the jaws. The ventral side of the gaster can also be bent under the thorax and can be used to stop small objects, principally brood, from slipping backwards as they are being manipulated by the forefeet and mouthparts.
The head is very flexibly articulated on to the thorax and the actual junction protected above by a stiff collar which projects forward from the thorax. The head can be moved downwards and rotated sideways but cannot be lifted very much. However it is arranged in the long axis of the body so that the jaws protrude, an obvious advantage in defensive situations.
The most important sense organs are the antennae (figs. 6 (#litres_trial_promo), 7 (#litres_trial_promo)). They are hinged so that they can be both extended well in front of the jaws or, in dangerous situations, folded back so that they lie close to the head itself. This hinge occurs at the junction of a long, thin, basal segment (called the scape) and a highly-subdivided club (called the funiculus). These organs undoubtedly help the ant to understand the size and shape of objects in its environment: they can be moved through a wide arc independently or the tips can be brought together on to small objects much less than a millimetre in diameter. A wide variety of chemicals can also be detected and recognized by the antennae and they may also respond to sounds and vibrations in the substratum.
Most ants have the usual insect compound eyes but they are never well-developed, even in males, and in those which spend most of their time underground or following scent trails above ground they may be quite absent. Each eye is formed of a group of small visual organs – ommatidia; in some insects like dragonflies there may be as many as thirty thousand of these but even in flying male ants one thousand is more like the maximum. In one of the wood ants, Formica cunicularia, there are 460 ommatidia in the worker caste; in the common black garden ant, Lasius niger, there are only 120 and in the related but soil-dwelling Lasius flavus a mere 45. Males and sexual females have three small, light-responsive organs on the top of their head (ocelli); their use is not fully understood but it seems likely that they play some part in activity, particularly flying.
The mouth, as in all insects, is surrounded and enclosed by several pairs of articulated appendages. The simplest and most prominent are the jaws (mandibles). These are hollow but have thick, tough walls and they hinge firmly on the front of the head in such a way that they can be either opened wide or closed tightly together (fig. 3 (#litres_trial_promo)). Their sharply serrate margins enable a strong grip to be taken on small objects, such as the leg of a fly. Not all the teeth are of equal size; the front two are larger and can pierce insect cuticle. The back ones are shorter and more frequently used in gripping the brood when it is being carried about. All these teeth wear quite blunt as the worker ages and one or two of the bigger ones may chip away. Between the jaws there lies another pair of jointed structures that carry small palps which are almost certainly used in tasting and feeling food. These maxillae, as they are called, can fold back and cover the mouth completely without interfering with the free movement of the jaws; this is an obvious advantage in fighting. Finally, there is a tongue (the labium) that can be ejected by blood pressure (fig. 2 (#litres_trial_promo)). It is quite manoeuvrable and is frequently used as a rasp since its tip is covered with fine striations. Liquid food flows up the tongue towards the mouth in saliva but before entering it must pass over a silt trap, known as the infra-buccal pocket, which takes out solid, largely indigestible granules from the food (fig. 2 (#litres_trial_promo)). This pocket is also used as a depository for dust from the body in general; dust is collected by rubbing the middle and hind legs over the body surface; combs on the front legs (fig. 6 (#litres_trial_promo)) are then used to remove it and these are then cleaned by passing them through the mouthparts. A lot of dust is collected by licking other ants and when the pocket is full its contents are thrown away into the rubbish area.
The mesosoma is the motor part of the body and carries the six legs and, in sexuals, the two pairs of wings. Each leg has five main joints but the outermost one is finely subdivided to make a flexible foot that ends in two curved claws. The articulation and manoeuvrability of each leg is remarkable; they are perfectly constructed for movement over rough and irregular ground and up vertical and overhanging surfaces; they can be lifted and rubbed over the body for grooming or so that, in the case of the young queen, the wings can be broken off. The forelegs, besides having a comb, are used for holding food and are placed round the head of a larva when it is being given liquid. The hindlegs are the longest and can be used to lift the body well clear of the ground; in this way the gaster can be brought under the mesosoma and venom ejected forwards or, when the individual is laying an egg, it can be taken off with the mandibles.
The mesosoma of the queen is much bigger than that of the worker and is divided into plates joined by elastic sutures that play a part in controlling wing movement during flight. In workers nearly all the sutures are sealed over and the mesosoma is a single rigid structure. At the hind end there may be spines which are thought to protect the petiole from damage by predators and other aggressive ants and there are two large pockets that carry a grease which is said to be spread over the body, to be antiseptic and perhaps to carry a species-characteristic smell. The gland cells which produce this are just below the inner surface of the pocket. On each side of the mesosoma there are three spiracles. These are entrances to complicated systems of air tubes (tracheae) that ramify both forwards into the head and backwards into the gaster. Gases are carried in this way to and from the tissues which absorb oxygen and emit carbon dioxide.
The most striking external feature of the gaster is its extensibility (fig. 4 (#litres_trial_promo)). Supple zones of cuticle are folded and tucked under tougher, but still comparatively flexible, zones in such a way that each segment telescopes into the one in front. A fully distended gaster shows the intersegmental cuticle as white bands between much darker segmental ones. Right at the tip is the anus, often surrounded by a ring of hairs and below this there is an opening for eggs. The only appendage is the sting and its sheath but in many groups of ant even these are missing. The sting is a simple, pointed object without barbs (such as occur in the honeybee). It is tough and quite flexible, not brittle and can be gradually pushed into the soft body parts of insects and through the skin of birds and mammals, provided the ant can get a firm grip. Queens have stings like workers but very little is known about how and when they use them. They do fight each other under certain conditions and may use their venom like honeybee queens do; it is most likely, however, that stings and venom are used in defence of brood during the early stages of colony foundation when there are too few workers to defend the nest unaided.
At the front of the gaster (in myrmicine ants) there is a sound-producing organ (fig. 4 (#litres_trial_promo)). The second petiolar segment has a very stiff hind margin that can be rubbed against a series of fine ridges in the cuticle on the dorsal surface of the gaster. When the gaster is lifted up and down a squeak is produced. It is just audible in ants of the genus Myrmica if they are held firmly by the head and thorax leaving the gaster free to move. Curiously, this sound seems to have no social function at all; other ants ignore it. The only plausible suggestion is that it shocks predators that have acute hearing and causes them to drop the ant, which then has a chance to escape.
In the integument, particularly at joints, there are many microscopic sense organs that send into the central nervous system information about the stance and position of the body relative to the substratum. These enable the ant to assess its relation to gravity and provide it with information important in navigation.
INTERNAL FEATURES
Gases are taken to and from the tissues in air tubes from the six spiracles which have already been mentioned. Fluids bathe all the internal organs and are circulated by a long, thin, tubular pump (the heart) which runs dorsally all the way from the tip of the gaster to the inside of the head just behind the brain; it has valves along the way which prevent back flow (fig. 4 (#litres_trial_promo)). These fluids then filter through the organs and tissues. Blood does not carry gases and is not piped to and from the tissues, as is the case in vertebrates.
Much of the head is occupied by muscles which work the jaws and the food pump (pharynx). The biggest muscles are those that close the mandibles and they are obviously capable of exercising a strong and tight grip; the muscles are fixed on the back of the head and run to the inner edge of each jaw (fig. 3 (#litres_trial_promo)). Seated on the ventral part of the head and running to the outer part of each jaw are the muscles which open them; they are much smaller. Other small muscles retract the tongue after it has been extended by blood pressure and there are some inside which manipulate it. The pharynx is a flattened part of the gut in the front of the head which can be enlarged and diminished by means of six muscles attached to it in various ways, some from above, some from the sides and some transversely (fig. 2 (#litres_trial_promo)). It pumps liquid food up after the silt and solid particles have been taken out and passes it down the oesophagus. This latter is a very thin tube, more or less circular in cross section that runs right through the mesosoma and petiole to the gaster. The brain is a ganglion of nerve cells and fibres behind the pharynx and over the oesophagus. It collects sensory data from outside and inside the body, works out correlations and associations and stores relevant information. This enables the insect to learn and react to quite complicated patterns of stimuli. The brain also has closely associated with it a number of ductless glands that control most internal chemical processes. Underneath the oesophagus is another ganglion, connected to the brain by thick nerve trunks, which is largely responsible for controlling motor activity; it co-ordinates the movements of antennae and mouth parts. In its turn it sends nerve trunks into the mesosoma.
FIG. 1. Worker of Myrmica rubra, sagittal section showing main organs and divisions of the body (after Janet).
A gland opens on each mouth part (figs. 1 (#litres_trial_promo), 2 (#litres_trial_promo), 3 (#litres_trial_promo)). The mandibles have glands that lie against the outer wall of the head, one on each side between the eye and the mandible base. A single layer of cells produces a secretion which is stored in a thin-walled reservoir and conducted by tube to the base of the mandible where it opens through a pore. These glands have several functions: they are partly digestive, partly they lubricate the joints of the mouth and they also contain volatile substances that alert other ants. The maxillary glands have no reservoir and open directly at the base of the maxillae; they probably produce a digestive fluid. The labial glands are not in the head at all; they occur in the front of the mesosoma and are connected to the tip of the tongue by a long, thin tube. They supply a watery lubricant which contains some digestive juices. The most striking gland in the head, next to the mandibular, is the pharyngeal gland; this consists of about twenty finger-shaped tubes, often containing a yellow oil. It is thought that this substance is separated from the food as it travels through the pharynx; it may be digested and used as an energy supply. The watery part of the food goes down the oesophagus to the crop which is in the gaster. From here it can, nevertheless, be regurgitated to larvae and queens and as it contains most of the proteins it is of great value to them, either for body growth or egg production, as the case may be.
FIG. 2. Worker of Myrmica rubra, head sagittal section showing main organs (after Janet).
In the narrow constriction between the head and the mesosoma there are six channels: the ventral nerve cord, the duct of the labial gland, the oesophagus, two air ducts and the tubular heart. In the mesosoma the ventral nerve cord forms into three ganglia (the last a compound one) that are largely concerned with co-ordinating leg and wing movements. It contains all the muscles operating the legs and, in the sexuals, the wings. In fertile queens these wing muscles degenerate and the material from them is used to make eggs which not only give rise to a brood of larvae but are also used to feed them. Later on, when the colony is well established, the space occupied by the muscles becomes filled with storage tissue called fat-body from the opalescent oil droplets that are the principal component. Muscular action uses a lot of energy and the mesosomal organ must be well-supplied with oxygen from the three pairs of air tubes. There is also a pair of large air sacs which may be ventilated during muscular movement in some way.
FIG. 3. Worker of Myrmica rubra, head section from a. to b. of fig. 2 (#litres_trial_promo) (after Janet).
Water-dissolved substances are carried through the long, thin oesophagus to be stored in a dilatation called the crop which lies dorsally in the front of the gaster. This can be distended to a huge size for it is used to carry nectar and honeydew back to the nest. There is a valve between the crop and the rest of the gut which prevents a good deal of loss from the crop but not all. In some ants this is held closed by muscular sphincters; in others it closes passively when the fluid pressure in the crop rises. It is thought that this second method of control is more efficient since it uses no energy. The crop contents are mostly very liquid and can be regurgitated to other adults and to larvae. This is probably done by telescoping the gastral segments inwards but perhaps by reversing the pharyngeal pump mechanism.
Once fluid has passed back into the midgut it can no longer be shared by other ants. The thick walls of the midgut (fig. 4 (#litres_trial_promo)) secrete digestive enzymes and absorb the small molecules into which the food is broken. Waste material passes back through another tube into a thin-walled hindgut which stores indigestible residues and probably extracts some water and water-soluble substances. Finally, the liquid waste is ejected in a special part of the nest some way away from the brood rearing zone, along with the refuse. Foragers probably eject their own residues outside the nest. It includes waste substances extracted from the body fluids by long, thin, hollow, thread-like tubules; they join the gut just after the midgut. Much of this excretion is synthesized into uric acid which appears as white granules. In workers the main storage tissue is in the gaster. Oils, glycogen and proteins are all stored there though most of it is oil.
FIG. 4. Queen of Myrmica rubra, gaster sagittal section showing main organs (after Janet).
A very important part of the gaster is occupied by the genital system (fig. 4 (#litres_trial_promo)). This consists of thin-walled egg tubes (many more in queens than in workers) that start just below the heart at the front of the gaster and pass backwards and downwards to the genital opening below the anus and the sting. Eggs start as small cells at the tip and move down, growing and maturing as they go. Part of the material for growth comes from nurse cells which are broken down and incorporated into the egg tissue but some is extracted from the blood by an enveloping layer of follicle cells. When the egg has been fully formed it is enclosed in a special skin (the chorion) and the egg passes into the oviduct to be fertilized. The oviduct is a simple tube with a thicker wall than the ovary. In queens it has two pouches on its dorsal side, one near the outside which received sperm during copulation and the other a bit farther in that stores the sperm alive for a decade or more. How it gets from one to the other is unknown. These pouches are not present in workers unless the ant is a very primitive species.
It remains only to mention two glands that open into the sting (fig. 4 (#litres_trial_promo)). One is the poison gland which consists of two closed tubes feeding a thin-walled storage vesicle and which contains a watery solution of mixed venoms. The other is a smaller gland called after Dufour which contains an oily material secreted by cells which surround the storage space. The poison glands vary quite considerably in different families of ant, depending upon whether they synthesize a thin liquid for squirting or injecting with a sting, or a sticky fluid for smearing.
In general the ant body is neatly divided for separate functions: the head carries the main sense organs and the brain and has in front a mechanism for food capture and food pre-treatment, the mesosoma is specialized for locomotion and the gaster has a region of absorption and food storage but also contains the reproductive organs. Defensive apparatus is disposed both in front and behind but the body is flexible enough for both to act forwards in concert.
LARVAL STRUCTURE
Ants’ eggs are oval and white; each egg weighs only ·00005 gm and is less than a millimetre long. In most of them a legless grub develops. This is of course no bigger than an egg at first and breaks its way out by means of its sharply pointed jaws. It is almost hairless and transparent so that a small, residual blob of egg yolk in its midgut can easily be seen. Though its cuticle is inelastic rather like polythene it is wrinkled and folded so that a good deal of room for expansion is allowed. There are three stages separated by moults, when the old skin is cast off and a new one, folded and wrinkled to allow for growth, is built underneath. Only the head has a firm cuticle that will not expand at all so that its growth only takes place during the moult between successive stages. The second stage has a good many more hairs than the first and the third and last stage is very hairy indeed. It has some hooked hairs as well as the simple ones which enable the grubs to interlock and so be carried about in one group. It is tempting to believe that the workers are able to distinguish the stages by feeling the degree of hairiness with their antennae, but again this has not been tested by experiment.
The head carries jaws with pointed, hardened tips. These are capable of piercing eggs and can masticate soft tissues a little. There are several sense organs of a very simple kind, including minute, unjointed, conical antennae; several other similar ones occur around the mouth and are probably used by the larva to feel and taste what it is eating, as well as to enable it to lock on to the worker’s mouth whilst it is being fed regurgitated liquid. The head, like that of the adult, contains a pharynx innervated by muscles (fig. 5 (#litres_trial_promo)). It is a pump that only sucks inwards; larvae cannot regurgitate food. The ‘brain’ consists of a dorsal ganglion and a ventral ganglion with nerve trunks running round the oesophagus to connect them. The ventral ganglion is the first of a long, chain-like series that runs down the body near the ventral wall.
FIG. 5. Larva of Myrmica rubra in third instar, longitudinal sections to show main organs: a. slightly off mid-line, b. on mid-line (sagittal).
Most of the body contents are connected with the digestion and storage of food. A very large, sac-shaped midgut consists of a single layer of large, glandular cells. This is connected to the pharynx by a thin-walled oesophagus and where this and the midgut meet there is a structure for secreting and moulding a thin membrane which encloses the food and is thought to protect the delicate gut cells from abrasion. However, there is no way through from the midgut into the much smaller hindgut and all the food residues collect and condense into a black pellet surrounded by food membranes which are ejected at the end of the larval stage. The hindgut is very thin-walled and collects the liquids extracted by excretory tubules that lie freely in the body fluids. It thus acts as a bladder by storing urine and, if the diet is rich in protein, white insoluble granules of uric acid. The bladder is emptied by a longitudinal contraction of the body; this usually occurs only in response to a light touch on the skin in the region of the anus so that normally there is a worker nearby to collect it up and carry it away. The larva has a single but large pair of tubular digestive glands that start in the front, pass backwards and then enlarge into thin-walled reservoirs which go forwards again well into the first body segment just behind the head before sending tubes that join and run to the lower lip. A great deal of fluid that is rich in protein-splitting enzymes is stored in these reservoirs, ready to be shed on food: as they are thin-walled and quite close to the body wall they are really a considerable hazard for if they are broken in an attack the juices escape into the larval body and digest its internal organs. Saliva may be ejected as a droplet if the larva’s head is pushed backwards.
A long, thin, tubular, valved heart runs mid-dorsally from just above the anus to just behind the brain and circulates body fluid, as in the adult (fig. 5 (#litres_trial_promo)). There are a number of muscles inside each larva which enable it to make some simple movements, such as bringing its head down towards its abdomen or retracting it into the thorax, moving its jaws and ejecting urine but they also contract in a way that maintains blood pressure against the body wall and hence turgidity. A large part of the body space is filled with storage tissue, called fat-body from the abundance of oil in it; both glycogen and protein are also stored there.
Rudiments of the adult are distributed throughout the larva in appropriate positions (fig. 5 (#litres_trial_promo)). At first they are simple, hollow sacs surrounded by thin-walled sheaths but as they grow they elongate and split transversely or longitudinally into segments. Thus the antennae are situated in front of the brain and are nearly spherical. The six leg buds are very similar in shape and are arranged in three pairs on either side of the nerve cord in the first three body segments. All of these grow in length and split transversely. The wings are buds, again almost spherical at first, which lie on the side of the second and third segments. They retain a flattened form but develop into a heart-shaped one by the growth of their ventral tips. The ovaries are exceptional. They are paired, solid, carrot-shaped organs fixed by the broad end to the heart not far from the end of the abdomen; their other end passes as a thin filament right down to the genital buds just in front of the anus.
When the chemical signal for metamorphosis is given these buds grow more quickly and begin to join up and assume their adult shape. Even the sheaths spread out and form, at least in the legs and antennae, some of the basal segments. The wings elongate but never split. The ovary, in the case of a queen-forming larva, splits longitudinally into about eight parts which grow along the filament towards the genital buds where they meet the oviduct as it grows upwards. In the worker this is a narrow tube and a broader one in the queen.
All this takes place in the larval skin after the larva has stopped feeding and ejected its food residues through the hindgut which breaks down where it touches the midgut. The membrane enclosing all the residues not only makes this easier but probably prevents gut bacteria getting into the body and starting an infection. The larval head is not big enough to hold the new pupal head and so this is formed in the first body segment; only the tips of the pupal antennae lie in the larval head. The young pupa escapes through a split in the larval skin which starts dorsally just behind the head. Somehow or other it manages to push off the larval skin as far as the tip of its gaster without any help from workers. Simultaneously the petiole is formed by a contraction of the second and third abdominal segments. This together with the inward telescoping of the last body segments seems to reduce the body volume and create enough blood pressure to inflate and smooth out the surface just before it sets.
Only one more moult is needed before the adult is produced. This forms with much less change in shape inside the pupal skin. The wings flatten, elongate and fold up and the legs and antennae and petiole segments narrow a little more. Colour appears as a gradually increasing brownness in the general body but the eyes change from pink to black. The adult emerges with a very soft flexible skin and the wings, if any, inflate, spread out and set hard.
The caste differences in the female do not develop until very near the end of larval life. In workers the wing buds stop growing when they are quite small and all traces vanish by the time the pupa is formed; their ovary never thickens and splits into egg tubes but simply elongates to meet the oviduct. Caste differences in ants thus depend quite simply on whether buds grow or not; there is no degeneration and reorganization as happens in the honeybee. It seems likely that the growth of wings and ovaries is specially delayed during the development of females so that caste determination can be left to the last moment.
In males, which have only one caste, the wing grows earlier and the testis is split longitudinally very early in the larval stage. Late caste determination presumably gives a more sensitive response to social conditions.
Males are either the same size or smaller than the females and they are usually darker. Their antennae are straighter and clumsier and cannot be folded back against the head. They have well-developed compound eyes and ocelli. Their thorax, like that of the sexual females, carries two pairs of wings linked by hooks and it is composed of plates separated by sutures but their petiole is not quite so well-developed. There is no sting at the end of the abdomen; in its place are appendages for locking on to the female during copulation. They do not possess large body reserves like the sexual females.
CHAPTER 3 (#ulink_61715572-728c-5e76-8891-58a4724f1aac)
TYPES OF BRITISH ANT (#ulink_61715572-728c-5e76-8891-58a4724f1aac)
IDENTIFICATION
Three keys are given here: the first to sub-families, the second to genera and the third to species. These are based on characteristics shown by workers and, in those species which lack them, queens; males help a great deal but for simplicity have been left out. To identify ants as far as the species is difficult; indeed, experts are often not in agreement about some very similar forms. Nevertheless the four main genera, Myrmica, Leptothorax, Lasius and Formica, have been here divided into their most common and easily identified species. The key is not based solely on structural features but includes in a few places reference to habitat, nest site and shape and other aspects of natural history. A low-powered stereoscopic microscope is best for assessing many characteristics but a hand lens may sometimes be adequate. Many distinctions are comparative, e.g., hairy or not hairy, and it is obvious that in these cases experience and reference to a reliable, modern collection is essential.
Key to sub-families
Key to genera
Key to species
FIG. 6. Worker of Myrmica rubra: a. head, b. foreleg, c. scape of antenna, d. Myrmica scabrinodis: scape of antenna. c. and d. are viewed from behind. Hairs are abundant on the head which is strongly corrugated.
FIG. 7. Worker of Lasius niger: a. head; b. scale on petiole from behind; c. side view of tail segments to show ring of hairs around the circular orifice. The whole body is covered with a light pubescence and there are short, erect hairs on the scape of the antenna but none of these have been shown.
CHARACTERISTICS AND DISTRIBUTION
Only four of the nine or so sub-families of the family Formicidae are represented in this country. Two of these, Ponerinae and Dolichoderinae, have only one genus here. Of the other two the Myrmicinae have ten and the Formicinae two genera. The Ponerinae and Myrmicinae have certain similarities and are grouped together in a poneroid complex whereas the Dolichoderinae and Formicinae are included in a myrmecoid complex (named after the basic Australian sub-family Myrmeciinae).
The Ponerinae contain a mixture of very primitive and highly-evolved forms which are mainly tropical and Australian in distribution. In southern Europe there are at present some nine species but fossil evidence shows that there were once many more. Primitive features are the possession of a sting in the females (as in wasps) and the structural similarity between queens and workers; the latter lack only wings and ocelli. All ponerines have a constriction between the first and second segments of the gaster; here the integument forms, on the underside, an organ for stridulating. They feed largely on small animals and show foraging behaviour that ranges from the highly individual to the advanced legionary type. Larvae are able to eat prey directly and even to move about the nest slightly in the less advanced genera.
Ponera coarcta has a worldwide distribution but occurs in only 13 of the 152 vice-counties of the British Isles, all in southern England. Its colonies are small and inconspicuous and usually live in woodland amongst the stones and moss of the soil surface. There is another species, Hypoponera punctatissima, that is commonly found in glasshouses and, very rarely, in sunny situations outside.
The Myrmicinae are thought to have evolved from ponerine ants; both groups retain stings and have a tendency to a thick, wrinkled cuticle with spines on the mesosoma. The myrmicine workers have a much simpler and smaller form than the queens. Their waist comprises two segments; this waist gives extraordinary flexibility to the gaster, and enables the sting at its tip to be brought round under the body and pushed forwards in front of the head. The integuments of the second waist on the first gastral segments form a stridulatory organ on the upper side instead of the underside, as in the ponerines. Only in the more primitive genera do the workers lay eggs. Seed-eating is common in this sub-family; it also includes the only group to have perfected a method of culturing and eating fungi. Many genera have evolved social parasitism; of the ten indigenous here half show this tendency.
Undoubtedly the most widespread genus in the British Isles is Myrmica. It is a brownish-red ant found in many different habitats, in small colonies that rarely exceed 3000 workers. They sting effectively and painfully if disturbed. Myrmica reaches into every one of our islands. There are eight species; one, Myrmica ruginodis, is apparently the only ant to have colonized Shetland and the only species so far which has been found in all of the 152 vice-counties. At the opposite extreme there is Myrmica speciodes found only in Kent and Sussex, although it is more common in Europe. This is one genus which shows hardly any geographical bias here, for of the eight species which occur in southern England, six are also found in Scotland.
The next most common genus is Leptothorax, although only one of its four species (Leptothorax acervorum) is widely, though patchily, distributed throughout the British Isles. It is a small, brown ant living in small colonies rich in queens, nesting in quite hard wood or in twigs or under the surface crust of soil. The other three species are restricted to southern England.
Tetramorium caespitum is the only species of this genus (which is predominantly African) in the British Isles. It is a small, black ant spread widely over central Europe but restricted here to the south and farther north to coastal zones. Such habitats have a high incidence of sunshine in spring so that the soil surface where the ants nest is warmed early in the season. Furthermore, temperatures do not fall very much in winter, especially on the western coasts. Tetramorium caespitum makes large, highly-organized colonies in lowland heath in the south. In autumn it collects and stores the seeds of heather and grass for spring feeding.
Two myrmicine genera are parasitic on Tetramorium caespitum in this country. One, Strongylognathus (one species Strongylognathus testaceus), has workers of about the same size and shape as its host but they are pale brown and have curved, toothless mandibles. The sexuals are not much bigger and contrast strikingly with the large black queen of Tetramorium. It is rare, even where it is known to exist in Dorset and Hampshire. The other parasite of Tetramorium is Anergates atratulus. It has no workers and the males are wingless and curiously shaped. Again, this species has only been found in the middle south of England. Very few of the nests of Tetramorium are parasitized; this is so, even in France where both host and parasite are quite widespread.
Stenamma westwoodii is a rare ant limited to the south and rarely seen, even on the Continent. Myrmecina graminicola is a dark, thickset species which nests deep in the soil in sunny places in southern England; sometimes it makes galleries in Myrmica nests where it probably preys on the brood. Formicoxenus nitidulus is a small ant with a highly polished cuticle and lives in nests with wood ants. Solenopsis fugax is a small, yellow ant living underground near Lasius or Formica nests, eating their brood; it is restricted to the south in this country. Finally comes a completely parasitic, workerless species, Sifolinia karavajevi, only recently found here in a colony of Myrmica sabuleti. Emery first caught a female flying near Siena in 1907. The species has been found in Europe and Algeria on a few occasions but the number of colonies parasitized by species of Sifolinia in the whole hemisphere must be remarkably small.
Next are the two myrmecoid sub-families, Formicinae and Dolichoderinae. Both have only one joint in the waist which often carries a well-developed scale, particularly in the former sub-family. The gaster is unconstricted and contains no sting. Instead there is a poison apparatus opening in the Formicinae in a circular orifice, usually surrounded by hairs just below the anus. A jet of fluid consisting largely of formic acid can be shot for several centimetres from this after it has been bent round under the mesosoma and directed forwards. The Dolichoderinae do not produce a liquid jet but in most cases a sticky toxic chemical is extruded from their slit-shaped anus. The difference between queens and workers in the Formicinae is considerable, though the workers do lay eggs in some genera and may produce females as well as males from unfertilized eggs. Other primitive features are the retention of a cocoon to enclose the pupa, the presence of visible ocelli in workers (in some genera) and the incompletely-fused mesosomal sutures. Often, too, they are highly individual and forage singly. Most seem to have a well-developed valve between the crop where imbibed food is stored before regurgitation to larvae and the midgut where food is digested. This is kept closed by the presence of fluid in the crop and does not need a persistent muscular effort, as it is thought to do in the Myrmicinae. Often these ants have quite good vision through the usual insect compound eyes but some species rely entirely on chemical and tactile senses. All make use of nectar and honeydew as well as hunting prey but they do not appear to eat seeds very much (unless these have an oily caruncle).
There are only two genera of formicine ants represented in the British Isles, Lasius and Formica. Lasius are smaller than Formica and have less well-developed eyes. They form a large, diverse genus that is widely distributed throughout the temperate parts of the Northern Hemisphere. The eight British species have a slightly southerly bias: there are only five in Scotland and none at all in the Northern Isles (Hebrides, Orkney and Shetland.) Some are yellow and live entirely in the soil, others are jet black and forage in files up trees for honeydew. Their colonies are often enormous, extending to tens of thousands, and they may have only one queen. In general Lasius are very skilled at making nests out of soil or wood pulp. Undoubtedly the most striking is the jet black, large-headed Lasius fuliginosus which forages up tall trees from a carton nest in a rotting stump. It occurs sporadically in southern England. The queens are not much bigger than the workers and are unable to found colonies alone; they parasitize Lasius umbratus and other species as a first stage in colony formation. Lasius umbratus also has small queens and seems to be an obligate social parasite of Lasius niger. It is not often seen above ground, as its name implies, but is said to accompany Lasius fuliginosus up trees from mixed nests. Lasius brunneus, too, nests in old trees, usually oak, in open country; it has a curious distribution in the south Midlands, in part of which it has taken to entering the timber of buildings. Lasius alienus, a small, brown species, lives in subsurface galleries in the warm soils of heathland, limestone or chalk; it ejects the excavated soil to form characteristic craters in spring. Very common in Europe it extends like Tetramorium caespitum into Ireland and Scotland along the coasts. The behaviour of this ant seems to vary geographically: thus in England it rarely ascends into bushes, as it does in the Mediterranean area, but in America it lives in woodland. Lasius flavus is even more confined to the soil and in many parts of western Europe it builds large mounds of soil which are permanently covered with vegetation and which become a conspicuous feature of the landscape in areas of uncultivated grassland. It ranges throughout the British Isles except for parts of northern Scotland and Ireland. In many places, however, it is unable to form mounds: on steep slopes, in areas of high rainfall, in dry sandy soil or in cultivated grassland.
One of the most commonly encountered ants is Lasius niger. It is widely distributed throughout the British Isles but appears to be absent from some places in Ireland and Scotland. Bushy scrubland and gardens or wet places are its favourite habitats and it inhabits grassland only when stones or the mounds of Lasius flavus, which is a much more skilful builder, are available for it to nest in.
Formica is a genus of big, long-legged ants that spend their foraging lives in shrubs and trees and may build large mound nests of plant debris, although some only excavate galleries and chambers in the soil. It is widely distributed throughout the cooler parts of the Northern Hemisphere. There are eleven species in this country, nine in southern England, five in northern England, six in Scotland, five in Wales and three in Ireland. It thus shows a considerable southern bias.
The simplest Formica species are Formica fusca and Formica lemani. Both are black, have relatively small colonies and live in simple excavated nests in soil. They have strong workers that hunt and forage in bushes alone and are capable of carrying large prey back to the nest. These two species differ in several small ways; Formica fusca is less hairy, has up to half its pupae bare and is said to have fewer queens; it seems to be absent from all of Scotland except the Western Isles. Formica lemani occurs farther north than Formica fusca and is the only Formica in the Northern Isles. Both the species are common and widespread wherever they occur and in areas where they overlap Formica lemani tends to live in the cooler zones with the more northerly aspect.
There are also a number of rare Formica in this country, allied to Formica fusca; Formica cunicularia occurs in England and is slightly browner and hairier than fusca and often collects some plant material to make a small mound nest; Formica rufibarbis is quite reddish and very local. These are both common on the mainland of Europe. Formica transkaucasica, a jet black and shiny ant, is very rare, even in the south, but extends widely into Asia and is said to be the species which lives highest in the Himalayas. In England it is a specialist bog liver and covers its nests with small domes of cut grass, often on the tops of Molinia tussocks. Formica sanguinea is a large, red ant allied to the wood ants and has the habit of collecting the pupae of Formica fusca; many of these are eaten but some hatch out and the fusca workers stay on in the sanguinea nests, co-operating in the nest work; they have misleadingly been called ‘slaves’. The queens enter Formicafusca nests and replace the normal queen, thus living temporarily as social parasites. Formica sanguinea is widely but patchily distributed in the British Isles; it occurs in Scotland and southern England but not in northern England, Wales or Ireland. Formica exsecta is a small wood ant with a distribution like that of Formica sanguinea; it can be recognized by the cut-out scale and back of the head and by the fact that it builds mounds of vegetation in scrub and heath that are never very large and are really not much more than thatched soil mounds.
Finally in this cursory survey come the spectacular wood ants, well known for making huge mounds of vegetation debris with tracks to and up large forest trees on which they hunt for food. All have good sight and are expert with jets of formic acid which they can shoot several centimetres. There are three species. Formica rufa ranges over most of eastern and western England but is rare in the north and absent in Scotland and Ireland. Formica lugubris by contrast ranges from Wales and Ireland through northern England to Scotland, where it is widespread in the Highlands but apparently not in the Lowlands. The third, Formica aquilonia, is almost confined to the Scottish Highlands but has been recorded from one place in Ulster. It is an inhabitant of northern Europe and the High Alps. The differences in ecology between these species, so far as they are known, will be discussed later. On the European mainland there are two other species; one, Formicapolyctena, is very like rufa but has many more queens. Another, Formica pratensis, is less wood-bound than rufa and lives in meadows and roadsides, where it makes rather small nests. In central southern England it has recently been extinguished by suburban development.
The sub-family Dolichoderinae is represented only by the species Tapinoma erraticum, an active small, black ant with many queens in its colonies. This species makes nests in heathland; they are a mere 10 cm across and are covered with, and in part constructed of, vegetable debris. The ants seem always to be moving from one to another during the summer. Though widely distributed in Europe this species is confined to the central south of this country.
SPECIES RICHNESS
It is quite obvious from what has been said that the south is richer in species than the north. To be precise, of the 42 so far found in the British Isles, 33 occur in Dorset, 31 in Hampshire, 29 in Surrey, 27 in the Isle of Wight, 26 in East Kent and South Devon and 24 in Berkshire (see fig. 8 (#litres_trial_promo)). The regional divisions into which the Watsonian system groups its vice-counties show that Channel has 37, Thames 31 and Severn only 25 species. South Wales (24), Anglia (23), Trent (20), North Wales (18), and Lakes (18) come next. Not far behind are Humber (16), Mersey (15), Tyne (14) and the Scottish Lowlands West and East (14). The number rises to 15 in the West Highlands and 16 in the Eastern Highlands but drops again to 14 in the Northern Highlands. There are only 4 species in the Northern Isles. In Ireland, Leinster (18) and Munster (17) are twice as rich as Ulster (9); Connaught with 14 is intermediate.
This southerly tendency can be seen in ant distribution over the whole world. The humid Tropics have by far the greatest number. No doubt temperature is the most important single factor in this but rainfall, soil type, vegetation and humidity are subsidiary and of course highly interrelated. In this country the temperature of the soil where the ants live is probably more important than the air temperature and ant distribution is strongly influenced by the hours of sunshine in spring; this can be seen very clearly from fig. 8 (#litres_trial_promo) in which the hours of sunshine in May have been plotted over the number of species per vice-county.
FIG. 8. The number of species of ant per vice-county and the zones where daily sunlight in May averages 6 to 7 hours. Black: over 30 species; densely dotted: over 20; lightly dotted: over 10; blank: between 1 and 10. The information on ants was obtained from the Transactions of the Society for British Entomology, Volume 16, Part 3, pages 93–121, Collingwood, C. A. and Barrett, K. E. J. The information on sunlight is from the Climatological Atlas of the British Isles, published by H.M.S.O. in 1952.
CHAPTER 4 (#ulink_6ef1f923-a888-59c9-9c91-dc87ba637834)
FEEDING (#ulink_6ef1f923-a888-59c9-9c91-dc87ba637834)
EARLY ants lived on soil insects and this is still true of some primitive Ponerines. As simple predators ants were rather a long way from the primary source of food, the green plant, and their scope for population growth and spread and evolution was limited. The use of plant carbohydrates for energy, saving protein-rich foods, cannot have been long delayed as nectar-gathering is well established in the other primitive branch, the Myrmeciinae. Nectar is the common source of sugar in nature but other sources such as honeydew and fruit, both of which are quite easily recognized from their sugar and organic acid content, are more often used by ants. Seeds, though they are a very valuable source of food, are not eaten much, perhaps because their nutritive value is less easily recognized; they are, after all, covered in a tough skin. Fungi also have food value but are hardly used at all, though some of their threads which enter nest cavities from the surrounding wood or soil or which grow on their rubbish heaps may be cut and eaten. Only one group of ants, in tropical America, eat fungi regularly and systematically and these are cultivated in the nest and fed on vegetation which the ants collect regularly.
PREDATION AND SCAVENGING
Most predatory ants have a varied diet which usually includes a lot of small invertebrate animals of about their own size and an occasional vertebrate corpse. They are not impressive as hunters but not a great deal is in fact known about the circumstances in which they catch their prey and there is much work to be done in this field. From what is known so far it appears that they can detect other animals from a distance of a few centimetres. Thus wood ants are known to be able to see movements 10 cm away. In soil spaces and perhaps in foliage ants may be able to detect the vibrations made by small animals moving through the substratum and they certainly have an acute sense of smell. Once they have received a distance signal they approach slowly, alert for others, probably mainly smells, before actually attacking. As they get nearer to the potential prey they orientate with their head forward, jaws open and antennae retracted and make further exploratory movements. Clearly, they must not throw themselves into the jaws of an enemy and a secure grip on prey is an obvious advantage. Finally they pounce and snap. If a hold is obtained, usually on a leg or antenna, they quickly bring round their gaster and inject poison with their sting if they have one, otherwise they spray penetrating or adhesive toxins. Not many ants are really alone when hunting, there are others nearby usually that help by flushing out prey or gathering round to help pin one down; they are more dog-like than cat-like on the whole.
Many small invertebrates are easily caught and immobilized; the small larvae of flies and moths and beetles, for example. Many others, particularly adults, have escape mechanisms. Thus, springtails (Collembola), which are very much valued as food by Myrmica and Lasius, can usually escape instantly by jumping but are easily caught while they are changing their skins, stuck in a water droplet or in some way damaged. The larvae of many sawflies and moths can flick their bodies smartly, others are so hairy that ants find them difficult to get hold of and many caterpillars can simply slip away on a thread of silk; some protect themselves with a case of vegetable material. To escape, other insects kick or produce repellents or sticky exudates from special skin glands, such as the cornicles of aphids. Those with a hard, shiny cuticle, like beetles, may be difficult to grip and impossible to sting. The great variety of these defensive mechanisms make it highly probable that many prey animals are only caught when they are incapacitated, perhaps through age, perhaps by mechanical damage (such as being trodden on), by wing failure, by the necessity to moult or even by transient low temperatures or weak light (some insects can only fly in sunlight). Bugs, flies and spiders comprised 80–90% of the number of prey caught by species of Myrmica. Small spiders that live on and near the soil surface are a major constituent of this food (11–38% in different years). In Polish grassland from 1700 to 4000 spiders were caught in a year in one square metre. In May, June and July, when feeding peaks, between sixteen and seventy-four spiders may be taken each day from one square metre.
Undoubtedly, an important element in the diet of most ants is other ants. Each summer they eat large numbers of sexuals, both of their own and other species, especially those that are unlucky enough to descend after the mating flight on to already occupied territory. Also, in spring when food is scarce and catastrophic fires destroy the vegetation, a wider search for food often leads to fighting between workers of different colonies and species in which a lot die. The corpses are taken back to the nest and sucked dry: an economical way of adjusting the population to a sharp drop in food supply.
Information is very much needed about the food of different colonies and species at different times of year. Some ecologists have taken samples of foragers on their return to the nest and identified what they were carrying. One has recently invented an ingenious trap which has been used to study the food of the wood ant, a species with two advantages: well-defined tracks above ground and large workers. A whole nest was surrounded by a barrier soaked in repellent oil and workers were induced to pass over this on specially constructed wooden bridges. One was used for incoming and the other for outgoing traffic. This was possible because although ants are prepared to drop a few centimetres from the end of a bridge on to the soil below they cannot reach up to return the same way. So, it was only necessary to place strips with the drop outside the barrier to take outgoing traffic and the drop inside the barrier for ingoing traffic. Those returning dropped into a box from which they could only escape by small holes little bigger than their bodies and they left behind anything they had been carrying in their jaws. Perhaps some day, traps on a similar principle will be devised for ants which forage underground, but this is likely to be much more difficult.
Wood ants eat many invertebrates which they catch both in the trees and on the ground. A large number of these are flies (including midges and crane flies (Tipula spp.)) and many kinds of aphids; also, in season, winged ants, particularly of the genera Lasius and Myrmica. A lot of these insects are forest pests and the establishment of wood ant populations has become, at least in Europe, an important part of woodland management. In years when defoliating insects are very abundant and trees are stripped of their leaves in summer, conspicuous green islands are left around areas where ants nest. These defoliators include the larvae of various moths and sawflies that feed on oaks, pine, spruce and larch. Pupae and adults are also eaten. There are indications that wood ants will attack moving things in preference to quiet, still ones; yet it was found by trapping that they collected a lot of prey at night. There are of course a great many flying insects that settle in the foliage of trees and bushes during darkness; these could perhaps be easily located by smell. A wood ants’ nest was surrounded by guttering into which they threw their refuse. This not only consisted of empty cocoon fragments as expected but of many other insects and other odd items that were collected but not eaten; evidently they take in a great many more things than they use.
Food collection by Formica aquilonia has been studied in detail in an old Caledonian forest. Some five or six trackways leave each nest and go to trees on which prey are caught and aphids are tended for honeydew. After leaving the nest, ants pass round the perimeter and then leave on any one of the tracks. There is just a slight tendency for individuals to use the same track out as they use in. This is oddly at variance with other results which have shown taat Formica rufa and its allies come near to partitioning their foraging grounds between groups of workers that are fairly fixed in individual composition. Different species, different types of food collection or just different times of year may explain this apparent contradiction. Many of the foragers of Formica aquilonia leave the trackways to forage in the neighbouring herbs and observations show that this ‘leakage’ occurs at a constant rate and that the search for prey is quite random until very high prey densities are encountered. Then a recruitment mechanism increases the number of ants entering the area. Temperature affects the rate of flow of traffic on the trackways; a rise from 8 to 18% increases it 5 times. This is probably due to a greater availability of prey at high temperatures, as well as to a greater number of foragers joining in.
Rate of traffic flow is also very much affected by obstacles. If the leaves and twigs are swept from a track the walking rate rises as much as 10%; at 20° C it is normally about a metre a minute. In sections where the tracks pass between rocks traffic density is often so high that collisions are frequent. This causes some delay as the ants stop to examine each other with their antennae. There seems to be very little organization of the flow near the nest: a slight tendency exists for incoming workers to move on the outside and outgoing ones in the centre of the track. In the July of the study there were about seventy thousand foragers active; one in five brought an insect back and it was estimated that about a hundred thousand insects were collected each day.
All this information is not very well received by entomologists primarily interested in the insects which are destroyed. They claim, not without reason, that wood ants impoverish the fauna, but the interrelationships between insects are so complicated and numerous that it is more likely that they merely prevent any one type from predominating and thus preserve a richer mixture at a lower density. As has been pointed out there is plenty of evidence that they concentrate on prey that is momentarily superabundant. Birds of course do this too, and as there are many which live on insects in forests they might be expected to compete with ants. Curiously, the evidence is to the contrary and it is suggested that ants dislodge many insects whilst hunting which they lose and these are caught by birds. This sort of situation is well known in the Tropics where some birds subsist largely by collecting the prey which escapes from the devastating columns of army ants.
The food of Lasius flavus was a mystery until quite recently but it is now known that they eat a great many soil animals, including soft-bodied mites, beetle larvae (notably two species of wireworm), woodlice, other workers of their own species and, in season, queens caught after the nuptial flight. Interestingly enough, they eat more of their own species of queen than of Lasius niger. This last ant is more aggressive and larger than Lasius flavus and it forages both above and below the ground. No doubt as a result it has a much wider range of prey which includes several species of ant, larvae of beetles (again, frequently wireworms), caterpillars, bugs, earwigs, harvesters and woodlice. It has been seen collecting Cabbage White caterpillars in gardens.
In late summer, after a period of dry weather, fires may destroy all the vegetation above the soil surface in heathland. Then the ants Lasius alienus and Tetramorium caespitum eat many soil invertebrates, predominantly the long, slender centipede, Geophilus, and several kinds of wireworm. As already mentioned, food scarcity causes them to search more widely and they meet neighbouring ants much more often, fighting ensues and finally cannibalism. Myrmica eat aphids, springtails, fly larvae, adult flies, spiders and many other small creatures. Some species differences in food must exist as Myrmica scabrinodis is known to hunt nearer the soil in shorter vegetation than Myrmica ruginodis. These ants also remove flesh from the carcasses of dead birds and mammals.
A matter of considerable interest is whether ants control the number of honeydew aphids by butchering and eating surplus ones, and, if they do, how they recognize those that are surplus. Formica rufa is known to kill aphids that crawl away from the main clusters. They might only wander in this way if their food supply was overloaded; it happens particularly after storms which must of course be disturbing and at certain times of year for unknown reasons. Lasius niger, though it frequently collects honeydew from the Black Bean aphis (Aphis fabae) on broad beans, never takes any back to its nest, according to one investigator. Others have watched both this species and Lasius flavus carry dead aphids nestwards and feed them to their brood. The tendency with Lasius niger is probably to destroy aphids that can no longer produce honeydew, especially if these try to defend themselves with wax from their cornicles. This species also tends an aphid (Protrama flavescens) underground and kills and eats the parasitized ones selectively. Lasius flavus is now known to eat seven species of myrmecophilous aphids. As no other types of aphis are eaten so extensively it seems likely that a special predator/prey relationship has evolved. As they are taken in both young and old stages the basis on which the cull is made is unlikely to be senescence. There seems good evidence that these ants, after removing and eating all the honeydew they need, kill the surplus aphids from protein hunger. One possibility is that too much honeydew is produced when aphids are surplus, with the result that it leaks out and smears them and is decomposed by bacteria so that they are no longer recognized and protected by the foraging ants. Other ecologists have suggested that the honeydew which they offer deflects the ants’ aggressive actions by satisfying another facet of their appetite. There is some support for this in laboratory experiments with Myrmica which have shown that if sugar solution is provided, fewer flies are killed. There is also some evidence that clusters of aphids farthest from the nest are considered more expendable than clusters nearby. Clearly, our understanding of this relationship is rudimentary as yet; it seems that any abnormal state or activity, particularly unnatural movements, positions or smells, may cause the ants to attack instead of protect the aphids and that these abnormal conditions tend to arise more often when the aphids are overcrowded and in need of culling.
SEED EATING
