Collins New Naturalist Library

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

Whereas prey provides mostly protein and oil and decomposes quickly, seeds are also rich in starch and they store well, provided germination can be prevented. Seed storage is a regular feature of ants that live in deserts where seed-producing, ephemeral plants may be quite common and the main source of food. Even in this country Myrmica, when it lives in acid grassland, collects the seeds of Potentilla erecta, but whether they are eaten or merely put in the nest is uncertain. Tetramorium caespitum on the other hand collects a great many seeds of grass and heather in late summer, both from the plant and after searching on the ground. Each seed is picked up and carried away individually; curiously enough, ling (Calluna vulgaris) is favoured, even though it has the smallest seed. Vast numbers are stored in galleries some 10–30 cm below the soil surface and are fed to the growing grubs in spring after a little preliminary mastication by the workers. They form a staple part of the diet and contribute substantially to the growth of the sexual brood in spring. The seeds never germinate during winter, this may simply be due to the low temperature or because they need light, for there is no evidence that the ants treat them in any way. Seed-eating myrmecines from semi-desert areas apparently take a great deal of trouble to keep their stores dry but this is certainly not the case with Tetramorium caespitum whose galleries are soaking wet for most of the year.

Although formicine ants do not collect seeds systematically records of this do exist. Lasius alienus collects the seeds of the dwarf gorse, Ulex minor; it only eats the oily caruncle, the rest is rejected undamaged and will germinate satisfactorily. The suggestion that the gorse is distributed in this way is reasonable. Lasius niger has been seen with viola and primula seeds; again, it only eats the stalk and the caruncle which contain oil and the rest presumably germinates after being thrown away by the ants. Wood ants, too, will take some kinds of seeds if these are put in their tracks; they, too, prefer the oily ones. The trapping results with Formica polyctena already mentioned show that a great many seeds are taken in along with other vegetable matter, such as buds and twigs; even pebbles coated with vegetable oil were picked up and carried away but soon rejected.

NECTAR AND HONEYDEW

Ants lick up many juices that exude from plants, particularly from extra-floral nectaries. A good example is the zone near the base of the growing bracken frond which secretes an attractive solution on the frond pushing up through the soil in early spring. Both Lasius alienus and Lasius niger are very fond of this; they excavate a space around the base of the frond and from positions here they interfere with the passage of other species. A number of species collect from ling, the principal heathland shrub; unlike most plants this has a little protein in its nectar and it is just conceivable that this could help larval growth. Ants are frequently found in flowers that do not produce nectar. Lasius niger may be found in the flowers of the wild poppy which produces only pollen, almost certainly of no use to them. Of course they may be lying in wait for insects attracted by the pollen. Honeybees might be caught by Lasius niger if several ants acted together but there must be many smaller insects that could be overpowered easily. Pollination by ants is not thought to be frequent but it could happen; they might transfer pollen to the stigma in their wanderings and this might be important for some plants; little is known about this. Nor is much known about how plants protect their nectaries from ants; it is usually suggested that a long corolla tube, down which only a bee can get its tongue, is in itself quite adequate, but surely ants could easily bite through to the sugar, at least one species of bumble-bee does. Honeydew is not strictly a plant exudate, for in passing through the body of the bug (usually an aphid or a scale insect) it undergoes considerable change. Aphids receive the sap more or less passively as it is under some pressure from inside the plant; they control its flow by means of a valve in their head; others feed actively as well. Though the food-carrying channels of the plant (sieve tubes of the phloem) are the most usual place from which the bugs obtain nutriment, there are some which feed on other parts, such as the leaf tissues. Honeydew has had some food substances taken out and some excretory products added. It is now known to contain a mixture of sugars, organic acids, alcohols, plant hormones, salts, vitamins, amino acids and amides. It is difficult to quantify this as it varies with the age of the plant and the aphid position on the plant, as well as the current weather and season but 10% dry material is probably a fair average. Most of this is carbohydrate but about one-third may be nitrogenous. Not only are the ordinary plant sugars present (glucose, fructose and sucrose) but some additional ones appear to be synthesized by the aphid itself; such is melezitose. The nitrogenous compounds are mainly amino acids coming directly from the plant sap, of which at least twelve kinds are common. There are no proteins. This means that honeydew can provide many of the growth substances needed but in rather small quantities. Laboratory experiments have shown in fact that auxiliary protein sources are needed for normal growth. However, during winter, honeydew from root-feeding aphids may yield enough amino acids to maintain the protein reserves of adult ants in a very satisfactory state.

The relationship between these bugs and ants has evolved from one of casual contact in which the latter merely scavenge for bug excretions lying on leaves, as do honeybees, wasps and very many other insects, to one involving a very high degree of mutual dependence. The relationship between the common Lasius niger and the Black Bean aphis, Aphis fabae, on the broad bean, has been carefully studied recently. The ant keeps the bean free of sticky honeydew in which moulds are likely to grow and prevents the aphis cast skins from adhering. This is because instead of ejecting their excretions freely, as they do if unattended, these aphids wait until an ant touches them and then release a droplet of honeydew. They quite clearly save this up, for if the ant refuses to collect, it can be and often is withdrawn. Aphids also wave their hind legs in the air less when ants are about. All these and other behavioural changes persist for some time after all the ants have been taken away. They vanish only gradually.

Other aphis species that are more often associated with ants have lost their ability to excrete independently. Instead of a large tail-piece (cauda) that helps in the ejection of fluids they have a small one and a ring of hairs round the anus that holds excrement until the ants can suck it up. Also, instead of a pair of cornicles at the rear of the abdomen that secrete quick-setting, deterrent waxes, they are without such weapons. Underground aphids do not usually shoot their excrement away; it is collected on wax plates and put in the soil crevices, but those which are regularly associated with ants simply have the circle of hairs round the anus.

Ant-attended bean aphids produce about twice as much honeydew as unattended ones and Aphis sambuci with wood ants give three times as much honeydew as normal. No doubt this arises partly from the extra contacts that the ants provide, exciting the aphids and making them eat more and hence, of course, excrete more. Possibly, too, the instant removal of these liquids prevents accumulation starting an inhibitory back action. Other factors are certainly involved; thus, the part of the plant on which the Bean aphis feeds is actively influenced by Lasius niger. They remain for much longer on the younger, more productive tissues where presumably they obtain better food. The aphids are also kept on the underside of the plant and cluster more tightly into larger groups. Both a rich food supply and high density are known to increase the tendency for sexual forms to be produced in ant-free conditions and yet when ants are present this increase in reproductive rate is very much delayed. The complete answer to this question is not yet known; at the moment it is thought that indirect influences on the movement of aphids are more likely to be important than direct ones which act physiologically.

It has often been noticed that Lasius niger protect their aphids from some of their enemies; they remove the larvae and even the eggs of ladybirds and hoverflies; the adult ladybird, though too big for them to lift, can be driven away. Not all observers agree about this, and it is true that some ladybirds and hoverflies can avoid workers of Lasius niger; so, too, can a number of insect parasitoids and indeed, one of these animals has evolved the ability to parasitize aphids underground and even take honeydew from them whilst ants are in attendance; the ants may even feed it. Such a special case in no way invalidates the generality that ants reduce interference with aphids by their enemies. The degree of ant vigilance and sensitivity probably decreases with the distance of the group of aphids from the ant nest and with the current state of their food supply. Much variation is also found between ant species in this respect, for Lasius flavus is undoubtedly able to exclude aphid enemies more effectively than Lasius niger, though this might simply be due to the fact that their nests are closed and their aphids live underground. The wood ant is also known to repel, if not attack, parasitic insects but is said by some ecologists to be less effective against hoverflies and ladybirds. Many aphids and coccids attended by ants have shelters of soil and vegetable particles built around them. These must help the ant to keep off dangerous animals as well as providing a more equable microclimate, not only for the benefit of the bugs but also for the ants which are reluctant to forage in wet and windy weather. Formica rufa cannot construct aerial chambers and loses a lot of aphids during storms but it does excavate underground and some of its aerial aphids may be found there.

Many underground aphids have evolved high degrees of dependence on ants, so much so that they are never found alone. They may be moved about and cared for in the same way as the brood. Some ants may bite through tough plant roots so that aphids can find the soft nutritious inner layers but one cannot be sure that this is not an incidental consequence of nest excavation. There is some evidence that some aphids that overwinter in an active state can only survive in ants’ nests where they are protected from fungi and cold. As already suggested they probably supply the ants with easy food at this season. Even the eggs of some species are collected and stored over winter in the nest. Lasius flavus