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
