Collins New Naturalist Library. M. Brian V.

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      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 They quite clearly save this up, for if the ant refuses to collect, it can be and often is withdrawn.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