MANY insects can behave as parasites or, alternatively, can harbour parasites at every stage of their life-history. Some insects, such as the ichneumons, lay their eggs in the larva; of other insects, and, when the eggs hatch, the young larva; feed on the tissues of the host larva;. The important parasites are included in the groups of beetles, ants, bees and wasps, and flies. An insect may be parasitised by another which is also parasitised and so ad infinitum, and a balance may exist which ranges between limits favourable to all the parasites in question. Two parasites on the same insect have to bargain for a place in the sun, even if the bargaining entails force and the subsequent annihilation of one or other of the bargainers.
The presence of parasites on an insect host tends to control the numbers of that host, and as parasites are often limited to one kind of host, their range is limited and the host species tends to become more and more infested with the parasite until it is exterminated or the parasites cancel themselves out. Where parasites have other parasites living on them, the parasites themselves are controlled, with the result that the host survives. Parasite control has been used by entomologists for the control of insect pests; the process has been spectacular in its success in some cases, but great dangers and complications may attend it—for example, a parasite introduced into
new conditions for the purpose of controlling a pest may fasten on another and beneficial species instead, thus becoming a pest itself.
Much might be written about the question of parasitism, for there are grades of it each with different implications. Certain insects change their habit of living very easily, and when they do this, corresponding changes take place in their structure to enable them to carry on their new mode of life successfully. In this way new races are formed, and these races, which may be interchangeable, are referred to as biological races, since change of habit brings about structural changes. The body and head lice of man is a case in point. The difference between these two varieties lies chiefly in the structure of the claws. One can be converted into the other in two or three generations.
LODGERS THAT DO NOT PAY FOR THEIR KEEP IN addition to body parasites, there are what are called social parasites—solitary insects which take advantage of the social insects by living in their colonies and receiving board and lodging at their expense. They may indeed actually acquire a superficial resemblance to their hosts. There are solitary bees which build cells and provision them with pollen for their larva?. There is another solitary bee which builds no cells but exploits the cell-building bee by laying eggs in its cells. This habit is a parallel case to that of the cuckoo which lays its eggs in the nests of other birds, and hence the bee is called a ‘cuckoo bee.’ The larva? of the cuckoo bee hatch before those of the host, and they live on the pollen which the host bee had collected for its young. When the host’s larva? hatch, the cuckoo bee’s larva? attack and consume them.
Certain beetle larva? are found in flowers, and one, Meloe, lies in wait for a visiting bee. When one arrives, the larva clings to hairs of its legs and is carried to the bee’s nest; the bee brushes it off with the pollen with which the hairs are covered. The larva continues its development at the expense of the bee’s provisioning and young.