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Ichneumon Flies — Part 3

The number of Pimpla larvae nourished by a single tussock moth caterpillar varies from one to ten, with perhaps an average of three or four. This is the case in the summer time, but in the autumn more are found. Ten or fifteen in a single cocoon are not unusual at this time of the year, while in one case the writer has seen twenty-three male Pimpla cocoons in a single cocoon of the tussock moth. This particular cocoon mass was cross-sectioned and is shown at c in the accompanying figure. When fall comes and the feeding and breeding season is past, all living individuals of this particular parasite will be found snugly packed away in the larval state within the bodies of some host insect and the winter is passed by them mainly in the larval condition in their cocoons. With the approach of spring, however, they transform to pupae and about the time when the young host caterpillars begin to hatch from their eggs the adult parasites emerge ready for the summer's campaign.

In this act of emerging it is interesting to notice that as with many other parasitic Hymenoptera and in fact with many other insects the males as a rule issue before the females and wait with impatience for the companionship of the fair sex. From one lot of cocoons studied by the writer in the spring of 1896, forty-nine males issued between March 3d and 11th before a single female put in an appearance. On March 13th and 14th a single female issued each day; on the 17th eight more; on the 18th three more; on the 19th fourteen more, and on the 20th twenty more. In the meantime the number of males issuing had rapidly fallen off and they eventually ceased to make their appearance. The number of members of both sexes was approximately equal.

A curious fact may be noticed concerning the cocoons from which these early spring individuals issued, that is to say, the over-wintering cocoons. We have seen that an entire generation may be produced in a space of fifteen days in midsummer, but, of course, in colder weather the development is slow and the time may be much extended. Again, in the absence of proper host insects the females may live for several weeks without having opportunity to deposit their eggs. The midsummer cocoons are rather fluffy and although more closely spun than are the cocoons of the tussock moth caterpillar, they are still rather loose and the outer silk in particular is loosely spun. With the over-wintering cocoons, however, it is at once noticed that they are of a closer, tougher and more parchment-like consistency. There is less of the loosely spun silk with the individual although more of this loose silk surrounding the whole mass in any given host cocoon. Thus it seems as though the parasitic larvae in preparing for the winter appreciated the degree of cold which they would have to experience and wisely prepared for it by making their habitations thicker and tighter against the inclemencies of the weather.

As abundant and hardy as this species seems to be it does not escape the attacks of enemies of its own. Certain soldierbugs have been seen to capture the adult females of the Pimpla when they were engaged so assiduously in egg-laying that they were blind to their surroundings. This is not surprising since one can approach them during this process so closely as to be able to study them well with a small hand lens. Then, too, secondary parasites have been reared from their cocoons, that is to say, internal parasites of their own, and still more strange to relate, Hymenopterous parasites of these secondary or hyperparasites have also been reared from the Pimpla cocoons. An extraordinary chain of links in the development of species is thus brought about and may be studied by any one during almost any summer in one of our northeastern cities. When the tussock moth caterpillars appear in great numbers on our shade trees during a given season, it will almost invariably be found that this Pimpla is present also in great numbers and that the majority of the caterpillars are stung by it. This means that the following season there will be an unusual number of adults of the parasitic insect which is now termed the primary parasite. So great is this abundance that the first generation of tussock moth caterpillars is practically wiped out of existence. Then comes the second curious fact, that the secondary parasites become enormously abundant and kill off the abundant Pimplas. At the end of the same season or at the beginning of the next the tertiary parasites put in their appearance and the secondary parasites are destroyed, thus giving relief again to the primary parasites which once more begin to be abundant and ready for the next case of super-abundance of the host caterpillar. This little chain of species depending upon species offers one of the easiest and most interesting series of observations which may be made by any school class in the part of the country indicated.