The physiological problems connected with Insect parasitism are of great interest to the entomologist; the modes of nutrition and respiration of these encaged creatures could not fail to be most instructive were we fully acquainted with them. It is obvious that when an Insect-egg is laid inside another Insect's egg, and the parasite has to undergo the whole of its growth therein, it is in the strangest condition as regards nutrition. It is unnecessary for the intruded egg to have yolk of its own; moreover, the embryonic mode of nutrition may be continued during what would, with other Insects, be the larval period. And it seems to be the case that both these conditions are actually met with in the lives of egg-parasites. The embryology and post-embryonic development of parasitic Hymenoptera have already been ascertained to be of the most extraordinary nature. Great variety, however, will no doubt be found to exist, as will be readily understood if we tabulate the conditions of the early life of various parasitic Hymenoptera.
1. The egg may be laid outside a larva, and the embryonic and larval developments may both be passed on the exterior.
2. The egg may be laid and the embryonic development passed through, outside the host, but the parasite on hatching may enter the host, so that the post-embryonic development is passed in the lymph of the host.
3. The egg may be laid inside the host, both embryonic and post-embryonic developments being gone through in the fluids of the host.
4. The egg may be laid inside another egg, the embryonic and post-embryonic developments being passed therein.
We shall find that all these conditions exist in the Insects we are about to consider.
We shall treat the series as composed of ten families; but we must remind the student that this great subject is still in a very unadvanced state; the combined efforts of generations of naturalists will be required to perfect it. Of the ten families five are comparatively insignificant in number of species. Many of the Cynipidae are not parasitic in habits, but live in galls. After what we have said as to the mode of nutrition of parasites it will be understood that the physiological conditions of life may not be so different in a gall-dweller and a parasite as would at first be supposed; and it is perhaps not a matter for much surprise that good characters cannot be found to separate the gallicolous from the parasitic forms.
Fam. I. Cynipidae—Gall-flies.
Wings with very few cells, with no dark patch (stigma) on the anterior margin; pronotum fixed to the mesonotum, and at each side extending back to the point of insertion of the front wing. Antennae not elbowed but straight, composed of a moderate number (12-15) of joints. Early stages passed either in galls or as parasites in the bodies of other Insects.
