1. Parasitica.—Trochanters of two pieces, female with an ovipositor.
2. Tubulifera.—Trochanters undivided; abdomen consisting of only three, four, or five visible segments.
3. Aculeata.—Trochanters undivided; abdomen consisting of six or seven visible segments; female furnished with a retractile sting.
In the absence of any clear distinction between sting and ovipositor, these groups are merely conventional. The character furnished by the trochanters is unfortunately subject to some exceptions, there being a few parasitic forms in which the trochanters are not divided, and a few aculeates in which the reverse is more or less distinctly the case; moreover, the division, when it exists, is in some cases obscure, and the two pieces are of unequal size. Ratzeburg calls the upper division, which is frequently much larger than the other, the trochanter, and the lower division the apophysis. There is much reason for believing that the apophysis is really merely a secondary division of the femur. The Tubulifera are a comparatively small group, and will probably be merged in one of the other two, when the anatomy and morphology of the abdomen have been more thoroughly elucidated.
Fig. 345.—Divided (ditrochous) trochanter of an Ichneumon: a, coxa; b, the two divisions of the trochanter; c, femur. (For monotrochous trochanter see Fig. 335, A, c.)
Hymenoptera Parasitica or Terebrantia.
This is one of the most extensive divisions of the class Insecta. There can be little doubt that it contains 200,000 species, and possibly the number may be very much greater than this. It is, however, one of the most neglected of the great groups of Insects, though it is perhaps of greater economic importance to mankind than any other.
Insects derive their sustenance primarily from the vegetable kingdom. So great and rapid are the powers of assimilation of the Insect, so prodigious its capacity for multiplication, that the Mammal would not be able to compete with it were it not that the great horde of six-legged creatures has divided itself into two armies, one of which destroys the other. The parasitic Hymenoptera are chiefly occupied in destroying the tribes of vegetarian Insects; the parasites do this by the simple and efficient device of dwelling in the bodies of their hosts and appropriating the nutriment the latter take in. The parasites do not, as a rule, eat the structures of their host,—many of them, indeed, have no organs that would enable them to do this,—but they absorb the vegetable juices that, in a more or less altered state, form the lymph or so-called blood of the host. The host could perhaps starve out his enemies by a judicious system of abstention from food; instead, however, of doing this, he adopts the suicidal policy of persistent eating, and as the result of his exertions, furnishes sufficient food to his parasites, and then dies himself, indirectly starved. Ratzeburg considers that the traditional view that the larvae of parasitic Hymenoptera live by eating the fat-body of their host is erroneous. They imbibe, he considers, the liquid that fills the body of the parasitised Insect.[[428]]
The wide prevalence of Insect parasitism is appreciated only by entomologists. The destructive winter moth—Cheimatobia brumata—is known to be subject to the attacks of sixty-three species of Hymenopterous parasites. So abundant are these latter that late in the autumn it is not infrequently the case that the majority of caterpillars contain these destroyers. Although Lepidoptera are very favourite objects with parasitic Hymenoptera, yet other Insects are also pertinaciously attacked; there is quite a host of Insect creatures that obtain their sustenance by living inside the tiny Aphididae, or "green-flies," that so much annoy the gardener. A still larger number of parasites attack eggs of Insects, one or more individuals finding sufficient sustenance for growth and development inside another Insect's egg. As Insects have attacked Insects, so have parasites attacked parasites, and the phenomena called hyperparasitism have been developed. These cases of secondary parasitism, in which another species attacks a primary parasite, are extremely numerous. It is also pretty certain that tertiary parasitism occurs, and Riley is of opinion that even quaternary destruction is not outside the range of probability.