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Proctotrypoid Parasites — Part 1

(Super-family Proctotrypoidea.)

With the consideration of the insects of this group we first meet with forms which were formerly grouped together in a section called the Hymenoptera parasitica, the true parasitic Hymenoptera. In the old system this included the families Ichneumonidae, Braconidae, Chalcididae and Proctotrypidae. Other groups have been added since, and entomologists now, following Ashmead, consider the Proctotrypoidea, in spite of their invariably parasitic habits, to be more closely allied to the Vespoidea and Cynipoidea than to the Ichneumon flies and the Chalcis flies. This is undoubtedly true as to structure, and it will be remembered some of the Vespoidea which we have just been considering are parasitic in their habits, while, as will be seen when we take up the Cynipoidea, some of these insects too, although most of them are gall-makers, are truly parasitic in their life.

How internal parasites live.--The development of the larvae of those parasitic insects which live within the bodies of other insects has been the subject of much speculation and some investigation. How these creatures breathe, nourish themselves, move, cast their skins, and pass their excrement have been mooted points. Cuvier thought that these larvae breathe by placing their spiracles in relation with those of the insect in which they live. Ratzeburg showed that some of them have a curious caudal appendage with very thin walls, and this he thought acted as a blood gill, oxygen being gained through its walls from the purified blood of the host insect. Boisduval concluded that they do not take nourishment through the mouth; that they do not breathe, and that they void no excrement, the larvae being analogous to the foetus in mammals, which lives the life of the mother. Newport described the larvae of certain Ichneumon flies as having no anus, the rectum and its orifice being rapidly developed at the final molt of the larvae. The older authors thought that these larvae attack only the fatty tissues of the host, or that they nourish themselves exclusively on the lymph.

Through the work of a recent French author, Seurat, we now understand better how these internal parasites live. They have very sharp jaws, and use them in order to pierce the tissue of the host. The digestive tube has a very voluminous stomach which is closed behind, and which serves as a storehouse for food which is digested later. A small quantity of food digested at once suffices for the immediate wants of the larvae. The voiding of the excrement takes place only in the interior of the cocoon, the stomach not opening until the larva changes to pupa. The parasite larva feeds upon the fat and blood and the lymph, and in most cases devours everything except the skin and the air vessels. It respects the viscera up to the last limit, and only sacrifices them at the end. In the youngest of these internal parasites the air vessels have not appeared, and they breathe by absorbing oxygen through the skin from the blood of the insect in which they live. The skin is very delicate and the oxygen passes through by osmosis. A little later the air vessels begin to appear and become very complicated, branching all through the skin. The skin in fact appears as if carpeted with an extremely rich network of fine breathing tubes. In no place do these breathing tubes open through the skin but the oxygen is absorbed into them through the skin from the blood of the host. When the parasite leaves its host its spiracles or breathing holes open, and the outside air enters through them into the air vessels. Contrary to the old idea, these internal parasites sometimes molt, and the cast skin slips from the anal end of the body into the open space behind it. A remarkable observation has been made by Marchal, who states that a single egg of Encyrtus laid in the egg of a little Tineid moth divides up into a great number of embryos which develop into individual larvae in the larva of the host.

These remarks are general, and apply in a way to all of the parasitic Hymenoptera. In the Proctotrypoidea a curious hypermetamorphosis occurs in certain egg parasites in which there are three distinct forms of the larva of the same species, one form resembling the little aquatic animals of the genus Cyclops.

The eggs of the Proctotrypoids are ovate, with a peduncle at the end. Nearly all of the larvae live within other insects. Very many of them live in insect eggs, undergoing their transformations within these eggs, sometimes a half-dozen or more being found within a single egg-shell. Still others live within the larvae or the pupae of insects, in which case they gnaw their way out before spinning cocoons, or sometimes transform within the body of the host, in which case there is usually no cocoon, the skin of the host giving ample protection. The insects of one genus, Trichacis, develop in the nervous system of the larvae of one of the little gall midges, while the larvae of another genus, Polygnotus, develop in the digestive tract of the same insect.