The Proctotrypoid Parasites 



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 void- 

 ing 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 ap- 

 peared, and they breathe 

 by absorbing oxygen 

 through the skin from 

 the blood of the insect in 

 which they live. The skin 

 o '^ is very delicate and the 



Fig. 25.— Pelecinus polyturator. OXygen paSSeS through 



(After Packard.) ^^y 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 breath- 

 ing 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 Qgg 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 



50 



