ORDER HYMENOPTERA 295 



unable to withdraw the ovipositor. Presumably they 

 deposit a single egg in each perforation. There are two 

 common parasitic species, Thalessa atraia and lunator, 

 which are the natural check upon the multiplication of this 

 form and which ordinarily keep its numbers down to narrow 

 limits. Without this check the species would probably mul- 

 tiply to a great extent. The parasites are representatives 

 of another family, Ichneuimmidcp. They are rather slender- 

 bodied and with extremely long ovipositors. In Thalessa 

 atrata they are four or five inches long. These are driven into 

 the burrows of the tremex and the eggs are deposited in the 

 burrow and presumably at some point close to the tremex 

 larva. The larva of the Thalessa is said to attack the tremex 

 larva externally and not to be internally parasitic — a little 

 variation from the general habit of parasitism for the family. 



Cynipidse. — The Cympidoe are in one sense a vegetable- 

 feeding group, though some are hosts or guests of gall-makers. 

 The primary feature is that they make galls and the larvse 

 develop within these galls. Galls are not a normal plant 

 product, nor a profluct of the insect alone, but a combina- 

 tion product of the insect and the plant. A stimulus given 

 to the plant cells results in a definite specific form of growth 

 which is just as specific as the form of the leaf or the fruit 

 of the plant on which it is formed. Species can be deter- 

 mined accurately from the form and structure of the gall. 

 The most familiar of these galls are the oak apples. These 

 are rich in tannic acid which gives the gall a decided flavor. 

 A globular structure is built out by a great growth of plant 

 cells. The gall grows and develops more rapidly than the 

 larva. The larva gets its growth by eating the cells which 

 are close to it and all the rest of the structure is apparently 

 developed purely as a protective device for the larva. 

 The greatest number of species occur on oaks, some on 

 roses, etc. There are dozens of dift'erent kinds on oaks and 

 many different species on a single species of oak and some 

 restricted to one certain species of oak alone. 



Parasitica includes several families: Ichnemnonidw, a large 

 group iBraconidcEiChah-id idee, a large group ;and Proctotriqndw. 



