ICHNEUMONS AND THEIR ALLIES 109 



hardly looks like an insect at all, yet the fly is unmistakably 

 Hymenopterous. 



The various families of parasitic Hymenoptera differ materi- 

 ally in structure, being sometimes more closely allied to a non- 

 parasitic group than to any other parasitic one. Only a few 

 families are altogether parasitic upon other insects ; the rest 

 include gall-flies or vegetable-feeders. These facts suggest 

 that the parasitic habit may have been independently acquired, 

 and that more than once or twice. 



Encyrtus fuscicollis, a Chalcidid, infests the larva of the 

 small ermine-moth. From 50 to roo eggs or larvae may 

 be found within a single caterpillar. They are at first in- 

 vested by a common branching envelope with a cellular lining. 

 The air-tubes of the caterpillar ramify upon the envelope, and 

 supply the parasites with air. All the larvse inhabiting one 

 caterpillar are of the same sex. This is wonderful enough, 

 but a still more curious fact has lately come to light.* The 

 branched sac with its many embryos proceeds from a single 

 egg, introduced by the female Encyrtus. The embryo buds, 

 like a polyp or a plant. This process of embryonic fission, as 

 it is called, is probably not uncommon in Chalcididae. It is 

 not peculiar to insects, but occurs in animals of several different 

 classes. Even a mammal (Praopus, an armadillo) is known 

 regularly to produce several embryos from one ovum, and here, 

 too, all the brood are of the same sex. 



* Mar&hal, Comptes Rendus, 1898, p. 662. 



