198 THE BIOLOGY OF INSECTS 



parasitic Hymenoptera. In this strange mode of develop- 

 ment, as observed in Encyrtus of the Chalcid family by 

 P. Marchal (1904), the female lays, in the egg of a moth, 

 her minute egg so that it becomes enclosed in the body of 

 the growing embryo as this develops into the caterpillar. 

 The Encyrtus egg undergoes a curious kind of development, 

 the polar nuclei persisting and multiplying at one end, 

 while the egg nucleus, which may be fertilised or not, 

 segments in the hinder region of the egg-substance and forms 

 blastomeres. Ultimately the polar cells give rise to a nutrient 

 capsule which spreads around the embryonic cells ; these 

 by a process akin to budding form a large number of embryos, 

 in some cases over a hundred resulting from a single egg. 

 Growth is slow during the winter while the host larva 

 develops in the egg-shell, but after hatching, when the 

 caterpillar begins to feed, the embryonic mass of the parasitic 

 chalcid increases rapidly in size, and assumes the form of a 

 sinuous thread extending through the caterpillar's fat-body, 

 the nutrient membrane being now enclosed in a sheath 

 derived from the host's tissues. At length the Encyrtus 

 grubs become free in the caterpillar's body- cavity and finally 

 eat their way out through its dried skin and cuticle to pupate 

 and assume the adult form. Allied forms, which have 

 been found to undergo a similar course of development, 

 are described by F. Silvestri (1908) and R. W. Leiby (1922). 

 The result is to bring about a hundredfold multiplication 

 between the single egg laid in the egg of the moth by the 

 tiny chalcid fly and the enormous family at the close of the 

 completed transformation. R. W. Leiby and C. C. Hill 

 (1923, 1924) have shown that in species of Platygaster 

 (belonging to the Proctrotrupidae), parasitic on gall-midge 

 (" Hessian Fly ") larvae, there may be the usual direct 

 development of the egg into one larva, or a poly- 

 embryonic development resulting in the production of six 

 or eight parasitic grubs (Fig. 55). This condition, suitable 

 to a host-larva of small size, suggests an early stage 

 towards the abnormal fecundity of Encyrtus. 



In face of such facts as these, the student cannot but feel 



