POLY EM BRYONY AND PARTHENOGENESIS 131 



P. vernalis. More recently Parker (1931) discovered that the braconid 

 Macrocentrus gifuensis also undergoes polyembryonic development with 

 the production of about 10 individuals from a single egg. M. ahdominalis 

 may belong in the same category. 



Since the polyembryonic development of several species is discussed in 

 some detail in the chapter on Hymenoptera in Part II, it suffices here to 

 give a brief review. 



Polj^embryonic species are all parasitic upon either Hj^menoptera or 

 Diptera. They oviposit in the eggs of their hosts and emerge as adults 

 nearly always from the fully grown host larva. From one to eight eggs 

 are deposited b}' the female parasite at a time within the host egg. Some 

 species rarely deposit more than one egg in a host egg ; others deposit two 

 as a rule; and still others deposit from four to eight eggs. Both fertilized 

 and unfertilized eggs develop, the former giving rise to females, and the 

 latter to males. At times, according to Leiby, one fertihzed female may 

 deposit both fertilized and unfertilized eggs in a host egg at one insertion 

 of the ovipositor, and the brood developed therefrom will consist of both 

 male and female individuals. 



Whether the egg is fertilized or not, maturation takes place in which 

 the polar bodies are not thrown off. Instead they are retained to form, 

 together with a differentiated part of the egg, a thickened membrane 

 known as the " trophamnion " (Figs. 263, 272) which invests the embry- 

 onic portion of the egg during early development and later functions as an 

 envelope capable of extracting from the host such elements as are neces- 

 sary to bring about the development of the parasites from the fertihzed 

 or unfertilized egg nucleus to the primary larval stage. Henceforth the 

 growth of the parasite is by direct feeding of the larval stages within 

 the host larva which has meanwhile hatched from the egg and is gro\^^ng 

 to reach the pupal stage. The posterior region of the parasite egg con- 

 taining the fertilized or unfertilized nucleus becomes enlarged by the 

 division and subdivision of the oocyte nucleus. If it is limited to a single 

 division, only two embryos are formed, but in most of the species thus far 

 studied division and subdivision of the embryonic and daughter embry- 

 onic nuclei continue to a point where from 1,600 to 1,800 daughter nuclei 

 are formed, which result in the development of as many embryos. 



Embryonic development having been completed to the primary larval 

 stage, by means of host elements fed through the trophamnion, the 

 larvae then feed directly upon the internal tissues of the host. Their 

 feeding upon the host kills it just as it is about to transform to the pupal 

 stage, the parasite larvae having meanwhile matured. They then trans- 

 form to pupae within the carcass of the host larva and later emerge 

 simultaneously as adults from their individual cells in which they have 

 passed the pupal stage. 



