288 EMBRYOLOGY OF INSECTS AND MYRIAPODS 



escape through a sht between head and prothorax of the puparium and 

 then crawl over the body of the host wasp or bee. The triimguHnid now 

 finds a larva or a nymph of its host species, quickly bores into it, and 

 begins its parasitic life. It undergoes its metamorphosis within the body 

 of its host, there completing the life cycle. The finding of the host larva 

 by the triungulinid is easily understood in case of the social insect hosts, 

 since the original host may carry it to the nest, but in other cases it is 

 possible the triungulinids creep over flowers visited by the host and thus 

 make the transfer. 



Immature eggs of Stylops are more or less spherical. Later they may 

 be found free in the body cavity of the female instead of in the ovary. 



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Fig. 217.— Stylops. Two-cell stage. 



Surrounding the egg is a syncytial membrane which contains a few nuclei, 

 and outside this are a few scattered cells. A chorion is lacking as in 

 viviparous aphids. The eggs are small: those of Stylops parvula measure 

 43 microns in diameter, those of *S. ovina 56 microns, the average being 

 50 microns. The sparse yolk is a fatty substance with but little albumi- 

 noid matter. The species studied by Noskiewicz and Poluszynski (1927) 

 is not parthenogenetic. Polyspermy is common. In maturation the 

 nucleus migrates to the surface where two polar bodies are formed ; then 

 the pronucleus goes back toward the center of the egg to meet the male 

 pronucleus. Schrader (1924) has demonstrated that in Acroschismus 

 wheeleri, also, unfertilized eggs do not develop. Immediately after the 

 union of the male and female pronucleus, cleavage begins (Figs. 217-219), 

 giving rise to the two-, four-, and eight-cell stages. Cleavage does not 

 extend to the central yolk. In either the 16- or the 32-cell stage the 

 nucleus of one daughter cell migrates into the central yolk mass. Seven 

 strictly synchronous divisions take place, the nucleus within the yolk also 

 undergoing two divisions, thus forming 120 or 124 blastomeres and a four- 

 nucleate yolk syncytium. Further cell division gives rise to a partial 



