472 AETHROPODA. 



tozoa from it. Since a firm shell or chorion is developed around 

 the egg in the ovary, access of spermatozoa is only possible by the 

 existence of a micropylar apparatus, a system of tubes penetrating 

 the chorion at one end of the egg. 



Oviposition occurs in many insects by means of an ovipositor 

 which may project free from the body (fig. 509) or may be re- 

 tracted into it. It consists of four or (Orthoptera) six parts or 

 gonapophyses developed from the eighth and ninth abdominal 

 segment, which form a tube. In many Hymenoptera this struc- 

 ture has become modified into a sting (aculeus), and is provided 

 with poison glands, making it an efficient weapon of defence. 

 From its nature the sting is of necessity confined to the females. 

 In the males there is usually a protrusible penis which is frequently 

 composed of the same parts as the ovipositor ; in others of metamor- 

 phosed somites. Further sexual differences lie in the form of the 

 antennae, shape and color of the wings, modifications of the eyes, 

 etc. 



In many insects the eggs may develop parthenogenetically. 

 Plant lice and scale insects reproduce for generations asexually, and 

 parthenogenesis is widely distributed among Hymenoptera, Lepi- 

 doptera, and Neuroptera. The conditions among the bees are 

 especially interesting, since here the determination of sex rests with 

 the existence or non-existence of fertilization (pp. 142, 487). 

 Much rarer than the ordinary parthenogenesis is that special form, 

 known as paedogenesis, which occurs only in certain Diptera like 

 Miastor. In the female Miastor larva (fig. 498) the eggs develop 



FIQ. 498. Larva of a Cecidomyid with psedogenetic daughter larvae. (From Hatschek, 



after Pagenstecher.) 



before the appearance of the ducts, so that the young can only 

 escape by rupture of the mother. After several paedogenetic 

 generations there appear at last larvae which pupate and produce 

 adult male and female flies. 



With the exception of these paedogenetic forms, the Pupipara, 

 many Aphidae and a few other viviparous species, the Hexapoda 

 are oviparous. The development begins, after oviposition, by a 

 superficial segmentation of the egg. Later there appear two em- 

 bryonic structures, the yolk sac and the amnion; the first, in con- 

 trast to the vertebrate structure with the same name, is dorsal. 



