30 ECONOMIC ENTOMOLOGY 



Several generations of females only which bring forth living young 

 may occur, but at intervals males appear and fertihzed eggs are laid. 



In some species of the Cecidomyiidae the young are produced by 

 larvae. Such a method is called pcsdo genesis. After several genera- 

 tions, however, the last larvas pupate and form normal male and female 

 flies. Examples are Tanytarsus dissimilis and Miastor americana 

 (Fig. 2>2,a). 



Another method of asexual reproduction, called polyembryony 

 occurs in certain parasitic insects, e.g., Polygnotus, belonging to the 

 Hymenoptera. Each egg produces many embryos, instead of one, 

 which develop into as many adult insects of the same sex. 



Pig. 33a. — Young pasdogenetic larvae of Miastor in the body of the mother larva. 

 Greatly enlarged. {After Pagenstecher from Folsom.) 



The Development of Insects 



All insects that reach maturity pass through two distinct stages 

 of development — the embryonic changes within the egg, and the 

 changes after leaving the egg until the adult condition is reached. 

 The growth of the embryo within the egg progresses from the seg- 

 mentation of the ovum to the formation of the blastoderm with its 

 ventral plate and germinal groove, and the gradual growth of the 

 ectoderm, mesoderm and entoderm, from which layers the various 

 organs of the body arise. 



(a) Embryology. — The egg or ovum is a single cell containing — 

 (i) The nucleus or germinal vesicle. (2) The yolk, or nutritive material. 

 (3) The cytoplasm. (4) The cell wall or vitelHne membrane. (5) 

 The egg shell or chorion. (6) The micropyle or opening in the chorion 

 to admit the spermatoza (Fig. 34). 



When the sperm nucleus unites with the nucleus of the egg, and 

 forms a segmentation nucleus, fertilization is accomplished. 



By division of the segmentation nucleus a large number of nuclei 

 are formed many migrating outward toward the margin of the egg. 

 There a layer of cells internal to the yolk membrane called the blasto- 



