PHYLUM ARTHROPODA. INSECTS 



239 



Testis- 



Vas deferens 



Ovary 



Accessory giand 

 Seminal vesicle 



Oviduct with 

 egg pouch 



Seminal receptacle 

 (spermatheca) 



Spermathecal gland 

 Accessory gland 



Ejaculatory duct^ 



Penis 



Vagina 

 Vulva 



Figure 139. Diagrams of the reproductive systems of insects in general, that is, typical repro- 

 ductive organs found in various insects; but all the organs shown are not present in all species. 



Embryonic development 

 and growth 



The eggs are fertilized at the time they 

 are deposited by the entrance of sperma- 

 tozoa through an opening called the micro- 

 pyle in one end of the eggshell. One sperm 

 nucleus unites with the nucleus of the ma- 

 ture egg; a blastoderm is formed around the 

 periphery of the egg from which an embryo 

 develops (Fig. 41). The young grasshopper 

 that hatches from the egg is called a nymph 

 (Fig. 140). It resembles its parent but has a 

 large head compared with the rest of the 

 body and lacks wings. As it grows its body 

 becomes too large for the inflexible exoskelc- 

 ton, and the latter is shed periodically. Wings 

 are gradually developed from wing buds, and 

 the adult condition is finally assumed. This 

 type of development is called gradual meta- 

 morphosis. 



LIFE CYCLES OF INSECTS 



The most conspicuous differences in the 

 life cycles of various types of insects are as- 

 sociated with the kind of metamorphosis 

 involved. There is no metamorphosis in cer- 

 tain species, a gradual metamorphosis in 

 some as the grasshopper, incomplete meta- 

 morphosis in others (dragonfly or mayfly), 

 and a complete metamorphosis in the most 

 specialized groups. As variations in insect 

 life cycles are almost infinite, we select a 

 few typical common species for descriptive 

 purposes. 



A primitive wingless insect 

 without metamorphosis 



Campodea staphylinus (order Thysanura) 

 is a delicate, whitish species (Fig. 140) 

 about Vs inch long, that lives under stones 



