Chap. III.] 



INSECT A. 



75 



developed behind the region of the thorax ; one pair 

 of appendages form the prse-oral antennae, and the 

 metameres do not exceed twenty in number. They 

 are sharply divisible 

 into two great sub- 

 divisions, according as 

 they are or are not 

 provided with wings ; 

 with the latter, of 

 course, we must as- 

 sociate those in which 

 wings are found in 

 one sex only, or are 

 rudimentary, or of 

 whose ancestral ex- 

 istence (as in the case 

 of parasites), we have 

 sufficient evidence. 



A. Aptera, or 

 true wingless forms 

 such as the spring- 

 tails (Podura), and 

 bristle-tails (Lepisma). 

 In the simplest of 

 these the mouth or- 

 gans can work either 

 from side to side, or 

 from before back- 

 wards ; the tracheae, 

 however well de- 

 veloped, and they are 

 often only poorly so, 



never anastomose with one another (Fig. 34). 



B. Pterygota. Here belong all the remaining 

 insects, which are either winged, that is, provided 

 with two pairs of membranous dorsal outgrowths in 

 the region of the thorax, which can be moved by 



Fig. 34. Orchcsella cincta, enlarged. 



