INSECTS 



281 



ninth abdominal sterna. The eggs are laid in cases, each 

 of which contains sixteen ova and some spermatozoa from 

 the spermathecse. The young are like the adults, save 

 for the absence of the wings, which appear after several 

 moults. Thus the metamorphosis found in so many insects 

 is practically absent. 



Fig. 185. — A portion of the tracheal tissue of a cockroach, highly 

 magnified. Only parts of the tubes are in focus. 



c«.j Cuticular lining with spiral thickening ; ««., nuclei of the protoplasmic layer ; 

 ppm., protoplasmic layer continuous with the epidermis (" hypodermis ") of the 

 surface of the body. 



Insects. 



The number of different kinds of insects is enormous. 

 Probably it is not less than 500,000 and equals 

 that of all other animals together. All insects 

 resemble the cockroach in the main features of their 

 anatomy, but many of them depart widely from it in 

 detail, the differences affecting principally the mouth-parts, 

 the wings, and the life-history. The mouth-parts vary in 



