66 GENERAL ZOOLOGY 



The Myriapoda (Fig. 35) show many points of progress 

 when compared with our hypothetical ancestral arthropod 

 (Fig. 27). The metameric kidneys have been lost and 

 replaced by Malpighian tubules; tracheae have been de- 

 veloped for breathing; the appendages are all uniramous 

 except one pair of mouth parts; and the different orders 

 show a progressive tendency to make walking legs into 

 mouth parts. 



CLASS 4. INSECTA 



The class Insecta, once known as the Hexapoda (six 

 legs), contains an enormous number of species exceeding 

 all the rest of the animal kingdom. The class is not only 

 large but versatile as well. Insects show all sorts of peculiar 

 adaptations for specialized modes of life. They are found 

 in all parts of the world and live in almost every conceiv- 

 able habitat. They vary in size from a twentieth of an 

 inch to nearly seven inches in length. 



The body of an insect has three distinct regions: head, 

 thorax, and abdomen. The head bears a single pair of 

 antennae, usually two compound eyes, three simple eyes 

 or ocelli, and four different kinds of mouth parts the 

 labrum, which is single and not one of the series of matemeric 

 appendages; the mandibles, maxillae, and labium, which 

 are paired. The thorax is always composed of three seg- 

 ments prothorax, mesothorax, metathorax. Each seg- 

 ment is protected by four exoskeletal plates a dorsal 

 tergum, a ventral sternum, and two lateral pleura. Each 

 thoracic metamere bears a pair of walking legs, and each 

 of the last two frequently bears a pair of wings. The ab- 

 domen typically consists of eleven free segments which are 

 without appendages, except accessory reproductive organs 

 or a sting at the posterior end. 



The mouth parts vary greatly but conform mostly to 

 two types: (1) for biting, as in a beetle; or (2) for sucking, 

 as in a bug. Some insects, however, like the honey bee, 

 may have very specialized mouth parts which may be used 



