APTERYGOGENEA 



271 



and sometimes compound eyes. An ocellus may also be pres- 

 ent. There is a mandible and two pairs of maxillae. Each 

 segment of the thorax bears a pair of appendages. The only 

 evidence of abdominal appen- 

 dages to be found are a pair of 

 stiff bristles attached to the 

 ventral surface of the fifth 

 segment and projecting forward 

 beneath the body, and a pair of 

 hooks beneath the third seg- 

 ment. By means of this ap- 

 paratus the animal makes 

 springing movements. Hence 

 they are called spring tails. The 

 respiratory organs are tracheae 

 except in some groups where 

 respiratory organs are wanting. 



578. Class 10. Insecta. In 

 the Insects the body is divided 

 into head, thorax of three seg- 

 ments (prothorax, mesothorax 

 and metathorax) and an ab- 

 domen of 9-10 segments. The 

 appendages of the head are a 

 pair of antennae, a pair of man- 

 dibles, a pair of maxillae and 

 a labium which represents a 

 second pair of maxillae. Each 

 segment of the thorax bears a 

 pair of legs and in addition 

 the meso- and metathorax also 



each have a pair of wings. The abdominal segments bear no 

 appendages. 



579. The mouth opens into a narrow oesophagus with which 



FBG. 162 Tracheal system of an 

 insect, a, Antenna; b, brain; I, leg; 

 n, nerve-cord; p, palpus; s, spiracle; 

 st, spiracular, or stigmatal branch; 

 /, main tracheal trunk; v, ventral 

 branch; vs, visceral branch. (From 

 Folsom, after Kolbe.) 



