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 ^ , , 



; 1.1 Fbg. 162 — Tracheal system of an 



a labium which represents a insect. a, Antenna; h, brain; /, leg; 



second pair of maxilla. Each "; nerye-co'-d; p, palpus; . spiracle; 



^ si, spiracular, or stigmatal branch; 



segment of the thorax bears a i, main tracheal trunk; V, ventral 



[. , , . -I I'j' branch; vs. visceral branch. (From 



pair of legs and in addition Folsom, after Kolbe.) 



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 



