68 



GENERAL ZOOLOGY 



entire skin is shed at intervals. After each moult the body 

 enlarges rapidly for a short time, but when the exoskeleton 

 once hardens there is no more growing until the skin is shed 

 again. Insects show three conditions of larval life and 



Fia. 36. Insect respiratory systems. A, a. flying insect with exparisions.or 

 air sacs, on the trachea! tubes; B, an insect which never flies; C, a portion of a 

 tracheal tube enlarged. 



metamorphosis. Ametabolous insects have no metamor- 

 phosis. They are much like the adult in form when hatched 

 from the egg. Heterometabolous insects hatch as nymphs, 

 which are at first wingless, but have gradually larger wings 



