14 THE BIOLOGY OF INSECTS 



perforated floor and transmitting blood to the heart by means 

 of paired ostia. The organs of nitrogenous excretion are 

 elongate, tubular outgrowths of the hind-gut. The nervous 

 system consists of a double chain of segmentally arranged 

 ventral ganglia with longitudinal trunks connected with an 

 anterior complex brain. Sensory nerve-endings are affected 

 through modified cuticular areas or outgrowths. Insects 

 are of separate sexes (dioecious), the female producing large 

 eggs which may, in some cases, develop parthenogenetically ; 

 secondary sexual characters are often conspicuous. Growth 

 is necessarily accompanied by a series of ecdyses in which 

 a less or greater degree of metamorphosis is connected with 

 the acquisition of wings and other structures characteristic 

 of the adult creature. 



