SUB-ORDERS 503 



is modified for the division of labour, while in Termites both 

 sexes undergo this change. Hence it is impossible to suppose 

 that the remarkable analogies that exist between the societies of 

 ants and those of Termites are due to any common origin. It 

 is probably to some similar physiological susceptibilities in the 

 ancestors, at an extremely remote epoch, of both groups that we 

 must look for an explanation of the interesting resemblances in 

 the social lives of ants and Termites. 



The Hymenoptera are no doubt one of the largest Orders of 

 Insects, the species of the paiasitic tribes being apparently 

 innumerable. No doubt 250,000 species of the Order exist, and 

 possibly the number may prove to be very much larger. Up to 

 the present time 25,000 or 30,000 have been discovered. No 

 remains of Insects of this Order, of older age than the Lias, 

 have been brought to light ; it is indeed doubtful whether the 

 fossils considered to be Hymenopterous of the period referred 

 to are really such. 



The Order, as already mentioned, consists of two very distinct 

 sub-Orders, viz. : — 



1. Hymenoptera Sessiliventres. — Insects with the abdomen broad at the base, 



its first segment not completely amalgamated with the thorax. 



2. Hymenoptera Petioliventres or Petiolata. — The abdomen connected with 



what appears to be the thorax by a slender joint, the posterior 

 part of the apparent thorax consisting of an abdominal segment. 



Hymenoptera Sessiliventres. — This group has been variously 

 called Hymenoptera phytophaga, H. securifera, If. sessiliventres, 

 H. serrifera, H. symphyta. "We prefer an old term, taken from 

 a character that enables us to recognise at a glance which group 

 a species belongs to. The division or sub-Order may be formally 

 defined as follows : — 



Abdomen nearly continuous in outline with the thorax, the tivo 

 parts having a broad connexion instead of a small highly 

 mobile articulation. Anal lobe of hind vnngs usually of 

 considerable size. Trochanters ditrochous {transversely 

 divided into two, Fig. 345). Extremity of body of female 

 furnished with saws or boring instruments, usually concealed, 

 in some cases visible in part. Larvae luith complex mouth- 

 parts ; th7'ee pairs of thoracic legs (imperfect in Cephidae and 



