270 The Orders of Insects 



development of social life to be found among in- 

 vertebrate animals (see below, pp. 331-7)- The Ants, 

 and some of the Wasps and Bees live in large 

 communities, the majority of the members consisting 

 of undeveloped females known as " workers " or 

 "soldiers," who build the nest, provide food for the 

 larvae, defend the colony, and manage its affairs. 



Classification. — The division of the Hymenoptera 

 into Terebrantia and Aculeata according as the 

 female possesses an ovipositor or a sting cannot be 

 maintained. The sting is only a highly modified 

 ovipositor, poison glands are present in all the families, 

 and several large Ichneumon flies, classed in the 

 terebrant (ovipositor -bearing) division, can inflict a 

 painful sting. The Hymenoptera are now divided 

 into two sub-orders distinguished by the relation 

 between the thorax and the hind-body (3). 



Sub-Order A. Sessiliventres. 



The Sessiliventres are those Hymenoptera in which 

 the first abdominal segment is incompletely fused with 

 the thorax, and is not followed by a marked constriction 

 or " waist," the lateral outline of the body being con- 

 tinuous. The anal lobe of the hind-wing is large and 

 the trochanter of each leg is divided into two segments. 

 The females have an ovipositor adapted for boring or 

 cutting. The larvse often have, in addition to the six 

 thoracic legs, many pairs of prolegs on the hind-body ; 

 they feed on the tissues of plants (l57)- The Sessili- 

 ventres may be divided into three families. 



Cephidse. — The CephiJa or Stem-sawflies are small, slender insects 

 distinguished by a free and elongate pronotum — a primitive character 

 not found in any other of the Hymenoptera. The first abdominal 

 segment has its tergite interrupted centrally by a small triangular 

 membrane whose apex is in front. The front shin has a single spine. 

 The ovipositor of the female is composed of two saw-edged processes by 

 means of which holes are bored into the stems of plants where the eggs 



