96 A BOOK OF INSECTS 



tumours; but the larvae, on hatching, escape from the 

 host's body. 



Order XIX. — Hymenoptera. 



This order includes the saw-flies, ichneumons, ants, 

 wasps and bees — insects with two pairs of membranous 

 wings. Except in some tiny species the wings of 

 each side are united during flight by a series of minute 

 hooks, on the anterior margin of the hind-wing, which 

 engage with a corresponding fold in the fore-wing. The 

 mandibles retain their biting function ; but the other 

 mouth-parts are often adapted for licking or sucking. 

 The first, or basal, segment of the abdomen is more 

 or less closely united with the thorax — the characteristic 

 "waist" of such an insect as an ant or a hornet coming 

 not between the thorax and the abdomen, but behind the 

 first abdominal segment. The females are provided with 

 elaborate ovipositors, which in many families are modified 

 into poison-injecting stings. Hymenoptera undergo a 

 complete metamorphosis, the larva being always a variant 

 of the eruciform type, while the pupa is free, usually 

 enclosed in a silken cocoon. Two sub-orders are 

 recognised. 



Sub-Order 1. — Symphyta. 



In this group, which includes the saw-flies, the abdo- 

 men is not basally constricted to form a " waist." The 

 ovipositor of the female is adapted for cutting or boring 

 but never for stinging. The larva?, except in those species 

 which burrow into the tissues of plants, resemble the 

 caterpillars of butterflies and moths ; but they have more 

 than five pairs of prolegs. There are three families. 



The stem saw-flies (Cephidce) are small, slender insects 



