224 INJURIOUS AND USEFUL INSECTS 



shows the very same parts. It has been long debated 

 whether the paired constituents of the ovipositor represent 

 Hmbs or not. This is certainly more than can be affirmed 

 at present ; recent study of the early stages of development 

 complicates instead of simplifying the problem, and we know 

 little or nothing of the original form and function of the 

 parts of the ovipositor. 



Life-Mstory. — All Hymenoptera are metabolic. The larvse 

 of saw-flies travel in search of food, and have the form of 

 caterpillars, with much resemblance to those of Lepidoptera. 

 They differ, however, in the number and position of the 

 pseudopods (false feet). The larvae of the wood-wasps, being 

 unable to leave their galleries, have only vestiges of limbs. 

 All the petiolate Hymenoptera (with narrowed waist) produce 

 footless larvse, which depend for food upon the exertions or 

 the forethought of the parent. Here, as in Diptera (p. 8i), 

 high intelligence and activity in the parent may bring about 

 simplification, even amounting to degeneration, in the larva. 

 This is one extreme of a series which includes many grades. 

 The other extreme is found in insects where there is 

 no transformation at all, and where the larva runs about 

 and feeds after the manner of its parent, from which it differs 

 only in size (Thysanura and CoUembola). The Hymenopterous 

 pupa is inactive, free-limbed, and usually enclosed in some 

 kind of cocoon or cell. 



Parthenogenesis — i.e. reproduction by the female without 

 the co-operation of the male — is frequent, the unfertilised eggs, 

 contrary to what is observed in Hemiptera, usually yielding 

 males only. The parthenogenesis of Hymenoptera is not, 

 however, of this kind only. Sometimes it yields only males, 

 or only females (certain gall-flies never produce males at 

 all), sometimes both males and females in a regular succession 

 of sexual and asexual generations. One species of gall-fly 

 (Cynips) found on the oak exhibits an alternation of unisexual 

 and bisexual generations. The winter eggs produce females 

 only; these in summer form galls, from which male and 

 female flies proceed, and thus the fertilised winter eggs are 

 produced. The two generations differ structurally, and are 

 contained in galls of different appearance. Parthenogenesis 

 seems to be connected, like viviparous reproduction, with 

 abundance of food. It is observed to occur where the 



