102 INJURIOUS AND USEFUL INSECTS 



activity in spring; those that are hatched in early summer 

 complete their feeding before autumn. It is probable that, 

 as in many other Hymenoptera, there is a resting larva stage, 

 of uncertain duration, for the flies sometimes emerge from 

 wood which has been long stored. The pupal stage is passed 

 in a slightly enlarged gallery near the outside of the trunk, 

 and there is no cocoon. When the wasp emerges, it has 

 still to make its way to the outer air. The long winding 

 larval gallery, narrowing at length to a fine egg-hole, could not 

 possibly give passage to a large fly. There is nothing left 

 but to gnaw a fresh hole, leading straight to the air. Guided, 

 we may suppose, by the grain of the wood, perhaps, also, by 

 the curvature of the rings and their different degrees of hard- 

 ness, the wasp takes a short, straight course through the wood, 

 and soon becomes free. It leaves the trunk riddled for many 

 feet together by winding cylindrical passages. If many larvEe 

 have lived in the same tree, it is probably only fit for firewood. 

 The short, wide passages by which the wasps escape, are 

 sometimes plainly seen on standing or felled trees. An ex- 

 perienced eye can sometimes detect the attack of the wood- 

 wasp in an early stage by means of the pin-holes from which 

 resin distils, and which mark the places where the borer of 

 the female has been at work. 



Two species of Sirex are found in Britain — (i) Sirex gigas, 

 which is yellow and black ; (2) Sirex juvencus, which is rather 

 smaller, and has a shorter ovipositor. The female of Sirex 

 juvencus is steel-blue, with reddish legs ; the male has some 

 of the abdominal segments coloured red. When pine or fir- 

 trees are attacked, the only remedy known is prompt destruc- 

 tion of infected trees. Felled trees must not be allowed to 

 lie on the ground, and all mined trunks should be burnt as 

 soon as possible. 



22. ICHNEUMONS AND THEIR ALLIES 



The great majority of insects, as of animals in general, are 

 vegetable feeders. A glance at the common plants of our 

 fields, woods, and hedges, in summer, or autumn, shows how 

 many of them provide subsistence to insects. Some insects 

 gnaw the roots, others run their galleries through the wood, 



