io ON THE ORIGIN AND [CHAP. 



the sawfly, but resembling a needle. With this she 

 stings or punctures the surface of leaves, buds, stalks, 

 or even roots of various plants. In the wound thus 

 ptoduced she lays one or more eggs. The effects of 

 this proceeding, and particularly of the irritating fluid 

 which she injects into the wound, is to produce a 

 tumour or gall, within which the egg hatches, and on 

 which the larva, a thick fleshy grub (Plate II., Fig. 7), 

 feeds. In some species each gall contains a single 

 larva ; in others, several live together. 



The oak supports several kinds of gallflies : 

 one produces the well-known oak-apple, one a 

 small swelling on the leaf resembling a currant, 

 another a gall somewhat like an acorn, another 

 attacks the root ; the species making the bullet- 

 like galls, which are now so common, has only 

 existed for a few years in this country ; the 

 beautiful little spangles so common in autumn 

 on the under side of oak leaves are the work 

 of another species, the Cynus longipennis. One 

 curious point about this group is, that in some 

 of the commonest species the females alone are 

 known, no one yet having ever succeeded in 

 finding a male. 



Another great family of the Hymenoptera is that 

 of the ichneumons ; the females lay their eggs either 

 in or on other insects, within the bodies of which the 

 larvae live. These larvae are thick, fleshy, legless 

 grubs, and feed on the fatty tissues of their hosts, 

 but do not attack the vital organs. When full- 

 grown, the grubs eat their way through the skin of 



