432 



INTRODUCTION TO ZOOLOGY 



CHAP. 



B 



Other forms equally injurious are the Currant 

 Saw - fl y (Nematus ribesii) and the Gooseberry Saw- 

 fly (TV. ventricosus), forms which at times literally 

 strip the bushes of their leaves. The Turnip Saw-fly (Atlialia 



spinarum) has small black larvae 

 which do great damage to the 

 leaves of turnip crops. 



Trichiosoma betuleti (Fig. 324) 

 is a Saw-fly the larva of which 

 feeds on the hawthorn in July and 

 August ; it is green with minute 

 white spots, and on pupation it 

 makes for itself a silky brown 

 case (Fig. 324,^4), which is opened 

 at one end of the perfect insect 

 when it emerges. The fly is large 

 and covered with reddish -brown 

 hairs, and the tibia of the legs 

 -e of a characteristically dusky 

 colour. 



The larvae of some saw-flies remain within 



jfo^Ses^ the ^issues of the leaf where the eggs were laid, 



and cause the formation of galls in the leaf, as 



in the case of the reddish "bean galls" on crack willow 



leaves (Plate IV.), caused by Nematus gallicola. The larvae 



when full grown, usually in early November, leave the 



galls and make their way into the soil, where they become 



pupae. 



The Saw-fly Pea Gall (Plate IV.) is formed by Nematus 

 salicis-cinereae on the under side of the leaves of various 

 smooth-leaved willows. 



The Wood Wasp or Horn-tail (Sirex gigas}. 1 



The Wood Wasp is a conspicuous insect which may be 

 1^ inches long, and is coloured with black and gold bars. 

 It belongs to a family closely allied to the true saw-flies, and 

 having the same sessile abdomen, but it is peculiar in 

 possessing a cylindrical boring apparatus which always pro- 

 jects from the end of the body. The larva lives in the wood 

 1 See account iu Miall's Injurious and Useful Insects for further details. 



