GALLS CAUSED BY SAW-FLIES 23 



the leaf, but may be distinguished at once by its velvety 

 appearance, being covered with white hairs (Plate II., 

 Fig. 4). Fig. 5 on the same plate shows the spot on the 

 upper surface of the leaf; Fig. 6 a section showing the 

 frass within ; Fig. 7 is a magnified larva. Cameron con- 

 siders this gall to be only that of P. salicis occurring on 

 a hairy-leaved Willow. It must be remembered that the 

 various species of saw-flies belonging to the genus Pontania 

 (Nematus) are very much alike, differing but little in 

 structural details, sculpture, pubescence, and coloration ; 

 consequently the arrangement and classification of them 

 is a work of very great difficulty. 



The gall caused by Cryptocampus medullavius in the young 

 stems of the Bay-leaved Willow is the size and shape of a 

 hazel-nut, and is remarkably woody compared with other 

 saw-fly galls. Cameron remarks that, " besides the outer 

 bark layer, there is next to it a wide layer of cellular tissue 

 before the layer of woody fibre is reached. This, again, is 

 succeeded by the spongy mass representing the medullary 

 ray on which the larvae feed." Frequently through the 

 fusion of adjacent galls a large plurilocular structure the 

 size of a walnut is formed. 



Pontania vacciniella attacks the leaves of the Cowberry, 

 giving rise to oval bean-like galls, which are green at first, 

 becoming brownish when old. In general structure and 

 shape these galls do not differ from those of P. salicis. 



HYMENOPTERA PETIOLIVENTEES 



This suborder contains ten families ; gall-causing insects 

 occur in only two of them, the Chalcididae and the Cynipidae. 

 Very few of the Chalcididae are gall-causers ; the majority 

 are either parasites on gall-causers or inquilines living in 

 the galls on sufferance only. The members of the genus 

 Isosoma axe, however, truly phytophagous ; the larvae of 

 certain species live in the stalks of corn, and in some 

 countries cause much damage to crops. 



