GALLS CAUSED BY SAW-FLIES 
2 3 
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 medullarius 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 vaccinidla 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 PETIOLIVENTRES 
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 are, however, truly phytophagous ; the larvae of 
certain species live in the stalks of corn, and in some 
countries cause much damage to crops. 
