GALLS CAUSED BY SAW-FLIES 21 



(i) Unilateral rounded swelling, attaming the size of a 

 small nut {Cryptocampus medullarius). 



Leaf Galls 



(a) Petiole with fusiform swelling, a mm. in diameter ; 

 sometimes the base of the midrib is involved {Cryptocampus 

 venustus). 



(b) Margin loosely rolled inwards {Pontmia Uucosticta, 

 P. scotaspis, P. vimincdis). 



(c) In the blade, showing on both surfaces — 



(i.) Elongated swelling {Pontania femoralis): 

 (ii.) Smooth, bean-like, greenish or brown {Pontmia 

 vesicator). 

 (iii.) Often corrugated, red, reniform {Pontania proxima). 



{d) On the under surface of the blade — 



(i.) Glabrous and spherical {Pontania salicis). 

 (ii.) Hairy and spherical {Pontania pedunculi). 



The best known of all the saw-fly galls on Willows is that 

 caused by Pontania proxima, perhaps better known to British 

 cecidologists under the name of Nematus gallicola. The leaf 

 buds are attacked in May. The larva is at first white and 

 transparent-looking, with a shining blackish head. It soon 

 becomes green. At maturity the head has a greenish-white 

 semicircle on the face. The pupa is white. These galls are 

 often very abundant in June and July on many species of Wil- 

 lows. As a rule, there are two or three on a leaf (Plate II., i), 

 but occasionally there are as many as six. It is the excep- 

 tion to find a gall on the apical third of the leaf. In its 

 early state the gall is almost solid, excepting a little space in 

 the centre containing the egg. It usually appears almost 

 equally on both surfaces of the leaf. The larva feeds 

 around the interior, and by the time it is full fed the gall is 

 a mere shell. The gall is yellowish-green at first ; later it 

 takes on a red tint, which is always more pronounced on 

 the upper surface, and most vivid in leaves fully exposed to 



