328 Transactions of the Canadian Institute [vol. ix 



The close restriction of sawfly gall producers to definite species of 

 Salix can be illustrated by means of the forms mentioned in this paper. 

 The host plants of the species are: — 



Euura S. gemma Walsh 

 Pontania desmodioides Walsh 

 Pontania (undescribed) 

 found on Salix humilis Marsh, 



Pontania hyalina Norton 

 on 5. alba L. 



Pontania pomum Walsh 

 on S. cordata Muhl. 



Euura (undescribed) 

 on S. serissima Fernald. 

 Gall (undescribed) 

 on S. lucida Muhl. 



Pontania pisum Walsh 

 on S. discolor Muhl. 

 In this locality I have not found the above species on any other host 

 than that mentioned. When the type of gall is higher it would seem to 

 be axiomatic that the relations between the host plant and producer 

 would be more intimate than when the gall does not stand so high in 

 the scale and as a consequence the restriction to one host plant would be 

 a necessity. Yet the galls produced by the Cynipidae are often found 

 on two or three different hosts; as, for example, Amphibolips inanis O.S. 

 on both Quercus rubra L. and Q. coccinea Muench, Dryophanta palustris 

 O.S. on Quercus rubra L. and Q. coccinea Muench. Aulax nabali Brodie 

 on Prenanthes alba L. and P. altissima L. 



Euura S. gemma Walsh. 

 "On Salix humilis. The lateral bud of a twig enlarged so as to be 

 twice or thrice as long, wide and thick as the natural bud before it begins 

 to expand in the spring ; its external surface otherwise entirely unchanged 

 both in texture and colour. Internally, instead of the normal downy 

 embryo leaves, it contains early in the autumn a homogeneous grass- 

 green fleshy matter, which is afterwards gradually consumed by the 

 larva, leaving nothing at last but a mere shell partly filled with excre- 

 ment. The gall is monothalamous, sometimes one only on a twig, some- 

 times two or three or more at irregular intervals, very rarely as many 

 as three or four formed out of three or four consecutive buds. 

 Length .17 to .36 inch 

 Breadth .10 to .17 inch. — Walsh." 



The anatomy of this gall presents little differentiation of tissue. A 

 cross section shows that the entire mass of the gall consists of small 



