﻿110 lOetober, 



That the former is done in compliance with a general law can hardly be 

 doubted, when we consider how effectually and persistently this singling out of 

 weak specimens is carried out, for the evident purpose of their ultimate destruc- 

 tion and replacement of healthier individuals. 



As a case in point, I offer in this paper an account specifying the long con- 

 tinued attacks of Cecidomyia marginemtorquens, Bremi, on a plant of Salix viminalis. 



My diary records that— 



1866, August 3.— I found the marginal rolls of this Cecidomyia, coloured 

 alternatively red and yellow, in great numbers on the leaves of a straggling bush 

 of Salix viminalis, in the hedge-row of a garden at Penge. The willow has a 

 ragged appearance, branches have been violently torn off, the bark removed in 

 parts and the leading shoot is destroyed. The bush looks as if it had suffered 

 repeatedly by the ruthless passage of man and beast, it being situated in the 

 weakest part of the hedge. 



1866, August 17. — Still abundance of tenanted rolls. 



1867, May 22. — Same willow in the hedge nearly trodden down by cattle. 

 Fresh rolls on many leaves of the intact shoots. 



1868, June 5. — Plenty of rolls on the willow in the hedge on the few branches 

 remaining ; many shoots are clipped off. 



1868, June 7. — Bred from such rolls both sexes of Cecidomyia marginem- 

 torquens, Bremi, in numbers. 



1868, September 27. — Noticed chains of fresh rolls on the same bush in 

 abundance. 



1869, May 23. — Found first leaf this year bordered with several rolls on each 

 side. 



1869, July 25. — Kolls in excessive numbers j majority of leaves thus infested. 

 Scarcely any young shoots of the year growing healthily. 



During all these years I have diligently searched the other bushes of this 

 willow in the same hedge for these galls, but can find none. In one of the neigh- 

 bouring gardens there grows a fine tall tree of the same Salix ; thinking that 

 the straggling bush in the hedge might annually receive its settlers from there, I 

 have repeatedly, and at various times of the year, examined it, but always with a 

 negative result. Hence I am driven to the conclusion, that having accidentally 

 been peopled with this gall-midge from a distance, this straggling bush, although 

 plenty more grow near, is annually fixed upon by the laying females as the most 

 suitable for their purpose, and will eventually succumb to the ravages of their 

 progeny as one brood of larvae succeeds the other, at intervals of about four weeks 

 from May to October, the last brood hybernating in their cocoons in the fallen 

 leaves till spring. 



C. salicis. — From galls, encompassing some twigs of the preceding year's 

 growth of a plant of Salix cinerea, collected near Penge, on the 11th October, 

 1868, I bred a few specimens of this midge on the 26th June, 1869. In this 

 particular instance each gall harboured but one larva, and all I met with were 

 full fed or nearly so. They remained in their larva state until the middle of 



