552 [December 



ner described by Mr. Wm. Couper, (Proc. Enf. Soc. Phil. I. p. 373.) 

 May it not be possible that the Willow-galls attributed to Aphides by 

 Hartig are of a similar nature ? Whenever a particular genus of plants, 

 common to both N. A. and Europe, is infested by a particular genus of 

 ■ gall-making insects, it is generally the case that the same genus of In- 

 sects occurs upon the same genus of plants both in the Old and New 

 World. Now if there really exist in the U. S. Aphidse that produce 

 galls on our willows, I can scarcely believe that they should have all 

 managed to escape my notice. Still, like all other negative arguments, 

 such reasoning as this is not entitled to much weight. 



The genus Cecidoini/ia diifers from most other genera of gall-pro- 

 ducing insects, in that it occurs on very numerous and widely distinct 

 genera of plants. In Osten Sacken's excellent Memoir on this Family, 

 without the assistance of which I should not have ventured upon this 

 Paper, N. A. Cecidomyidse.^ many of them known only in the larva 

 state, are enumerated as occurring on Hickories (Carya) of different 

 kinds, on the red Maple (Acer), on the Ash (Fraxinus), on Oaks of 

 different kinds (Quercus), on the Hornbeam (Carpinus), on the Tulip 

 tree (Liriodendron), on the Willow (Salix), the Grape-vine (Vitis), the 

 Locust (Robinia), the Alder (Alnus), the Gooseberry (Ribes), the 

 Blackberry (Rubus), and the Pine (Pinus), besides Vaccinium, (or Gay- 

 lussacia?), Solidago, Impatiens, Agrostis, Chrysopsis, and the cereals 

 wheat, rye, &c., {Dipt. JY. A. 188—190.) Even the twelve N A. 

 species referred to the genus Cecidomj/ia^ where the perfect insect is 

 known as well as its larva, occur on eight distinct genera of plants. 

 {Ibid.) What a contrast with Ci/nips^ of which there are now about a 

 score described N. A. species, all found on different species of Oak I 

 We may observe, however, that as in the Cynips of the Oak, so also in 

 the Cecidomyia of the Willow, it is a very general rule that the galL 

 when it grows on a twig, kills that twig unless it is pretty large, so that 

 the presence of either of these two genera operates here as Nature's 

 own pruning-knife. 



But the most remarkable feature about the Cecidomj/idai is the gene- 

 rally dull, monotonous character of their coloration and ornamentation, 

 the extreme similarity of many species, and the apparent identity of 

 others, which we yet know, from the wide difference of the galls pro- 

 duced by them, must be distinct species. Hence Loew has remarked 



