﻿74 THE ENTOMOLOGIST. 



tinued life and vitality of the plant is beneficial to the larvae, and 

 the larger or more perfect the gall, the greater the amount of 

 available food. Hence natural selection will have preserved 

 and accumulated the gall-forming tendencies, as not only 

 beneficial to the larvae, but as a means whereby the larvae can 

 feed with least harm to the plant. So far from being developed 

 for the exclusive benefit of the larvae, it is easy to see that, 

 allowing a tendency to gall-formation, natural selection ivould have 

 developed galls exclusively for the benefit of the plants, so that 

 they might suffer a minimum of harm from the unavoidable 

 attacks of insects. 



But here it may be questioned — Have we proof that internal 

 feeders tend to form galls ? In answer to this I would point out 

 that gall-formation is a peculiar feature, and cannot be expected 

 to arise in every group of internal feeders. But I think we can 

 afford sufficient proof that wherever it has arisen it has been 

 preserved ; and further, that even the highly complex forms of 

 galls are evolved from forms so simple that we hesitate to call 

 them galls at all. 



Let us first take the Hymenoptera, which form so many 

 galls. The North American oak-galls, formed by species of 

 Cynipidae, afford every gradation of character. They may be 

 divided into sections ; the galls in the first section being so essen- 

 tially part of the branch that they cannot be removed without 

 taking away part of the branch or twig with them. The second 

 section comprises galls on the branches, but of a different 

 substance from the branch, so that they can be removed without 

 taking a portion of the branch with them. Then of leaf-galls we 

 have again two sections, one consisting of galls which cannot be 

 separated from the leaf, the other of galls which are separable 

 from the leaf-tissues.* Finally, there is a section comprising 

 galls on the roots. 



To take individual instances : the gall of Andricus cryjrtus, 

 Ashm., is hidden under the bark, so as to be invisible externally. 

 The gall of A. gemmarius, Ashm., is tubular or fusiform. But 

 a third species in the same genus, A. pomiformis, Bass., forms 

 a globular polythalamous gall. A. piger, Bass., forms a swelling 

 of the leaf; but A. virens, Ashm., forms a true globular leaf-gall. 

 So that, although we cannot actually observe the evolution, its 

 stages have been preserved for us in certain species. Diastrophus 

 fusiformans, n. sp., forms merely fusiform swellings of the stems 

 of Potentilla in Custer County, Colorado ; but D. potentilla, 

 Bass., forms oblong spongy galls. 



The rose-galls also show us various stages of evolution. In 

 Custer County, Colorado, we have three kinds of leaf-galls on 

 roses. Bhodites roscefolicz, mihi, forms galls which are nothing 



* See ' On the Cynipiclous Galls of Florida,' by W. H, Aslirnead, 



