﻿THE ENTOMOLOGIST. 



Vol. XXIIL] MARCH, 1890. [No. 322. 



THE EVOLUTION OF INSECT-GALLS. 

 By T. D. A. Cockerell. 



Prof. G. J. Romanes, in ' Nature,' November, 1889, p. 80, has 

 a letter on the subject of galls. After quoting Mr. Mivart's 

 remarks on the "disinterested" nature of galls, he himself also 

 says that he has always considered gall-formation " one of the most 

 important facts in organic nature with reference to the theory of 

 natural selection ;" and this because it is " the one and only case 

 in the whole range of organic nature where it can be truly said 

 that we have unequivocal evidence of a structure occurring in 

 one species [the plant] for the exclusive benefit of another" 

 [the gall-insect]. 



The assumption then is that the various forms of galls are 

 formed solely, and have been evolved only, for the benefit of the 

 gall-insects themselves, a phenomenon which Prof. Romanes justly 

 regards as very exceptional. But the exception seems to me to 

 be only apparent and not real. 



Let us consider the probable phases in the evolution of galls. 

 Doubtless there were internal plant-feeding larvae before there 

 were galls ; and, indeed, we have geological evidence that boring 

 insects date very far back indeed. The primitive internal 

 feeders, then, were miners in the roots, stems, twigs, or leaves, 

 such as occur very commonly at the present day. These miners 

 are excessively harmful to plant-life, and form a class of the 

 most destructive insect-pests known to the farmer : they fre- 

 quently cause the death of the whole or part of the plant 

 attacked. Now, we may suppose that the secretions of certain 

 of these insects caused a swelling to appear where the larvae 

 lived, and on this excrescence the larvae fed. It is easy to see 

 that the greater the excrescence, and the greater the tendency of 

 the larvae to feed upon it, instead of destroying the vital tissues, 

 the smaller is the amount of harm to the plant. Now the con- 



ENTOM. — MARCH, 1890. G 



