78 Annals Entomological Society of America [Vol.1, 



color and form of sand grains that when at rest it is quite invisible. 

 In the photograph shown the outline is indicated by shadows 

 which were really necessary in order to make it apparent, but it 

 may easily be seen, that without these it would be extremely diffi- 

 cult to discover the insect. Now none of these adaptations can 

 be thought of as primitive but rather as highly specialized forms 

 derived from an ancestry of more general habit. Ought this not 

 to be recognized in their classification ? 



GALL MAKING INSECTS. 



Another quite distinct line of divergence for insects is found 

 in the gall making habit, a habit which involves not simply the 

 action of the insect, but the stimulation by the insect of a certain 

 plant activity that results in an abnormal growth which is of ser- 

 vice to the insect either as a source of food material, or for pro- 

 tection, or both. That this adaptation occurs independently 

 in many different groups of insects is clearly evident if we consider 

 the distribution of the gall making groups. It is present even in 

 the Acarina, species of which produce a very great variety of galls 

 on many different plants. Among the true insects the aphides 

 contain a considerable number of gall making species, and in some 

 of these the galls are quite elaborate in structure. The Pemphi- 

 ginae found on elms and poplars show very distinctly formed 

 structures such as the poplar leaf gall. In the genus Phylloxera 

 which is distinctly a gall making genus we have the common grape 

 species and numerous species affecting the hickory, in all of which 

 there is a very distinct gall for each species. The Psyllidae are 

 distinctly gall making and the galls produced are very character- 

 istic, and the whole life of the insect is adapted to this method of 

 existence. 



Among the beetles we have a number of gall making species, 

 those of the genus Agrilus perhaps being the most distinctive, 

 but it is evident that the gall making habit in this group is entirely 

 independent of that in any other order, and even of any other 

 family of the group of beetles. In the Diptera several families 

 include gall making species, but the gnats are most distinctly 

 developed in this direction. We have, however, every gradation 

 in this family from species that are not gall makers up to those 

 which produce the most perfect and constant forms, for example, 

 the Hessian fly, willow galls and the grape filbert galls. 



