Ewing — Significance of Parasitism in Acarina. 55 



very similar, on the other hand some forms which phylo- 

 genetically were closely related, are now structurally 

 very diverse, because their habits, at least in some par- 

 ticular aspects, lead to peculiar environmental conditions. 

 This response, which has resulted in such peculiar spe- 

 cializations, has often brought about some very highly 

 evolved types. 



Prominent among those forms which exist under pecu- 

 liar environmental conditions might be mentioned the 

 gall mites, Eriophyidae (PI. VII, Fig. 29). These mites 

 live by sucking juices from various plants, but especially 

 from leaves of trees. Here their attacks have resulted 

 in the development of various malformations of the 

 leaves and in many cases in the formation of definite 

 galls. Not all species, however, cause galls and some 

 form only a distorted area in the leaf which is densely 

 covered with small hairs. The ancestors of these forms 

 evidently were simple plant feeders and probably were 

 closely related to the *'Eed Spiders." Certainly there 

 are many things which indicate their phylogenetic rela- 

 tionship to this group. As to just what kind of galls 

 were originally formed by these mites, probably we can 

 never know; yet there is the significant fact about the 

 galls now formed by them, that they all have openings 

 to the exterior through which the mites can pass. The 

 life in any of these galls with such small openings, or in 

 those of a pinlike formation, or even among the hairs 

 on the erineum, requires: that the body must be very 

 narrow in order that the mite may be able to move about. 

 In response to these conditions we find that these gall- 

 inhabiting mites have the body enormously lengthened 

 (PI. VII, Fig. 29), so much so as to suggest a super- 

 ficial resemblance to the worms. 



Again in other groups we find this same enormous 

 lengthening of the body in response to very different 

 conditions but each requiring this vermiform structure 

 of the body. This is true in the case of the Demodecidae, 

 the *' hair-follicle mites." These mites live in the hair 



