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THE AMERICAN NATURALIST [Vol. XL VIII 



posed a new scheme, based on my observations in the 

 field. In this scheme, Thysanoptera were divided into 

 two groups : (1) interstitial species, those living in closely 

 concealed situations, as among the florets of composite 

 flowers, or in clusters of young leaves; and (2) super- 

 ficial species, those living on exposed surfaces, for ex- 

 ample, the surface of leaves. The interstitial species 

 were further divided into an anthophilous division 

 (flower-dwellers) and a phlceophilous division (those 

 living under bark scales on trees). The superficial spe- 

 cies were either poephilous (on grass) or phyllophilous 

 (on leaves of plants other than grasses). The distinction 

 between poephilous and phyllophilous seemed warranted, 

 since grass-dwellers were found on many different 

 grasses, but rarely on other kinds of leaves. 



Such a classification undoubtedly describes the facts, 

 but does not explain why the habitats named are the ones 

 chosen(?). The factors determining habitat were be- 

 lieved by me at that time to be character of food, and pro- 

 tection afforded. In some species one of these factors 

 predominated, in other species the other factor, while 

 others may have been influenced largely by both. In the 

 light of recent ecological studies, however, the explana- 

 tion of local distribution in terms of such general environ- 

 mental factors seems inadequate. Largely owing to the 

 work of Shelf ord (1911) upon the tiger-beetles, much 

 emphasis is now being placed upon the ecological impor- 

 tance of physiological factors. With a view to relating the 

 distribution of Thysanoptera to the physiology (more 

 specifically, behavior) of the various species, and thus 

 explaining that distribution in more definite terms, the 

 experiments and observations recorded in this paper 

 were made. 



This work was done largely at the University of Michi- 

 gan Biological Station, at Douglas Lake, Michigan, sup- 

 plemented by observations at Ann Arbor, Michigan, in 

 Ohio and elsewhere. 



