INSECTS OF REVERSED BEHAVIOR. 397 



lamps. Anyone who has seen the snow storms of mayflies that 

 come to the street lamps of the lake ports realizes that the 

 subimago can have a reverse tropism under such conditions. 



\Ye are beginning to recognize physiological species among 

 insects those based on habits and habitats. In parasitic insects 

 particularly we recognize generic reactions to common hosts so 

 that we unofficially recognize physiological genera. There is no 

 reason why \\e should not, except the expediency of morpho- 

 logical char.K -ters. Viewed in this light /Eshna nevadensis and 

 /'.fj!n-mrr<i '^Httidata are physiologically outside of their respective 

 genera. 



\\lu-M the writer first thought through the habits and dis- 

 tribution of the North American species of the genus ^-Eslnia, 

 lii- coin'lu-ion was that the positive thermotropism of the lo\\ - 

 I.IIK! -pei ies was perhaps different in degree and distinct for each 

 species, thu> explaining the restriction of each species to its 

 >pn ilic tlitrm.il belt. Thus it appeared, at first, that this 

 difference in positive thermotropism accounted for the fact that 

 some . K-hnas lived in hot Arizona, others in the cooler northern 

 States, while still others are restricted to the northern parts of 

 i anada and Alaska. However on investigating further this does 

 not appear to be true. 



Tin- K -t rirtion of each species to its specific thermal belt is 

 l>r. .bably not due to a limitation of the positive thermotropism 

 of the adult to the narrow limits of the particular thermal belt 

 inhabited by that species. This is quite contrary to Merriam's 

 theory of distribution by thermal zones. 5 



This di-tnbution of the various species in different thermal 

 /ones ho\\e\er is a very striking thing and some of the thermal 

 conditions are easily sketched. Except for nuttala, californii 

 and multicolor, which are early spring species, the majority of 

 tin- species of .-Kshna are on the wing in August, so the writer 

 ha- \\orked out the flying temperatures for August at four points, 

 Yuma. Ari/ona, St. Louis, Missouri, St. Paul, Minnesota, and 

 Sika, . \la-ka. Each of these is representative of the flying con- 

 ditions for a restricted group of .^Eshnas. The Yuma tempera- 

 tures apply to jalapensis, multicolor and aridn. The St. Louis 



* Mcrriam, "Life and Crop Zones of the United States," Bull. U. S. Biol. 

 Survi-y, Nn. 10, pp. 1-79, 1898. 



