18 BIOLOGICAL SURVEY OF 



lived on this island for twenty years withont protection in 

 winter." 



In the remarks by Eand and Redfield on the plant distribu- 

 tion you will notice that an exception is made of the maritime 

 flora, otherwise there is a '' resemblance to the general flora 

 of the White Mountain region." This is, to a large degree, 

 equally true of the insects. The same trees and plants of 

 the two places have the same insect enemies, and these insects 

 in turn serve as hosts for the same parasites. The elevations 

 of the island are not sufficient for the arctic-alpine forms of 

 Mount Washington and the boreal species are more com- 

 monly found near the shores and in the vicinity of the lakes, 

 cold spring runs, and bogs, the more sheltered and southern 

 exposures probably harboring the greater number of austral 

 species. 



In regard to the distribution of aquatic insects, as pointed 

 out by Dr. R. H. Howe, Jr.,^ cold bogs and ponds that are 

 spring fed have less extremes in temperature than ponds that 

 are not thus supplied with water. Thus he found boreal and 

 austral dragon flies living together in the same pond far be- 

 yond their normal limits of distribution. The pond in ques- 

 tion had a bottom temperature of 51°F. in summer and 36°F. 

 in winter — a range of only 15°. 



The distribution of most of the maritime insects is governed 

 comparatively little by the corresponding vegetation, although 

 the two frequently show a similar distribution. Tempera- 

 ture and a suitable environment are the principal factors. 

 A low muddy shore with considerable marsh and pools of 

 brackish water has an insect fauna of its own, and it is 

 these species that often show^ a very wide distribution. To 

 the proximity of the ocean wdth its modifying etfect on the 

 climate is apparently due this extension of southern species 

 far beyond their normal habitat. On the other hand, cold 

 currents produce conditions favorable to boreal species. The 

 effect of the ocean on temperature is perhaps best shown at 



*"Tlie distribution of New England Odonata. " Proe. Boston Soc. Nat. Hist., 

 vol. 36, p. 110, 1921. 



