110 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY 
and a salinity up to 1.003 is endured lalliei- 1li;ni soufilit. A 
marked decrease after the severe wintei- of 1919-20 in Krijlhro- 
diplax berenice on Cape Cod supports this explanation. No one 
species has been found to be exclusively confined to bracldsh 
ponds, since all the supposed halophile species inhal^t both fresh 
and brackish ponds. This is true of the salt lakes of the interior 
as well (Schwartz, Can. Ent., 23: 235-241, 1891, and Osburn, Amer. 
Nat., 40 : 395-399, 1906). The proximity of the ocean is a stabiliz- 
ing temperature influence. 
The very evident congeniality or attraction of maritime condi- 
tions for species of plants and animals has long l)een noted, and as 
yet has met with no very satisfactory explanation. There are 
probably many unexplained links in the chain, which only a most 
intensive ecological study can solve. 
Seasonal Occurrence 
The very evident variability in the abundance of Odonata from 
season to season has been the common observation of field natural- 
ists (Walker, Can. Ent., 17 : 171-178, 1917). A warm, sunny, dry 
season always shows, so far as the author has observed, a decided 
local increase not only of individuals, but also of species in com- 
parison with numbers noted in a cold, sunless, wet season, pro- 
vided the ponds and streams are not low. This would seem to be 
due not only to wider flight activity, but also to unsuccessful 
transformations during wet weather. Another factor that may 
have a local influence is that some species remain in their larval 
state over a period of more than one year, and therefore in certain 
years more species are emerging than in others (Psyche, 27: 155, 
1920). 
Water Temperatures 
The primitive cedar swamps and cold-bog ponds have long been 
termed ''boreal islands" where northern species far south of their 
usual range find congenial habitats. In connection with the 
Odonata the cold-bog ponds might better be termed "extra- 
Imiital islands" for they are not only southern stations for boreal 
species, but northern stations for austral species. The presence of 
Leucorrhinia frigida and L. glacialis south of their normal ranges. 
