12 BIOLOGICAL SURVEY OF 



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 gov- 

 erned comparatively little by the corresponding vegetation, 

 although the two frequently show a similar distribution. 

 Temperature 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 with its modifying effect on 

 the climate is apparently due this extension of southern spe- 

 cies 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 

 Sable Island.^ '^A study of the temperature will show that 

 there are no extremes of heat or cold on the island ; that the 

 highest temperature during the past four years was 78° and 

 the lowest point reached by the thermometer during the same 

 period was 5° above zero." 



With these evident and interesting features bearing on the 

 zoogeography of the island before us, a special study has been 

 made each year of the halophilous insects frequenting the salt 

 marshes at the Narrows near the bridge. The number of 

 southern species that have been collected there is surprising. 

 Their abundance would also indicate that this is by no means 

 the northeastern limit of distribution of liiany of the species 

 and that a wide overlapping of northern and southern forms 

 occurs on this part of the coast. 



The abundance of the parasite wasps — Ichneumonidae — 

 and the parasitic flies — Tachinidae — indicates a normal or 

 balanced condition, these insects keeping in check the injuri- 

 ous species. The lady-beetles, lace wings, and syrphid flies, 

 the great enemies of the plant-lice, are also abundant and 



°" Sable Island with a catalogue of its vascular plants," by Harold St. John, 

 Proc. Boston Soc. Nat. Hist., vol. 36, p. 52, 1921. 



