APPENDIX I 429 



Jersey, has positively proved that during the early days of Feb- 

 ruary, in water just above the freezing temperature, the larva of 

 Culex canadensis^ hatches from the egg. A wingless snow gnat 

 (Chionea valga) is found only during the winter in the northern 

 United States and Canada, crawling on the snow with the ther- 

 mometer as low as 15 above zero. There are many other insects 

 which seem to thrive under similar conditions. 



Another feature which enables Diptera to withstand most un- 

 favourable climatic conditions is their diversity of habit ; aquatic, 

 parasitic, herbivorous, and carnivorous, they feed upon almost 

 everything from living tissue to the most putrid and decayed animal 

 and vegetable matter, and are thus liable to be widely distributed 

 through commerce. Many of the blood-thirsty species breed in 

 water, the larva of the mosquito living in swamps and stagnant 

 pools, while those of the black-fly frequent the rapidly running 

 streams. These conditions, existing to so great an extent through- 

 put the interior, present very favourable breeding places for these 

 insects, and render some districts practically uninhabitable by man. 



A great similarity prevails throughout the whole dipterous 

 fauna of the more northern regions. Many are circumpolar in 

 their distribution, others differ so slightly that it is almost impos- 

 sible to determine them from descriptions, and comparison with 

 European specimens is necessary. That they have not become 

 more differentiated is probably due to the uniform climatic condi- 

 tions under which they have existed. In numbers the Diptera 

 extend farther into the Arctic region than any other order of 

 insects, therefore presenting one of the best groups for tracing 

 boreal distribution. 



The flies include most of the many species of insects which 

 infest mammals and birds. Of these parasites some may be ex- 

 ternal, others internal. Their generally small size and the indif- 

 ference of trappers and most collectors of animals and birds to 

 their existence, is one of the principal reasons for our lack of 

 knowledge of these forms, especially from more northern latitudes. 

 It is doubtful if there is an animal or bird which is entirely free 

 from a parasite. While these are probably less numerous in the 

 colder region, the conditions are quite favourable, and they are 

 undoubtedly more abundant than is generally supposed. 



There are two species of flies of which we know but little, but 

 which we do know infest the caribou. They belong to the family 

 (Estrida3, popularly known as bot-flies. The habits of one of the 

 species are apparently similar to those of the sheep bot-fly. A 

 description, therefore, of what is known of the latter species may 

 aid in studying the life history of the one infesting the caribou. 



The fly of the sheep-bot is about one-half of an inch in length, 



