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- 
out 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 
(Estridz, 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, 
