DIPTERA. 37 
every situation ; some inhabiting woods, plains, fields, or banks 
of rivers; others preferring our houses. They each take their 
share of vegetation, preferring either the flowers, the leaves, or 
the stems of the trees of our woods, our gardens, or our planta- 
tions. Their food varies very much; and the formation of the 
sucker is regulated by it. Some imbibe blood, others live on the 
secretions of animals. Their chief nourishment, however, consists 
of the juices of flowers, on whose brilliant corollas the Diptera 
abound, either plundering from every species indiscriminately, 
or attaching themselves to some particular kind. They dis- 
play the most wonderful instinct in their maternal care, and 
employ the most varied and ingenious precautions to preserve 
their progeny. 
The Diptera, besides their variety and the number of their 
species, are remarkable on account of their profusion. The myriads 
of flies which rise from our meadows, which fly in crowds around 
our plants, and around every organised substance from which 
life has departed, some of which even infest living animals, are 
Diptera. 
The profusion with which they are distributed over the face 
of the globe, causes them to fulfil two important duties in the 
economy of nature. On the one hand, they furnish to insecti- 
vorous birds an inexhaustible supply of food; on the other, they 
contribute to the removal of all decaying animal and vegetable 
substances, and thus serve to purify the air which we breathe. 
Their fecundity, the rapidity with which one generation succeeds 
another, and their great voracity, added to the extraordinary 
quickness of their reproduction, are such that Linnzus tells us 
that three flies, with the generations which spring from them, 
could eat up a dead horse as quickly as a lion could. 
These Diptera, which are worthy of so much attention, and 
deserve so much study with regard to the part they play in the 
general economy of nature, are an object of fear and repulsion 
when one considers their relations to us and other animals. Gnats 
and mosquitoes suck our blood ; the gad-fly and the asilus attack 
our cattle. The order Diptera is composed of a great number of 
families, and these families are again divided into tribes, which 
themselves comprise several genera. We shall only speak of 
