214 NATURAL HISTORY. [cH. XV. 



of each to its allotted work in the grand system of 

 the creation. This is a point of view in which 

 zoologists in general seldom look at the subject, 

 their attention being rather directed, in the majority 

 of instances, to the peculiarity of structure of any 

 particular animal ; and if they do endeavour to trace 

 the intimate connexion between *' uses and things," 

 it is rather with a view of ascertaining the connex- 

 ion between a particular structure, and the function 

 which it is destined to perform : yet it will not be 

 denied, that in a far higher and more philosophical 

 spirit, the plan of the creation, or, as St. Pierre has 

 not unaptly expressed it, " the harmonies of nature," 

 must be studied with reference to the general agen- 

 cy, not of individuals, but of the varying masses 

 of living objects of which each species is composed. 



Leaving out of our present consideration the in- 

 teresting portion of this question which has refer- 

 ence to the proportion of vegetable feeding animals, 

 and limiting our inquiry to the legitimate object of 

 these volumes, we find that the order of two- winged 

 dipterous insects, although far below some of the 

 other orders in point of number of species, far ex- 

 ceeds them in the quantity of individuals. There 

 is, perhaps, not a spot upon the earth, not even ex- 

 cluding the polar regions, in which some one spe- 

 cies of gnat or fly will not be found, swarming in 

 every place, or filling the air at almost every sea- 

 son with their countless legions. We therefore 

 propose, in this and the following chapter, to bring 

 before the notice of the reader the natural history 

 of several of the more interesting species of which 

 this order is composed. 



Of all insects, perhaps the most widely distrib- 

 uted and abundant in number of individuals, is the 

 housefly, Musca domestica, a species which, from its 

 constant occurrence in every situation, will render 

 any precise description of it unnecessary ; and yet 

 many of our readers are, perhaps, unaware, that of 



