178 OUR HOUSEHOLD INSECTS 



accumulated on the window-sills. On examining the 

 heaps of bodies, a considerable variety of species was 

 discovered, and several were noticed whose presence was 

 quite unexpected ; still, out of many scores of specimens 

 examined, by far the largest proportion consisted of the 

 one species, Stomoxys calcitrans. 



As all five of the flies mentioned above belong to a 

 single family, the Muscidce, they exemplify the same 

 structural type, and, except in a few matters of detail, 

 one may be taken as representing all. To illustrate the 

 main points in a fly's structure, therefore, we will select 

 the bluebottle or blow-fly (CaUiphora), as it is the largest 

 of the indoor species, and can always be easily obtained. 

 We will suppose that we have before us one of these 

 flies, which has been killed without crushing or other- 

 wise damaging the body. This can be done by means of 

 the fumes of chloroform, cyanide of potassium, or crushed 

 laurel leaves, any of which will, in a few moments, render 

 such an insect insensible, while a longer exposure to the 

 poisonous vapours will kill it entirely, and leave it in a 

 good condition for examination. 



The distinguishing feature of the order is at once 

 noticed in the single pair of membranous wings placed 

 horizontally over the back when at rest, and extended 

 at right angles to the body when in use. As the Diptera 

 are the only order of insects in which a single pair of 

 wings is normally present, there is very little difficulty 

 in distinguishing them, and there are very few other 

 insects that can possibly be mistaken for them. Each 

 wing consists of a thin double membrane, strengthened 

 by six longitudinal hollow ribs or "nervures," of which 

 the larger contain breathing-tubes (tracheae) and nerves. 

 The nervures are not scattered at random, but always, 

 as in other orders, follow a definite course, which varies 



