to speak) inserted in the trunk, the two wires uniting without leaving a blank. It is diffi- 

 cult to describe how this is done ; but by tracing home one of the ramifications, one may- 

 see that it is performed most accurately — the circumvolutions of the trunk wire being 

 crowded and bent round above and below the insertion (like the grain of timber round a 

 knot), and the lowest turns of the branch wire being suitably dilated to fill up the hiatus. 

 The chemical name of the substance forming this wire is chitine. 



The tracheae terminate outwardly, as we said before, in spiracles, or trap doors, 

 arranged along the sides of the fly. They serve to allow the free entrance of air into the 

 tracheae, at the same time excluding dust and other foreign matter. These spiracles are 

 narrow oval orifices, which are closed sufficiently by means of minule delicate hairs, which 

 form a network over the entrance. 



The feet of Musca domesiica are also objects of interest. Each foot is furnished with 

 two large moveable claws, which it can affix to any little inequalities of surface ; but the 

 great bulk is composed of two large cushions or pads, or pulvilli, as they are technically 

 called. These pads are furnished with a great number of filaments, or soft hair-like bodies 

 situated on the margins. Many explanations have been given of the manner in which flies 

 walk on polished surfaces, especially if they are placed vertically. It was long supposed 

 that the jpulvilli were mere suckers, anS that the tiy sustained itself in unnatural positions 

 by forming a vacuum between these and the surface of the object ; in which case the 

 atmosphere would press with sufficient force on the outside of the sucker to hold the 

 weight of the fly. One writer and microscopist stated that the under portion of the pads 

 were beset with numerous bristles, or tenters, working in an opposite direction to the 

 large claws, thereby enabling the insect to take advantage of any slight irregularities of 

 surface. In the case of polished bodies of glass and such substances, he gratuitously sup- 

 posed it to be covered with a " smoky tarnish," into which these minute hairs might be 

 fastened. However, the accepted explanation now is, — that the small filaments belong- 

 ing to the pulvilli each terminate in a small fleshy bulb, which is kept moist by a viscid 

 liquid : these constitute the organs of adhesion. Although they are very minute, yet their 

 number is very great, and they expose considerable surface. 



Towards the close of autumn vast numbers of flies fall victims to a curious dis- 

 ease, which is highly interesting to the microscopist. Occasionally there may be noticed 

 numbers of dead flies adhering to the walls and windows, often so far retaining the atti- 

 tude of life that it is difficult, without touching them, to assure one's self that they are 

 not actually on the point of taking flight. Insects in dying usually draw up the legs and 

 cross them on the body, but in this case the dead body is supported on the outstretched 

 legs, whose feet seem still to retain their adhesive property. If the body be on a window 

 a halo may be observed around it, nearly an inch in diameter, and composed of a whitish 

 dust, which, on examination by the microscope, is found to consist of the spores of a fun- 

 gus. The abdomen is much distended, and the rings composing it are separated from each 

 other, the intervals being occupied by white prominent zones, constituted of a fungoid 

 growth, proceeding from the interior of the body. Further examination will show that 

 the whole of the contents of the body of the fly have been consumed by the parasitic 

 growth, and that nothing remains but an empty shell, lined with a thin felt-like layer 

 of the interlaced threads of this fungus, the name of which is Empvsa muscce. 



In conclusion, we will say a few words in favour of our " household pest." Most of 

 us have experienced in the summer time, during showery weather, the sharp bite of a fly, 

 which is usually supposed by most people to be the common house fly ; but, although their 

 appearance may seem similar, the two are perfectly distinct. They differ so much in struc- 

 ture and habits that entomologists have placed them in separate genera. The proper 

 name of the house fly is at the head of this article, while the bloodthirsty little creature we 

 have referred to rejoices in the title of Stomoxys calcitrans. 



C 



