THE FLY. 59 



in various parts^ where of course the circulation of 

 the air is then arrested ; and unless some means were 

 employed to re-open the tube, its lining membrane 

 would be very liable to adhere, and cause it to re- 

 main permanently closed. Nothing can be more de- 

 licate and simple than the contrivance by which this 

 obstacle is overcome, — a contrivance, that we find 

 constantly imitated m our every-day life. When one 

 of the larger tracheae is magnified about 400 diameters, 

 it is found to consist of a double integument, between 

 which a close continuous coil of hairlike fibre is situ- 

 ated, which supports the tubes in a similar manner 

 to the coil of wire in gas- or speaking-tubes. This 

 coU (PI. VI. fig. 5) adds considerably to the elasticity 

 of the air-vessels ; and when these collapse through 

 the movements of the insect, it prevents the opposed 

 sides from adhering permanently, and causes them to 

 resume their tubular form as soon as the pressure is 

 removed. 



In the distribution of the tracheae we find the same 

 wise arrangement that characterizes every other por- 

 tion of the internal economy of the Fly. The abdo- 

 men, being the heaviest section of the body, on ac- 

 count of the mass of viscera that it contains, has its 

 specific gravity considerably diminished by being fur- 

 nished with two large air-bladders (PI. III.), as w^eU 

 as two tracheal tubes of considerable size, that pass 

 along either side of the body ; whilst the thorax, on 

 which the wings are situated (and which of them- 

 selves add to the buoyancy of the insect), is only pro- 



