282 THE VITAL FUNCTIONS. 



frequently occasioned their being mistaken for blood 

 vessels. In the wings of insects the nervures, which 

 have the appearance of veins, are only large air- 

 tubes. Jurine asserts that it is by forcing air into 

 these tubes that the insect is enabled suddenly to 

 expand the wings in preparing them for flight, 

 giving them by this means greater buoyancy, as 

 well as tension. 



The tracheae are kept continually pervious by a 

 curious mechanism ; they are formed of three coats, 

 the external and internal of which are membranous; 

 but the middle coat is constructed of an elastic 

 thread coiled into a helix, or cylindrical spiral (as 

 seen in Fig. 372) ; and the elasticity of this thread 

 keeps the tube constantly in a state of expansion, 

 and therefore full of air. When examined under 

 water, the tracheae have a shining silvery appear- 

 ance, from the air they contain. This structure 

 has a remarkable analogy to that of the air vessels 

 of plants, which also bear the name of tracheae ; 

 and in both similai? variations are observed in the 

 contexture of the elastic membrane by which they 

 are kept pervious.* 



The tracheae, in many parts of their course, 

 present remarkable dilatations, which apparently 

 serve as reservoirs of air ; they are very conspicuous 

 in the Di/tiscus marginalis, which resides princi- 

 pally in water; but they also exist in many insects, 

 as the Melolontha and the Cerambyx, which live 



* According to the observation of Dr. Kidd these vessels are often 

 annular in insects, as is also the case with those of plants. Recon- 

 siders the longitudinal tracheae as connecting channels, by which 

 the insect is enabled to direct the air to particular parts for occa- 

 sional purposes. Phil. Trans, for 1825, p. 234. 



