RESPIRATION IN INSECTS. 223 



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 cylin- 

 drical spiral, (as seen in Fig. 372;) and the elasticity of this 

 thread keeps tl)e tube constantly in a state of expansion, and 

 therefore full of air. When examined under water, the tra- 

 cheae have a shining silvery appearance, from the air they 

 contain. This structure has a remarkable analogy with that 

 of the air vessels of plants, which also bear the name of tra- 

 cheae; and in both similar variations are observed in the con- 

 texture of the elastic membrane by which they are kept 

 pervious.* 



The tracheae, in many parts of their course, present re- 

 markable dilatations, which apparently serve as reservoirs 

 of air: they are very conspicuous in the Dytiscus margina- 

 Us, which resides principally in water; but they also exist 

 in many insects, as the Melolontha and the Cerambyx, 

 which live wholly in the air.t Those of the Scotia horto- 

 rum (Fab.) are delineated in Fig. 373, considerably magni- 

 fied. 



If an insect be immersed in water, air will be seen es- 

 caping in minute bubbles at each spiracle; and in proportion 

 as the water enters into the tubes, the sensibility is de- 

 stroyed. If all the spiracles be closed by oil, or any other 

 unctuous substance, the insect immediately dies of suffoca- 

 tion; but if some of them be left open, respiration is kept 

 up to a considerable extent, from the numerous communi- 

 cations which exist among the air vessels. Insects soon 



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

 in insects, as is also the case with those of plants. He considers the longi- 

 tudinal trachea as connecting channels, by which the insect is enabled to 

 direct the air to particular parts for occasional purposes. Phil. Trans, for 

 1825, p. 234. 

 f Lon Dufour, Annalea des Sciences Naturelles; viii. 26. 



