RESPIRATION IN INSECTS. 003 



giving them, by this means, greater buoyancy, as well as 

 tension. 



The trachea} 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 the tube constantly in a state of expansion, and 

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

 cheas 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- 

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

 texture of the elastic membrane by which they are kept 

 pervious.* i. 



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

 markable dilatations, wdiich apparently serve as reservoirs 

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

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

 in many insects, as the Melolontha and the Cerambijx, 

 which live wlK)lly in the air.t Those of the Scolia 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 suflbca- 

 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 witli those of plants. He considers the long-i- 

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

 direct the air to particular pai'ts for occasional purposes. Phil. Trans, for 

 1825, p. 234. 



t L^on Diifour, Annalcs dcs Sciences Naturellesj viii. 26. 



