RESPIRATION IN INSECTS. 313 



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 conspi- 

 cuous in the Dytiscus marginalis, which resides 

 principally in water ; but they also exist in 

 many insects, as the Melolontha and the Ceram- 

 hyx, which live wholly in the air.f Those of 

 the Scolia hortorum (Fab.) are delineated in 

 Fig. 373, considerably magnified. 



If an insect be immersed in water, air will be 

 seen escaping in minute bubbles at each spi- 

 racle ; and in proportion as the water enters into 

 the tubes, sensibility is destroyed. If all the 

 spiracles be closed by oil, or any other unctuous 

 substance, the insect immediately dies of suf- 

 focation ; but if some of them be left open, 

 respiration is kept up to a considerable ex- 

 tent, from the numerous communications which 

 exist among the air vessels. Insects soon perish 

 when placed in the receiver of an air-pump, 

 and the air exhausted ; but they are generally 



* 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 longitudinal tracheae 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. 



t Leon Dufour, Annales des Sciences Naturelles ; viii. 26. 



