RESPIRATION IN INSECTS. 285 



air is admitted by a separate external opening. 

 A, B, is the dorsal vessel, which is connected with 

 the pulmonary cavities by means of two sets of 

 muscles, the one set (m, m) being longer than the 

 other (m, m, m). The branchial arteries (v) are 

 seem ramifying over the inner surface of the pul- 

 monary cavities (r) on the right side, whence the 

 blood is conveyed by a corresponding set of bran- 

 chial veins to the dorsal vessel ; and other vessels, 

 which are ordinary veins, are seen at o, proceeding 

 from the abdominal cavity to join the dorsal vessel. 

 The membrane which lines the pulmonary cavities 

 is curiously plaited ; presenting the appearance of 

 the teeth of a comb, and partaking of the structure 

 of gills ; and on this account these organs are 

 termed by Latreille pneiimo-brcmchice . Organs of 

 a similar description exist in Spiders ; some species 

 have eight ; others four ; and some only two : but 

 there is one entire order of Arachnida which resj^ire 

 by means of tracheae, and in these the circulation 

 is as imperfect as it is in insects. 



It may here be remarked that an essential dif- 

 ference exists in the structure of the respiratory 

 organs, according to the nature of the medium 

 which is to act upon them ; for in aquatic respira- 

 tion the air contained in water is made to act on the 

 blood circulating in vessels which ramify on the 

 external surface of the filaments of the gills; while 

 in atmospheric respiration the air in its gaseous 

 state is always received into cavities, on the inter- 

 nal surface of which the blood-vessels, intended to 

 receive its influence, are distributed. It is not 

 difficult to assign the final cause of this change of 

 plan ; for in each case the structure is accommo- 



