RESPIRATION IN INSECTS. 



225 



pulmonary cavities (R) on the right side, whence the blood 

 is conveyed by a corresponding set of branchial veins to 

 the dorsal vessel: and other vessels, which are ordinary 



374 



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 ap- 

 pearance of the teeth of a comb, and partaking of the struc- 

 ture of gills; and on this account these organs are termed 

 by Latreille pneumo-branchiss. Organs of a similar de- 

 scription exist in Spiders, some species having eight, others 

 four, and some only two: but there is one entire order of 

 Arachnida which respire 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 difference ex- 

 ists in the structure of the respiratory organs, according to 

 the nature of the medium which is to act upon them: for in 



VOL. II. 29 



