I 30 RESPIRATION 



the tissues and form capillaries, while the aerating apparatus is 

 confined to one part of the hody ; in Insects the blood-circulating 

 system is very limited, and air is carried directly by complex 

 vessels to all parts ; thus the tracheal system is universally recog- 

 nised as one of the most remarkable of the characters of Insects. 

 Many Insects have a very active respiratory system, as is shown 

 by the rapidity with which they are affected by agents like 

 chloroform; but the exact manner in which the breathing is 

 carried on is unknown. In living Insects rapid movements of 

 contraction and expansion of parts of the body, chiefly the 

 abdomen, may be observed, and these body contractions are some- 

 times accompanied by opening and shutting the spiracular 

 orifices : it has been inferred that these phenomena are respira- 

 tory. Although such movements are not always present, it is 

 possible that when they occur they may force the air onwards 

 to the tissues, though this is by no means certain. It is clear 

 that the tracheal system is the usual means of supplying the 

 organisation with oxygen, but it appears to be improbable that 

 it can also act as the agent for removing the carbonaceous pro- 

 ducts of tissue-changes. It has been thought possible that car- 

 bonic acid might reach the spiracles from the remote capillaries 

 by a process of diffusion,^ but it should be recollected that as 

 some Insects have no tracheal system, there must exist some 

 other mode of eliminating carbonic acid, and it is possible that 

 this mode may continue to operate as an important agent of 

 purification, even when the tracheal system is, as a bearer of air 

 to the tissues, highly developed. Eisig ^ has suggested that the 

 formation of chitin is an act of excretion ; if so this is capable 

 of relieving the system of carbonic acid to some extent. Others 

 have maintained that transpiration takes place through the deli- 

 cate portions of the integument. Lubbock ^ has shown that 

 Melolontha larvae breathe "partly by means of their skin." 

 The mode in which the carbon of tissue -change, and the 

 nitrogen of inspiration are removed, is still obscure ; but it 

 appears probable that the views expressed by Eeaumur, Lyonnet, 

 and Lowne * as to inspiration and expiration may prove to be 

 nearer the truth than those which are more widely current. In 



^ See Miall and Denny, Cockroach, p. 158. 

 - Eisig, Mon. Capitelliden, 1887, p. 781. 

 3 Tr. Linn. Soc. London Zool. xxiii. 1860, p. 29. * Blowfly, etc. p. 376. 



