230 PHYSIOLOGY OF THE INVERTEBRATA. 



Respiration in tlie aquatic larva) of certain insects is 

 performed by means of tracheal gills or brancliia3. These 

 branchi;e are delicate folds of the integument, and are richly- 

 supplied with minute trachea?. The oxygen dissolved in 

 water, wherein or whereon these larva? flit to 8.nd fro, passes 

 into the trachea\ This phase of respiration among the 

 Insccta is suo-^estive of the branchia? of certain forms of the 

 Annelida, into which the vessels of the pseudo-hsemal systems 

 enter. 



The larva3 of Libdlula and JEsclina present yet another 

 form of respiratory organ. " Although they possess a pair 

 of thoracic stigmata, these appear to have little or no 

 functional importance, but respiration is effected by pumping 

 water into and out of the rectum. The walls of the latter 

 are produced into six double series of lamella?, in the interior 

 of which tracheae are abundantly distributed, and which play 

 the same part as the tracheal branchia? just mentioned. 

 These rectal respiratory organs, in fact, appear to be a com- 

 plicated form of the so-called ' rectal glands ' which are so 

 generally met with in insects." 



Besides the systems of tracheal tubes, tissue-respiration is 

 well marked in the Insccta, for MaclMunn has met witli 

 myohaBmatin in abundance in these animals ; and it is 

 probable that histohtematin is also present, and both of these 

 pigments have a respiratory function. 



The Arachnida. 



In these animals " trachea? may exist alone, or be accom- 

 panied by folded pulmonary sacs, or the latter may exist 

 alone, as in the Scorpion. In this case these lungs* are 

 supplied by blood which is returning from the heart." 



"The flow of air into and out of the air cavities is 

 governed by the contractions of muscles of the body, 

 disposed so as to alter its vertical and longitudinal dimen- 



* See also Macleod's paper in Bull. Acad, Boy. Scien. Bel//., vol. 3, p. 779. 



