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ON THE PHYSIOLOGY OF THE RECTAL GILLS IN 

 THE LARV^ OF ANISOPTERID DRAGONFLIES. 



By R. J. TiLLYARD, M.A., B.Sc, F.E.S., Linnean Macleay 

 Fellow op the Society in Zoology. 



(Plate xlvii.) 



The physiology of respiration is one of the most difficult prob- 

 lems still awaiting a solution in the biology of the Insecta. Most 

 insects breathe by means of spiracles, which are the apertures of 

 a complicated system of air-tubes or trachecE ramifying through- 

 out the body, and ending internally in numerous excessively fine 

 tubes or capillaries, which supply oxygen directly to all the cells 

 concerned in the metabolism of the body-tissues. Such a system 

 is known as an Open Tracheal System. There are, however^ 

 certain aquatic larvae in which spiracles are either absent, or, if 

 present, non-functional, except, perhaps, on very special occa- 

 sions, such as when the larvse leave the water just prior to 

 metamorphosis. In these forms, the required oxygen is derived 

 from the water either directly through the integument, or, more 

 usually, by means of tracheal gills. To this latter group belong 

 the larvae of Mayflies, Stoneflies, and Dragonfiies. This type of 

 system is called a Closed Tracheal System. 



It is not the purpose of this paper to discuss the various 

 theories of respiration which have been put forward to meet the 

 case of the Open Tracheal System. The question, whether 

 expiration is performed through the spiracles, as well as inspira- 

 tion, or whether the latter alone takes place there, does not enter 

 into the problem which we have before us For the fact, that 

 the tracheal system of the Dragonfly Larva is physiologically a 

 closed one, cannot be disputed, though this closed system shows 

 clearly a secondary derivation from an original open system, 

 resembling that of the imago. Spiracles are present both on the 

 thorax and the abdomen. Those of the thorax, at any rate, can, 



