226 PHYSIOLOGY OF RESPIRATION IN AQUATIC INSECTS. 



gases into the trachese through the rectal surface, he assigned 

 respiratory functions to certain tracheated fatty pads found at 

 the anterior end of the lumen of the rectum. With this the 

 present writer has no quarrel, since he found the same pads 

 and had independently assigned them the same function. ( Rich, 

 1918.) 



Bodine, however, goes on to say that the carriage of air into 

 the body-cells is not clear ; that it may be performed by the 

 blood or through the tracheae. It is at this point that issue is 

 joined. 



The first point of dispute is as to the necessity of postulat- 

 ing the blood as carrier of oxygen to the tissues and carbon 

 dioxide from them. Whoever is acquainted with the anatomy 

 and histology of insects is aware that the tracheal system, a 

 system of ramifying air-tubes, reaches all the tissues much as 

 the circulatory system does in vertebrates. The mechanism for 

 direct respiration of air by each cell, without intermediary, is 

 present. Not only is this true of terrestrial insects, but of water- 

 breathing forms as well. Cullen (1918) has shown that the 

 tracheal branches in the Agrionids pass in fairly large numbers to 

 the skin. This has been noted also in other water-breathing 

 insects, by earlier workers. 



It would appear that there is not the least necessity for 

 assuming any other respiratory mechanism to supplement the 

 tracheal system. The rectum is lined with exactly the same 

 material as the skin, namely, chitin. Dewitz (i8yo) has shown 

 that chitin is permeable to oxygen and to carbon dioxide. There 

 ii., therefore, no need to complicate the matter. Through skin 

 and through rectal lining and pads, the gases dissolved in the 

 water pass into the trachea, and the carbon dioxide in the trachere 

 dififuse outwards into the water. 



The second point of dispute is more vital. In the absence 

 of evidence to prove its presence, we may not assume for the 

 blood and assign to it either the presence of an oxygen-carrying 

 substance or the function of carrier in respiration. It is ex- 

 tremely improbable that two such divergent stocks as the verte- 

 brates and the insects should have the same method of respiratory 

 carriage. 



A third objection to allowing the blood any role in insect 

 respiration is the nature of the insectan circulatory system. 

 Unlike that of vertebrates, it is not a closed system. It c<Misists 

 of a heart pumping blood cephalad, discharging it into the 

 metacoele or body cavity, and then only of a chaotic series of 

 lacunae through which the blood oozes, and which are often mere 

 blind pockets of the metacoele. This is certainly a most in- 

 efifective mechanism for providing the rapid and continuous 

 supply of oxygen needed for the active life of the insects con- 

 sidered here ; but it does suffice for the carriage of nutriment 

 to the tissues. Moreover, the dis|X)sition of the tracheae is 



