442 MR. LUBBOCK ON THE THYSANURA. 



duridse, among which I have examined species belonging to most of the genera, though 

 I have not yet been able to obtain any specimens of the true Podura, to which genus the 

 species examined by Nicolet belonged. Close to the spiracles, the tracheae break up into 

 a great number of thin branches, which supply the head without much more subdivision. 

 There is also a very large trunk, which almost immediately divides into two branches, 

 the smaller one of which soon divides again, and supplies the anterior region of the 

 thorax, while the other gives off branches to the posterior legs and the abdominal 

 organs. In the manner of subdivision, the trachese of Smynthurus differ from those of 

 the true Insects, and agree more closely with the Myriapoda and tracheal Arachnida, in 

 the fact that they do not often give off branches nor form tufts, but generally divide 

 dichotomously, and run considerable distances without a separation. 



I have noticed no respiratory movements ; and the supply of oxygen is probably due 

 therefore principally to that diffusive power of gases, the laws of which have been so well 

 worked out by Dr. Graham, and even applied to the respiration of insects. " In the law 

 of diffusion of gases," he says, " we have therefore a singular provision for the full and 

 permanent inflation of the ultimate air-cells of the lungs. But it is in the respiration 

 of insects that the operation of the law will be most distinctly perceived. The minute 

 air-tubes accompanying the blood-vessels to every organ, and, like them, ramifying tUl 

 they cease to be visible under the most powerful microscope, are kept distended during 

 the most lively movements of the little animals, and the necessary gaseous circulation 

 maintained, wholly, we may presume, by the agency of diffusion." Though we must 

 attribute some influence to the respiratory movements exhibited by so many insects, the 

 above explanation seems to me to throw much light on the question, which I have already 

 treated at greater length in the ' Linnean Transactions ' for 1860. 



I should not have thought it necessary to allude again to the subject, but that Prof. 

 Rathke, in a posthumous memoir " On the Respiratory Process in Insects" (See Ann. 

 and Mag. of Nat. Hist. 3 ser. vol. ix. p. 105), appears to have overlooked these facts, and 

 thereby to have fallen into some errors. Thus he says, " Prom the absence of all such phe- 

 nomena, we might conclude that in the pupse of the above-mentioned insects (Coleoptera 

 and Hymenoptera) the tracheary respiration is entirely interrupted." And further on, 

 " In any case, it is certain that the respiration of pupse can only be very weak." It has, 

 I think, been sufficiently shown that the mere absence of respiratory movements does not 

 necessarily involve such a conclusion. 



While, however, in Smynthurus Buskii the mere presence of tracheae is easily detected, 

 difficult as it may be to ascertain their distribution, I have, to my great astonishment, been 

 unable to detect a trace of them in the genus Papirius. Remembering that though the 

 great Treviranus was unable to convince himself of the existence of tracheae in Lepisma, 

 they have since been discovered by Burmeister, and being only too well aware of the 

 difficulties attending the dissection of these minute animals, I long attributed the ap- 

 parent absence of tracheae to my own unskilfulness ; but this explanation is not, I think, 

 tenable ; and even if rudimentary tracheae l)e hereafter discovered, I feel at least convinced 

 that their arrangement and distribution will be found to differ altogether from those 

 which characterize Smynthurus. It must be remembered that the air in the tracheae of 



