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 tracheae 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 
tubes accompanying the blood-vessels to 
every 
and, like them, ramifying till 
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 Prot. 
It 
Kathke, 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 pupas 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 pupae can only be very weak 
I think, been sufliciently 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 Lepisnm 
they have since been discovered by Burmeister, and being only too well aware of t^ 
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 be hereafter discovered, I feel at least convinced 
that their arrangement and distribution will be found to differ altogether from those 
which characterize Smynth„rus. It must be remembered that the air in the trachea? of 
