78 EESPIEATORY OEGANS. 



(see Pagenstecher, ' Beitrage zur Anatomie der 

 Milben '), tliat, tliough in most mites the traclieal 

 orifices are situated at tlie base of the legs, yet in 

 Tromhidmm holosericeum the spiracles are two in 

 number, and, as in Smynthurits, are situated at the 

 lower side of the head, though not exactly in the same 

 place, since in this species they open on the inner side 

 of the basis of the mandibles. 



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 sub- 

 division the tracheae oi Smynthurus differ from those of 

 the true insects, and agree more closely with the 

 Myriapoda and Arachnida, in the fact that they do not 

 often give off brarches or 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 infla- 

 tion 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 until they cease to be visible 

 under the most powerful microscope, are kept dis- 

 tended during the most lively movements of the little 

 animals, and the necessary gaseous circulation main- 

 tained wholly, we may presume, by the agency of 

 diffusion." Though we must attribute some influence 



