Dr. T. Williams on the Tracheal System of Insects. Si? 



themuin. Although very short, the suspensor, in these plants, 

 bends in a remarkable manner in the middle, and is most fre- 

 quently attached to the embryo-sae, beside the point touched 

 externally by the twisted extremity of the pollen-tube. 



XXXI. — On the Tracheal System of Insects. By Tnos. Williams, 

 M.D. Lond., F.L.S., Physician to the Swansea Infirmary. 



To the Editors of the Annals of Natural History. 



GENTLEME>f, 



May I request that you will allow me the favour to announce 

 in your Journal the results of a series of minute dissections 

 which I have recently performed on the Tracheal System of 

 Insects and Myriapods. In consequence of a letter on this 

 subject from the late Mr. Newport, published in the ' Annals ' 

 of last year, calling in question the accuracy of my statements, 

 I have ever since felt anxious to rc])eat the observations upon 

 whicli those statements were made. That I have now done, and 

 with the utmost care. I find, that not only are the results then 

 stated true in every particular, but that they fall far short of 

 indicating the real distinction between the " membranous capil- 

 lary trachese," and those larger trunks in which the "spiral" 

 is visible. ]\Iy recent st\idies enable me now to state — 



1. That the "spiralled" or larger trachese are mere conduits, 

 like arteries or veins, and have nothing to do with, take no part 

 in, the ultimate act of respiration. 



2. That this function (that is, the interchange of the gases 

 concerned in the respiratory act) has its seat exclusively in the 

 capillary membranous trachese. 



3. That the peripheric or exti-eme distribution of the tracheal 

 system is conformable in plan to that of a blood-vascular system ; 

 that is, the capillary or membranous tracheae are always j)laeed 

 intermediately between larger trunks, the branches of which they 

 serve to connect, — standing to the larger trunks in the same 

 relation as the capillaries of a blood-vascular system do to arteries 

 and veins. 



4. That the trachefe can be discovered, in no single instance, 

 to end in csecal terminations, — always in mutual inosculations. 



5. That this anastomotic arrangement establishes a close 

 similarity between the tracheal system of Insects, Myriapods, 

 and the blood-vascular system of the Annulosa, — a homology 

 first theoretically suggested by Mr. Huxley. 



6. That the tracheal system, however, is distinguished from 

 the blood system in two striking anatomical particulars : in the 



