20 W. H. FURLONGE ON THE PULEX IRRITANS. 



At the second abdominal spiracle, tlie two main tracheal vessels 

 unite anteriorly and join the large trachea, which descends perpen- 

 dicularly from the first spiracle, and thence, from a little above the 

 point of junction, a large vessel — of course on each side of the 

 animal — proceeds into the thorax and head, supplying the smaller 

 vessels, which ramify from thqm to the various organs contained in 

 these portions of the body of the insect. 



I have here to refer to the pair of erectile spiracles mentioned 

 in my former paper as situated in the epimeron of the mezo-thorax. 

 It may be remembered that I described these as round dome or 

 nipple-shaped prominences, capable of protrusion and retraction, 

 and which, in fact, in the living animal are in a state of continual 

 movement. Tracheal vessels may be distinctly traced as being in 

 connection with and supplied by these erectile spiracles, and since 

 the reading of my previous paper I have satisfied myself that the 

 protrusion of these organs is not, as I believe was suggested, due 

 to the compression of the insect while under examination, but 

 that they normally present the characteristics I have described. I 

 may add that though I have not been able to make them out, I 

 confidently anticipate that similar pairs of spiracles exist in each 

 segment of the thorax, and I would invite the assistance of some 

 of our working members in searching for them. My reasons for 

 this conjecture will be stated presently. 



I also desire to refer to the remarkable sacs which I described 

 as existing in the upper tarsal joints of the third pair of legs, and 

 which I at that time considered to be contractile sacs. It may be 

 remembered that Mr. Lowne demurred to this view of their struc- 

 ture, and gave it as his opinion that these so-called sacs are really 

 expansions of the tracheal vessels supplying the limbs. I think it 

 right to say that I have since ascertained that Mr. Lowne's view 

 is correct, and I now exhibit one of these tracheal enlargements 

 carefully laid down by the aid of the neutral tint reflector. (Fig. 

 4.) I was led into the error of considering these vessels muscular 

 sacs, from the fact of their rhythmical contractions, wdiich I sup- 

 posed to be accomplished by muscular bands, for the striations of 

 which I mistook the spiral fibrous structure of the tracheal vessel 

 itself. Eut while I admit the error into wdiich I was betrayed, as 

 to the structure of these so-called organs, I must adhere to the 

 views I expressed as to their use and office, viz., that by the rhyth- 

 mical compression of these tracheal enlargements — however that 



