196 W. H. FURLONGE ON THE PULEX IRRITANS. 



apertures slightly depressed, and furnished with projecting covers ; 

 but whether the setae are rooted in the epidermis beneath, as I sus- 

 pect, or spring from the plate itself, I have been unable to deter- 

 mine. 



The mezo-thorax, or second segment, is likewise composed of 

 three plates, the superior embracing the back, and descending on 

 either side to about the medial line of the animal, where it terminates 

 in crescent shaped extremities, into which are fitted the two convex 

 lateral plates, which envelope and constitute the pyramidal 

 extremities of the mezo-thorax, to which the second pair of legs is 

 articulated at the posterior angles of their square terminations. I 

 am not quite sure that a fourth plate, uniting the lateral plates 

 beneath, does not exist. I have sometimes thought I could per- 

 ceive, that the chitinous segment formed a complete ring or collar 

 round the animal, but of this I am not certain at present. The 

 anterior half of the upper plate of the mezo-thorax, is attached to 

 the underlying skin, in a manner precisely similar to that already 

 described, and from the line of attachment proceeds a like row of 

 setaa. It may be noticed, that the chitinous plates composing this 

 segment of the thorax, are considerably stronger than those of the 

 pro-thorax, the additional strength being given by a marginal band 

 of brown chitin, of equal width, surrounding each of the three 

 plates like a frame. I may, in this place, advert to two extremely 

 curious organs, so far as I am aware, not heretofore described, 

 which are situated at the posterior portion of the soft part of the 

 epimeron. These organs are round, dome, or nipple shaped pro- 

 minences, which are capable of protrusion and retraction, and, in 

 point of fact, are, in the living animal, continually in movement, 

 sometimes being projected until the extremities assume an almost 

 pointed conical form, and at other times retracted until the extremi- 

 ties of the cone become truncated and nearly flat. In the centre 

 of each prominence is a cup-like perforation, which, it may be seen, 

 is connected with a large tracheal tube beneath, of which it appears 

 to form the external orifice. These curious organs are, without 

 doubt, spiracles, apparently of a very peculiar type, and regarding 

 them I shall have more to say in a subsequent part of this paper. 



The meta- thorax, or third segment, is much the largest and 

 strongest of the three, having to carry the third or principal pair 

 of legs, by means of which, the animal exerts the wonderful mus- 

 cular power, required for the accomplishment of its extraordinary 



