W. H. FURLONGE ON THE PULEX IRRITAXS. 19 



rounding the pygidium, and opening on either side just in front of 

 that organ. These large spiracles, in fact, appear to be the principal 

 orifices by which the main tracheae are supplied with air. The 

 lower main tracheae communicate with the external air by means of 

 short branches, which, rising vertically, cross the upper main 

 trachea, and terminate just beneath the cup or funnel-shaped ex- 

 tremities of the round spiracles, described in my former paper as 

 situated along the line of the abdomen. 



It is very remarkable that there appears to be a complete break, or 

 solution of continuity, between the extremities of all those spiracular 

 orifices, and the tracheae which they supply, with the exception of one 

 or two turns of the wire-like spiral fibre supporting the parietes of 

 the tracheal tubes, and which may be seen to pass from the spiracle 

 to the tracheal tube. I have endeavoured, by the use of very high 

 powers and careful illumination, to comprehend this curious peculiarity 

 of structure, and I have satisfied myself that there exists a very thin 

 transparent membranous envelope, surrounding the trachea, and 

 that at the points of junction with the extremities of the spiracles 

 this membrane is expanded into a sort of bag or corrugated sac, as 

 shown in the drawing, the obvious purpose of this method of union 

 being to permit the flexure of the animal's body in every direction, 

 without danger of rupturing the delicate tubes connected with the 

 external spiracles. 



The lower main tracheal vessel (B) presents the ajDpearance of a 

 series of loops* hanging down between the tubes which communi- 

 cate with the abdominal spiracles, and from the lower or convex 

 sides of these loops a series of large tracheal vessels descend per- 

 pendicularly, one of these vessels running downwards on each side 

 of every segment of the abdomen. From these large vessels, as also 

 from all parts of the main trachea, proceeds a wonderfully complex 

 system of minor vessels, ramifying in every direction, and proceed- 

 ing to every portion of the animal's organism in a series of filaments 

 constantly decreasing in diameter, the ultimate fibrils being so 

 minute as to demand the highest powers of the microscope to resolve, 

 and yet the smallest of these vessels exhibits its characteristic spiral 

 structure as perfectly as the largest of the main tracheae. 



* It is proper to state that the so-called loops of the lower main tracheal vessel 

 are shown much flatter than they are in reaHty. This is due to the distortion 

 arising from the compression of the insect, which has the effect of elongating or 

 stretcJung out this system of vessels. In their natural position the loops are much 

 deeper and are closer together than is here depicted. 



