496 THE ENTOMOLOGIST. 



molecule of human blood which has an average diameter of 

 3 jijjT inch. The distal orifice of the large oral tube of the 

 insect has a diameter only -r-gVir inch, while that of the 

 supposititious sucker, as described and figured by Hooke and 

 Swammerdara, certainly does not exceed one-fourth or one- 

 fifth of that of the oral orifice ; so that, estimating it at 

 one-fourth the size of the oral orifice, its diameter would not 

 exceed -5-5^^ inch, very little exceeding half the diameter of 

 a blood disk, 



Denny, in his excellent * Monographia Anoplurorura 

 Britanniae,' represents P. Vestimenti in pi. xxvi., and 

 exhibits, in figs, f^ g, and h, what he designates the 

 haustellum in a state of protrusion. I have not seen this 

 organ in the state he represents, but there is no doubt of its 

 existence, for at a very short distance within the two 

 spongeous masses, which occupy a considerable space in the 

 conical suctorial tube of P. Capitis, there is a third spongeous 

 mass, the long axis of which is at right angles to those of the 

 other two spongeous bodies. This third mass is similar in its 

 apparent structure, but rather smaller than either of the 

 others. From the middle of its posterior side there is a short, 

 stout ligament or pedicle projected backward for rather more 

 than the greatest length of the spongoid body, and terminating 

 in an irregular basal mass, from which two ligaments diverge, 

 passing towards the right and left sides of the head, each 

 terminating in a spindle-shaped muscular fasciculus. I could 

 not detect any prehensile teeth or hook on the spongeous 

 mass of this organ with a power of 700 linear, but this may 

 probably be accounted for by my having examined these 

 organs in their natural condition and in a state of repose, my 

 specimen having never undergone desiccation, while that of 

 Mr. Denny's may have been sketched from a dried specimen. 

 This third spongeous mass in the rear of the two formerly 

 described, and in conjoined action with them, would form an 

 exceedingly eflicacious compound piston-valve in such a 

 suctorial tube as that of P. Capitis. These organs are faith- 

 fully represented in the figure of the head of the insect by the 

 artist, but they are so minute that a lens of two or three 

 inches focus may be applied with advantage to the figure. 

 The third species of parasite infesting the human body is 

 Phthirius inguinalis (the crab-louse). Specimens in my pos- 



