190 W. H. FURLONGE ON THE PULEX IRRITANS. 



movements of the animal are in no way impeded, and perfect 

 flexure of the joints is permitted in every direction. This armour 

 is composed of exceedingly tough, thin, plates of chitin, of a pale 

 yellow colour, which assumes a deeper tint with age, and are ex- 

 quisitely marked with irregular striations. These plates are very 

 transparent,— in young specimens almost perfectly so, — highly 

 polished and lustrous. The shape and relative size of the male 

 and female flea are somewhat different, the former being always 

 smaller and shorter than the latter. 



The external structure of the flea, as of the insecta generally, is 

 divisible into three parts — the head, the thorax ? and the abdomen, 

 with their respective appendages. It will be convenient to describe 

 these parts in their order. 



The Head. — The head of the flea is singularly small in proportion 

 to the size of the animal. It is encased in a helmet of polished 

 chitin, composed of two pieces — the anterior, or clypeus, and the 

 posterior, or epicranium, — which are united bj a nearly perpen- 

 dicular suture, of a light brown colour. The exterior surface of 

 the head piece is pitted with numerous minute depressions, from 

 each of which a very small spicule projects backwards. The head 

 is attached so closely to the thorax, that at first sight it almost 

 appears to form its anterior segment ; but it is, nevertheless, 

 capable of considerable movement in every direction. Several 

 small square flaps, or neck plates, are hinged to the posterior edge 

 of the head piece, which slide freely over the first segment of the 

 thorax. 



The Eyes. — Undoubtedly, the most striking of the organs per- 

 taining to the head of the flea are the eyes. When examined under 

 a \ inch or 4-10ths objective, by reflected light thrown upon it by 

 the side parabolic illuminator, the eye of the flea presents an object of 

 singular beauty. It is found to consist of a highly refractive, single, 

 crystalline, lens, of great brilliancy, the retina pigment of which is 

 intensely black. Its form is probably spherical, or nearly so, but 

 it is so set that only a small portion of the lens is visible. It is 

 situated at the anterior, convex, margin of a deep cavity, having a 

 nearly semi-circular outline, the curved and diametric edges of 

 which incline downwards, to form a somewhat pyramidal and rather 

 deep, excavation, in the side of the head. Nearly one half of this 

 cavity is covered by a thin, fixed, semi-transparent plate, formed by 

 the extension of the anterior portion of the headpiece, which, 



