1S6 INSECTS. 



flea. Other species are found upon different animals, 

 as upon the dog, the rat (PL X. fig. 22) , the fowl, the 

 pigeon, &c. 



The head and the dark eye (fig. 21, head of the 

 human flea) are very evident. The antennae or head- 

 feelers are very minute and difficult to find, being 

 sunk in a little pit or fossa behind the eye ; they may, 

 however, generally be recognized by the detection of 

 the last joint, which is pectinate or cut like a comb. 

 The body, including the head, consists of thirteen 

 joints, one for the head, three for the chest or thorax, 

 and nine for the belly or abdomen, it being under- 

 stood that by "joint" is meant a segment, and not the 

 line of junction of two segments. The indication of 

 these joints is afforded by the horny integument, 

 which consists of a corresponding number of rings, 

 forming in fact the skeleton of the animal. This in 

 the flea, as in all insects and other Articulata, is ex- 

 ternal or cutaneous (cutis, skin), and consists of the 

 hardened skin, the peculiar animal substance of which 

 it is composed being called chitine (^trcov, tunic). 

 These chitinous rings overlap, and are composed of a 

 dorsal or upper, and a ventral or lower half; and 

 near the middle of each is a row of hairs directed 

 backwards. Along the sides of the body of the in- 

 sect may be seen a row of dots ; these are the spiracles 

 or orifices of the breathing-tubes (tracheae) . 



The legs are many -jointed, long, furnished with 

 numerous spines, and terminated by two slightly 

 curved claws, each with a little blunt tooth at its 

 base. The claws are not perfectly smooth on the 

 inside, but are covered with slightly raised lines, like 

 a file, so that a better hold can be taken of bodies. 



But the most interesting parts of the flea are those 

 of the mouth, with which it punctures the skin and 

 sucks the blood. There are nine of these; and they are 

 best seen when the head of the flea is pulled off with 

 the mounted needles, and the parts spread out and 



