70 ANIMAL PARASITES. 



frequently beset with acute horny teeth, with the lobes (malce] 

 on the inside, and maxillary palpi (palpi maxillares) on the out- 

 side ; the latter are rarely wanting, usually shorter than the 

 antennae, longer than the labial palpi, and in the number of 

 joints strictly fixed in every order of insects. 



The maxillae, which are usually clothed with hair internally, 

 grasp the food firmly. In the sucking insects they are con- 

 verted into piercing bristles, or into sheathing valves, or into 

 spirally rolled semi-canals, which form a tube by their contact. 



The labium with its palpi usually sits upon the chin (mentwri), 

 that is to say, a separate plate on which there is a valvular arti- 

 culation ; it is indented or notched in the middle, or even 

 divided into two parts, hairy, rarely smooth ; it closes the oral 

 aperture, and contains a soft membranous process, the tongue, 

 sometimes with independent processes, the paraglossa. The 

 labium and tongue sometimes form an open proboscidiform 

 sheath, sometimes a beak or rostrum, sometimes a sucking pro- 

 boscis, and sometimes a scooping proboscis. The retrogression 

 of one of these parts in comparison to the others produces a 

 great variety in their structure. 



2. The chest = thorax, which is usually the largest of the 

 three divisions of the body, bears three segments the prothorax 

 or fore-chest, the mesothorax or middle-chest, and the metatho- 

 rax or hinder-chest which are more or less mutually coalescent. 

 Each of these segments consists of a sternum (or breast-bone), 

 that is, the lower surface of the chest, and a dorsal surface. If 

 the prothorax be much developed it is called the corselet (thorax 

 = Halsschild) ; it never bears wings, but only the first pair of 

 legs. The mesothorax is most developed where the anterior 

 wings are the most important organs of flight, in other cases it 

 is even reduced to a scutellum (or little shield) upon the upper 

 surface. The metathorax is most developed where the posterior 

 legs are adapted for leaping, or the posterior wings are the most 

 essential. The wings are deficient in most of the human para- 

 sites, for which reason I pass over their structure. 



The legs consist of 1, a hip or coxa, which, with a cylindrical 

 or elongated head and femoral appendage (=^= trochanter), moves 

 in the manner of our humerus, by an imperfect ball and socket 

 joint, or a sort of rolling,in the socket of the segment; 2, of the thigh 

 ( = femur}, articulated by an imperfect ball and socket joint into 

 the coxa, and is cylindrical, spined, and very thick in the hind legs 



