THE INSECTA. 343 



limbs exclusively to the latter. The head possesses four pairs 

 of appendages, that is to say, one pair of antennas and three 

 pairs of gnathites ; and, as a general rule, there is a pair of 

 compound eyes, sessile upon the sides of the head ; sometimes 

 simple eyes are added to them. The first pair of gnathites 

 are the mandibles^ which are always devoid of a palp. The 

 second pair are the 7naxillce, which, in those insects in which 

 the mouth is least modified, are distinct from one another and 

 laterally movable ; while the third pair of gnathites are 

 united together in the median line, and constitute the lahiiun 

 of entomologists. In front of the oral aperture is a median 

 plate, the Icibrum, / while from the floor of the mouth formed 

 by the labium another median process, the lingua, is usually 

 developed. 



It is hardly open to doubt that the mandibles, the maxillre, 

 and the labium, answer to the mandibles and the two pairs of 

 maxillge of the crustacean mouth. In this case, one pair of 

 antennary organs found in the latter is wanting in insects, as 

 in other air-breathing Arthropods, and the existence of the 

 corresponding somite cannot be proved. But if it be sup- 

 posed to be present, though without any appendage, and if 

 the eyes be taken to represent the appendages of another 

 somite, the insect-head will contain six somites, the prjeoral 

 sterna being bent up toward the tergal aspect, as in the higher 

 Crustacea. 



The three somites which succeed the head are termed re- 

 spectively prothorax^ mesothorax^ and metathorax. A pair 

 of legs is normally attached to each ; and, when wings exist, 

 they are lateral expansions of the tergal region (correspond- 

 ing with the pleura of Cricstacea) of the mesothorax or the 

 metathorax, or of both. 



In the abdomen there are, at most, eleven somites, none 

 of which, in the adult, bear ambulatory limbs. Thus, assum- 

 ing the existence of six somites in the head, the normal num- 

 ber of somites in the body of insects will be twenty, as in the 

 higher Crustacea Arachnida.^ 



One of the commonest of insects, the Cockroach {JBlatta 

 (Periplaneta^ orientaUs) is fortunately one of the oldest, least 



1 It is open to question whether the podical plates represent a somite ; and 

 therefore it must be reoollected that the total numher of somites, the existence 

 of which can be actually demonstrated in insects, is only seventeen, viz., four 

 for the head, three for the thorax, and ten for the abdomen. 



