534 PHILOSOPHY OF ZOOLOGY. 



head, and termed mentum, corresponding, in position, with 

 the nasus of the labrurn. 



When all these parts bear such a proportion to one an- 

 other, as to be able to cut the substances used as food, and 

 convey them to the pharynx, they are considered as in the 

 most perfect condition. But all these parts in the different 

 classes, exhibit very remarkable modifications. In many 

 of the Crustacea, the labium has been converted into a 

 second pair of maxilla?, and the three pair of legs which, 

 in insects, constitute the organs of motion, are h ere chan- 

 ged into three pair of auxiliary maxillae, and close the mouth 

 below, in place of a labium. The locomotive organs are, 

 in reference to their position, abdominal legs. Modifications 

 of this arrangement prevail in the arachnida. There are 

 many species of insects which are destined to live chiefly or 

 exclusively on fluid substances, to whom such masticating or- 

 gans would be unsuitable. Those which hold a middle sta- 

 tion between the gnawers (inandibulata), and suckers, have 

 the labium transformed into a soft, fleshy plate, like the 

 tongue of quadrupeds (and hence frequently termed lin- 

 gua), capable of licking, and of being rolled inwards 

 when at rest, as in the Hymenoptera represented by 

 the bee. In the genuine suckers (Haustellata), the organs 

 of the mouth present two important modifications. In the 

 first, the labrum is nearly obliterated. The labium is pro- 

 duced and either crustaceous (rostrum), or membranaceous 

 (proboscis), with a groove on its upper side, for the recep- 

 tion of the four plates or hairs into which the two mandi- 

 bulae and maxillae have been converted. These appearan- 

 ces are exhibited in the Hymenoptera and Diptera. In 

 the second, both the labrum and mandibles are nearly ob- 

 literated, and the labium is short and fixed ; and the two 

 maxillae are produced and so applied to each other, as to 

 form a sucker. This suker is capable of being rolled up 



