216 LECTURE XVII. 



which are protected by a sheath formed by the equally elongated 

 labium, the upper groove of which at the same time serves to conduct 

 the liquid food into the mouth : the maxillary and labial palpi have 

 disappeared ; the latter may have coalesced with, and transferred 

 their properties to, the labial sheath. With such an instrument 

 the Cicada perforates the bark of the trees on which it lives, and 

 exhausts their sap ; and with a similar modification of the trophi, the 

 bug and flea pierce the skin and suck the juices of animals. 



In the blood-thirsty Diptera, as the gnat and forest fly, the labrum, 

 as well as the two latei'al pairs of jaws, are prolonged into lancet-shaped 

 organs, and are sheathed in a thickened lower lip, which is terminated 

 by two fleshy suckers : the maxillary palpi are attached to the base 

 of the maxillae. 



The singular spiral '' antlia " of the butterfly and other Lepido- 

 ptera is formed by the elongated slender maxillae, still characterised 

 by the minute palpi at their base. The inner margins of the max- 

 illae are concave, and the edges of the channels are in close contact, 

 or are confluent, so as to form a canal along which the juices of 

 flowers can be pumped ujd into the mouth. Each maxilla is likewise 

 hollow, and it is uncoiled or coiled by the varying tension of this 

 canal. The labial palpi are of large size, and defend the antlia when 

 it is retracted and coiled up. The labrum is a small triangular 

 piece, which bends down towards the mouth, and the rudimental, 

 conical, slightly bent mandibles are hidden by the labial palpi. 



The large curved piercing jaws of the Centipede are hollow, and 

 traversed by the duct of a poison-gland. 



The alimentary canal is most simple in the larvae of insects, in 

 which, as in worms, it usually extends, without convolutions from 

 one end of the body to the other; in a few larvae, as that of the bee, 

 it has only the anterior opening or mouth, and the opposite or anal 

 orifice is not developed until the pupa-state. In all mature insects 

 the alimentary tract presents the two distinct apertures; it is simplest 

 in the carnivorous larviform Myriapods ; presents more numerous 

 and distinct constrictions and divisions in the Hexapods, and increases 

 in complexity and length, as the food requires most pi'eparation in 

 order to its conversion into the animal nutrient fluid or chyle. 



The oesophagus of the Centipede is long, and dilated posteriorly, 

 where it communicates with the stomach ; this is a small muscular 

 cavity, bent upon itself, and lined by a longitudinally plicated horny 

 membrane. The intestine is long, straight, and wide, slightly saccu- 

 lated transversely ; it contracts, and is longitudinally folded near its 

 termination. In the lulus a short and wide cesophagus expands into 

 a shorter and wider muscular stomach. This is succeeded by a long 



