260 INSECTA. 



barbed piercer is denuded and plunged into the skin, where it" is 

 retained until a sufficient supply of nourishment has been obtained. 

 (301.) Inviting as the subject is, we are compelled by the strictly 

 general character of our investigations to abstain from entering upon 

 further details concerning the mouths of perfect insects, and conse- 

 quently to omit noticing innumerable secondary modifications in 

 the mechanical structure of the oral organs of these little animals. 

 When we turn our attention to the consideration of their internal 

 viscera, connected with the preparation and digestion of so many 

 different materials, we may well expect to find equal variety of 

 conformation ; and, in fact, the course, dimensions, and relative 

 proportions of the alimentary canal will be seen to be different to 

 a greater or less extent in almost every species. Considered as a 

 whole, the internal digestive apparatus of insects must be regarded 

 as a delicate membranous tube, in which the digestion of the sub- 

 stances used as food is accomplished, partly by mechanical and 

 partly by chemical agents : for the former purpose, gizzard-like 

 muscular cavities are not unfrequently provided ; and, to fulfil the 

 second, various fluids are poured into the canal in different parts of 

 its course : the arrangement of the cavities, and the nature of the 

 secreting vessels, however, will be modified in conformity with the 

 necessities of the case, and certain parts will be found to exist, or 

 to be deficient, as circumstances may require : it would be absurd, 

 therefore, to attempt to describe particular examples ; our observa- 

 tions must be of general application, and such as will enable the 

 reader to assign its proper functions to any organ which may pre- 

 sent itself to his notice. The first part of the digestive appara- 

 tus is disposed in the same manner in all insects, and is a slender 

 canal, arising from the mouth and passing straight through the tho- 

 rax into the cavity of the abdomen ; this portion represents the 

 oesophagus (fig* 116, a, a; 117, o). t The stomach and intestine 

 succeed to this, and, if the body of the insect be very thin, their 

 course also passes nearly in a direct line to the tail ; but in those 

 families which have the abdomen thick and largely developed, espe- 

 cially if herbivorous, the intestine becomes much elongated, and 

 winds upon itself in various convolutions : nevertheless, however 

 tortuous the canal may be, its windings are never sustained by any 

 mesentery or peritoneal investment ; the air-tubes, that, as we shall 

 afterwards see, permeate the body in all directions, form a sufficient 

 bond of connection, and one which is better adapted to the wants 

 of these animals. 



