FUNCTION OF DIGESTION. 17 



they arc short, their number is jnoportionally increased. Althougli Ihcst hepatic tubes are 

 generally quite uniform in diameter, there are some deviations from the rule ; but it is 

 perhaps unnecessary to describe particularly the few modifications that are known to occur. 

 The function of these tubes, as has been intimated, is to secrete or prepare a fluid analogous 

 to bile. They may be recognized in the caterpillar, when the abdomen is o])ened, by their 

 position, and by their white vermiform ai)pearance : they float apparently loosely in the 

 cavity of the abdomen. 



Among the secretions mentioned is the luiii.iry, which seems to be far less constant than 

 those already tlescribeil. When the urinary organs are present, they terminate in the rec- 

 tum : they have l>eeu found in certain carnivorous coleoptera. The fluid which has been 

 called the ttrine in insects, is caustic and odorous : it is often discharged by the carabici, 

 on handling them, in jets, which, when falling upon the skin, occasion a transitory burning. 

 In connection with this secretion, may be noticed that peculiar to the bombardiers, which 

 is discharged in explosive jets, and is supposed to be intended as a means of defence. The 

 name of the genus possessing the power of producing explosive jets of fluid, is Brachinus. 

 I am not aware of the fact, if it has been ascertained, that this fluid is similar in composi- 

 tion to that of tlie higher animals, or that it contains lu-ea. 



In concluding our remarks on digestion in insects, we may observe that the function is 

 performed in ways quite as numerous as in the higher orders of animals, though there is 

 no departiue from the general principles which prevail in the vertebrate and molluscous 

 types. There is always an apparatus for trituration, or mechanical separation of the food 

 into fine parts : it is thereby preptired for the action of the several fluids which concur in 

 the digestive process, and which exercise .some peculiar chemical influence that serves to 

 sei)arate the nutrient matter from the useless portion of the food. These fluids are more 

 or less acrid in their nature : thus the saliva injected into the wound made by a flea or a 

 fly, for example, by the iabaiius, occasions inflammation and itching ; and this irritation 

 is designed to favor the insect, inasmuch as it produces a flow of blood to the wounded 

 part. An eflect of saliva is seen upon leaves attacked by the caterpillar, which very soon 

 suflfer a loss of color ; and as the morsel swallowed passes along from one receptacle to 

 another, it is constantly undergoing changes : it is softened in the crop ; in the gizzard, 

 it becomes pulpy ; and in the true stomach the chyle is lbrmed,and is at once recognized 

 by the globules it contains. 



A controversy has been long maintained respecting the oflice of those tubes which have 

 been called kcpatic or bilianj. Some eminent physiologists have regarded them as urinary, 

 inasmuch as luic acid is sometimes Ibund in them ; but as this is not always the case, a 

 compromise seems to have been made by regarding them as both biliary and urinary, and 

 giving them a corresponding denomination tnino-bilianj. The circumstance that the same 

 tubes which have since been tbund sometimes to contain urine were regarded as biliary 

 [ Agricultural Report — Vol. v.] 3 



