16 FUNCTION OF DIGESTION. 



the passage of the canal, which it closes to prevent the backward movement of the digested 

 mass." The segment recognized as theosecum is short, and in direct communication with 



the anterior tube ; but in some cases it seems as it" joined laterally to it, and then it has the 

 appearance of a pouch. Fluids are supposed to be secreted by warty excrescences situated 

 upon the mucous lace of this intestine, designed to aid in the digestive process. 



The rectum is that part of the large intestines which terminates the alimentary canal 

 below : it is always short, and is furnished with a sphincter at its extremity. It is mus- 

 cular, has thicker walls than the ca-cum, and gives shape and form to the excrements. 



The. function of digestion requires, in addition to the formal tube through which the 

 parent matter must pass, a supply of certain fluids which shall possess a solvent power 

 upon the contained mass ; and more than one kind of fluid seems to be neccs-an . Though 

 these different fluids agree in use ami etl'eet with analogous ones found in the higher ani- 

 mals, the organs which secrete them have a very different form : indeed, the difference is 

 so great, that a student whose views of the function of digestion have been obtained solely 

 from the observation of the organs of a vertebrate animal would not at fust recognize the 

 parts which perform similar functions in insects. 



The secretory organs, and those which are subservient to digestion, are always tubular 

 in insects : the gland or parenchyma present in quadrupeds being entirely absent in in- 

 sects, these organs are reduced to their simplest form. The kinds of tubes taking their 

 names according to the nature of the fluid secreted, arc the salivary, biliary, and urinary. 

 The salivary apparatus is quite complicated : it consists of tubes, single and double, and 

 even triple, situated around the esophagus and mouth, and provided with bags that serve 

 as reservoirs. The tubes open into the mouth or esophagus by two or more excreti u-y orifices. 

 The salivary apparatus is most perfect in the haustellata or sucking insects ; the tubes 

 ascending into the abovenamed cavities in a meandering course, whereby the extent of 

 secreting surface is much increased. 



The operation of salivation is an important preliminary to digestion : mixed with the 

 food, the saliva serves to soften its consistence, while it also carries along with it much air 

 which appears to be necessary for some purpose in the economy. The bile is a still more 

 important fluid than the saliva : it is secreted in long capillary tubes, more or less flexuose 

 and folded upon themselves, which connect with the alimentary tube below the stomach, 

 or just behind the pylorus ; though they sometimes continue farther down, and enter near 

 the caecum. These tubes vary in number : there are usually two, especially in coleoptera, 

 hemiptera and diptera ; but in these orders they are not absolutely uniform, as in the 

 ccrambycidse there are three, and four in some dipte7-a, while in neuroptera there arc six, 

 and in certain neuropterous groups as many as eight. Where the number of the tabes 

 varies, however, it operates as a compensation for their length : for example, if the simple 

 tubes are each five or six times the length of the body, their number is less ; but when 



