ASSISTANT CHYLOPOIETIC VISCEKA OF INSECTS. 319 



(837.) We may now notice the secerning organs that pour fluids into 

 different parts of the digestive apparatus, beginning with those which 

 open into the ossophagus in the vicinity of the mouth, and examining 

 them in the order of their occurrence as we proceed backwards. 



(838.) The first are the salivary vessels, which terminate in the 

 neighbourhood of the mouth itself, into which they seem to pour a se- 

 cretion analogous to saliva. These glands are principally met with in 

 suctorial insects, but not unfrequently among the mandibulate orders. 

 Their form varies ; but they are generally simple slender tubes, that 

 float loosely among the juices of the body, from which they separate 

 the salivary fluid. There are, for the most part, only two of these 

 organs (fig. 160, s s) ; but in fleas (Pulex) and bugs (Cimex) there are 

 four, and in a water-bug (Nepa) there are six such vessels, all of which 

 open into the cavity of the mouth. The fluid supplied by the salivary 

 glands is usually merely intended to facilitate deglutition ; but there 

 are cases in which the saliva is excessively acrid and irritating, acting 

 as a kind of poison when infused into a puncture made by the mouth : 

 this is especially remarkable in many bugs and gnats, and is the chief 

 cause of the pain and inflammation frequently occasioned by their bite. 



(839.) Besides the proper salivary vessels, there are other glands, or 

 rather caeca, which open into the stomach itself, occasionally covering 

 that organ over its entire surface, as is the case in some water-beetles 

 (Hydropliilus} : these, no doubt, secrete a fluid subservient to digestion ; 

 but whether of a peculiar description, or allied to saliva in its proper- 

 ties, is unknown. 



(840.) The third kind of auxiliary vessels connected with the in- 

 testinal canal of insects are supposed to furnish a secretion analogous 

 to the bile of other animals, and consequently to represent the liver. 

 These bile-vessels (fig. 159, hh; fig. 160, #(7) are generally four, six, 

 or eight in number, but occasionally much more numerous ; they are 

 usually of great length, but exceedingly slender, and wind around the 

 intestine in all directions. "When unravelled, they are found to termi- 

 nate in the neighbourhood of the pylorus (fig. 160, h, ri), close to the 

 commencement of the intestine, at which point the secretion produced 

 by them is mixed with the food after it has undergone the process of 

 digestion. 



(841.) Appended to the termination of the alimentary tube, close to 

 its anal extremity, other vessels are met with in some insects, that are 

 looked upon by authors as being allied in function to the kidneys of 

 higher animals ; but apparently this opinion rests upon very doubtful 

 grounds. They indubitably furnish some secretion, the use of which is 

 perhaps connected with defecation ; but that it is of the same character 

 as the fluid separated by the renal organs of Vertebrata may well be 

 called in question, as no such parts are distinctly recognizable until we 

 arrive at much more elevated forms of life than the insects we are now 



