138 ANATOMY AND PHYSIOLOGY 



is placed in the head, the posterior in the thorax 

 The excretory duct is of such tenuity, that, if 

 exists, M. Leon Dufour was unable to detect it. The 

 prevailing number of these organs is two, one on 

 each side ; frequently there are four, and Burmeister 

 asserts that no fewer than six are to be found in the 

 genus Nepa, three on each side, all of them opening 

 into the cavity of the mouth. 



The peculiar fluid termed bile appears so indis- 

 pensable to digestion, both in the higher and lower 

 animals, that, in most cases, we find the organs des- 

 tined to secrete it very conspicuous. In insects, 

 what must essentially be regarded as such are always 

 present, as far as yet known, except in the genera 

 Chermes and Aphis. They assume the shape of fili- 

 form tubes, usually very long and flexuose, mean- 

 dering over the surface of the chylific ventricle and 

 the intestines. The point of the alimentary canal 

 where they are generally inserted is just behind the 

 pylorus ; in a few instances they open into the chylific 

 ventricle. In the whole of the order Hemiptera, the 

 point of insertion is the place where the slender intes- 

 tine meets the coecum ; an arrangement of which few 

 examples are to be met with elsewhere. Sometimes 

 they are inserted only by one extremity, the other being 

 free, at other times they are fixed by both ends. In 

 regard to number they vary extremely ; two is the pre- 

 vailing number among the Coleoptera, Hemiptera, and 

 Diptera, three occur among some coleopterous tribes, 

 such as the Cerambycidse, four in many Diptera and 

 a few Neuroptera, six in the Lepidoptera, eight in 



