138 
ANATOMY AND PHYSIOLOGY 
is placed in the head, the posterior in the thorax. 
The excretory duct is of such tenuity, that, if it 
exists, M. L6on 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 ccecum ; 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 Cerambycids e,four in many Diptera and 
a few Neuroptera, six in the Lepidoptera, eight in 
