INTERNAL ANATOMY OF INSECTS. Ill 



and when numerous, they are generally attached singly 

 though irregularly *. These vessels at their base do 

 not open into the cavity of the intestinal canal, but merely 

 into the space between its outer and inner tunicks, the 

 last being constantly imperforated b . 



With regard to their apex, the bile-vessels are some- 

 timesjm2 singly or connectedly to the intestine merely 

 by a few muscular fibres ; for they do not enter it, their 

 ends having no orifice. This structure is mostly to be met 

 with in the Coleoptera c . In caterpillars, the tops of these 

 vessels perforate the outer skin of the rectum, and pro- 

 ceeding in dense convolutions to the anus, become at last 

 so fine that their terminations cannot be discovered d . 

 In other cases, the extremities of a pair of these vessels 

 unite so as to form a double one: this may be seen in those 

 of Philonthus politus e , and probably other rove-beetles : 

 and lastly, in others the bile- vessels are free, hanging 

 down by the intestinal canal, without being attached to it 

 or to each other. This structure is constantly found 

 in the Orthoptera and Hymenoptera Orders, &c. f . 



With regard to their number, the bile- vessels vary from 

 two to upwards of one hundred and fifty, yet so that 

 their whole amount is constantly the product of the num- 

 ber two, at least as far as they have been counted : and 

 even when those on one side are not alike, a similar va- 

 riation takes place in the other, as may be seen in Gal- 

 leruca Vitellince, where on each side are two long ones 

 and one shorter * ; the most usual numbers are, /owr 

 six or many, that is, more than twenty 



a Ramdohr,*. xiii./. 13. b Ibid. 44. 



c Ibid. 45. A Ibid. 45. PLATE XXI. FIG. 3././. 



e Rhamdohr, Ibid. t. iii./. 6. E. 



f Ibid. t. If. 1. 5. 9, /, xiv./. 13. Ibid. 46. t. vi./. 3. 



