INTERNAL ANATOMY OF INSECTS. 129 



According to Ramdohr a , they consist of two trans- 

 parent membranes, between* which is found a yellow or 

 transparent jelly. The greater the quantity of silk em- 

 ployed by the caterpillar in the construction of its co- 

 coon, &c., the longer are the silk-secretors. Those of 

 the silkworm are a, foot long b , while those of the larva 

 of the goat-moth are little more than three inches c . 



Other insects spin silk with the posterior extremity 

 of their body. In the great water-beetle (Hydrophilus 

 piceus) the anus is furnished with two spinnerets, with 

 which it spins its egg-pouch d ; these are in connexion, 

 probably, with the five long and large vessels containing 

 a green fluid, described by Cuvier % which surround the 

 base of each branch of the ovaries. The larva of Myr- 

 meleon, which also spins a cocoon with its anus, differs 

 remarkably in this respect from other insects, since its 

 reservoir for the matter of silk is the rectum ; this is con- 

 nected with a horny tube, which the animal can pro- 

 trude, and thus agglutinate the silk and grains of sand 

 that compose its cocoon f . 



The web of spiders is also a kind of silk remarkable 

 for its lightness and extreme tenuity. It is spun from 

 four anal spinnerets, which never vary in number ; two 

 longer organs peculiar to some species have been mis- 

 taken for additional ones, but Treviranus affirms that 

 they are merely a kind of anal feeler. Their structure, 

 as far as known, has been before described . The web 

 is secreted in vessels varying in form. In some (Clubiona 



a Anat. der Ins. 59. b Ibid. 60. Malpigh. 20. 



Lyonet Anat. 111. d N. Diet. d'Hist. Nat. xv. 483. 



e Anat. Comp. v. 198. f Ramdohr, 60. t. xvii./. l./,g, h, r. 



g VOL. I. p. 403. Treviran. Arachnid. 42. 

 VOL. IV. K 



