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 e , 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 
' Anat. der Ins. 59. b Ibid. 60. Malpigh. 20. 
« Lyonet Anat. 111. d N. Diet, a" Hist. Nat. xv. 483. 
' Anat. Comp. v. 198. f Ramdohr, 60. t. xvii./. l.f,g, h, r. 
* Vol. I. p. 403 -. Treviran. Arachnid. 42. 
VOL. IV. K 
