STATES OF INSECTS. (Larva.) 221 
The most usual colour of silken cocoons is white, yel- 
low, or brown, or the intermediate shades. The whites 
are very pure in the general envelope of some species of 
Ichneumonidae, and yellows often very brilliant. But be- 
sides these more general colours, some cocoons are black*, 
some few blue or green, and others red®. Sometimes the 
same cocoon is of two different colours. ‘Those of certain 
parasites of the tribe of Chalcidites, &c. Latr., the mo- 
tions of one of which I noticed on a former occasion ®, 
are alternately banded with black or brown and white, or 
have only a pale or white belt in the middle, which gives 
them a singular appearance. In both cases the differ- 
ence in colour depends upon the different tints with which 
the silky gum is imbued in the reservoirs: the first por- 
tion of it is white, and with this the larva first sketches 
the outline of its cocoon, and then thickens the layers of 
silk considerably in those parts where the white bands 
appear: when these are finished, its stock of white silk 
is exhausted, and the remainder of the interior of the 
cocoon is composed of brown silk ¢. The circular oper- 
culum above mentioned as covering an acorn-shaped 
cocoon, is paler than the latter, and also ornamented by 
a zone within the margin of deep brown. The pale co- 
coon also of Attacus Paphia is veined with silk of a deep 
red. 
I have very little to say on the substance of the silk of 
cocoons. Though that of the silk-worm is composed of 
such a slender thread, that of many others is still finer, 
scarcely yielding in tenuity to the spider’s web. On the 
* T have a black one from Mr. Francillon’s cabinet. 
> N. Dict. d’ Hist. Nat. vi. 294. 
© See above, Vot. II. p. 296— 4 Reaum, ii, 436. 
