216 STATES OF INSECTS. (Larva.) 
in the silk manufacture. It is the continuous thread of 
the inner cocoon which is most valuable; the outer loose 
coating from its irregularity cannot be wound off, and is 
known in commerce by the name of floss silk. 
Manceuvres in their general principle similar to those 
of the silk-worm are followed by most of those larvze 
which inclose themselves in silken cocoons. Many spe- 
cies, however, adopt variations in the mode of procedure 
all of which it would be tedious to particularize, but 
some of them are worth mentioning. The larvae of Tor- 
trix prasinana, and other species of moths which form 
cocoons resembling a reversed boat, arrange their 
threads in layers, so as to construct two parallel walls 
gradually inclining towards the top and ends, where 
they finally force them to approach each other by means 
of an apparatus of silken cables*. And the larva of Sa- 
turnia Spint, though it forms the base of its flask-shaped 
cocoon by spinning like the silk-worm a number of in- 
terwoven zigzags, places the threads which compose the 
interior funnel-like opening of the apex nearly straight, 
parallel to each other, and converging towards the same 
point in the centre”. 
These last, as well as almost all larvee, constantly re- 
main in the zuside of the cocoon during its construction. 
But De Geer has given us the history of a minute cater- 
pillar of a species of moth (Zinea, L.) which feeds on 
the under side of the leaves of the Rhamnus Frangula, 
or Black Alder, that actually weaves half of its cocoon 
on the outside. This cocoon, which is very small, is 
beautifully fluted, consisting of several longitudinal 
cords, with the intervals filled by fine net-work, and 
* Reaum. i- 555—. b Prate XVII. Fic. 5, 6. 
