and by which gold and filver is drawn to any degree of minute-
nefs. Every thread proceeds from thefe two refervoirs, but are
united afterwards ; fo that if it is examined by the microfcope, it
will be found to confift of two cylinders, or threads, glued together,
with a groove in the middle ; a feparation may fometimes.
be perceived.
When the filk-worm has found a convenient fituation, it'fetsto
work, firft fpinning fome loofe thread, which ferves to fupport
the future fuperftruclure; upon thefe it forms an oval of a loofe
texture, confiding of what is called the flofs filk ; within this it
forms a firm and more confiftent ball of filk, remaining,
during the whole bufinefs within the circumference of the fphe-
roid that it is forming, reliing on it’s hinder parts, and with it s.
mouth and fore legs direfting and fattening the threads. Thefe '
threads are npt directed in a regular circular form, but are fpun
in different fpots, in an infinite number of zig-zag lines; fo that
when it is wound off, it proceeds in a very irregular manner,
fometimes from one fide of the cone, then from the other. This
thread, when meafured, has been found to be about three hum
dred yards long, and fo fine, that eight or ten are generally rolled
off into one by the manufadurers. The filk-worm ufually employs
about three days in finifhing this cone; the infide is
generally fmeared with a kind of gum, that is defigned to keep
out the rain; in this cone it affumes the pupa form, and remains,
therein from fifteen to thirty days, according to the warmth of
the climate. When the moth is formed, it moiftens the end of
this cone, and by frequent motions of the head loofens the texture
of the filk, fo as to form a hole without breaking it.
When
M ic r o s c o p ic a l E s s a y s . 213
When the filk-worm has attained it’s perfeft growth, the refervoirs
of filk are full, and it is preflfed by it’s fenfations to get rid
of this incumbrance, and accordingly fpins a cone, the altitude
and fize of which are proportioned to it’s wants : by traverfing
backwards and forwards, it is relieved, and thus attains, by an
innate defire, the end for which it was formed.
The fize of the cone is not always proportioned to the fize of
die caterpillar; fome that are finall conftruft larger cones than
other caterpillars which are greater in fize.
There is a caterpillar which forms it’s filken cone in the fhape
of a boat turned bottom upwards,, whence it is galled by M. de
Reaumur the. “ coque en batteauthe conftruftion is complicated,
andfeems to require more art than is ufually attributed to
this infea. It confifts of two principal parts, fhaped like Ihel-ls,
which are put together with confiderable fkill apd propriety;
each fhell, or fide, is framed by itfelf, and formed of an innumerable
quantity of minute filk rings ; in the fore part there is a
projeftion, in which a finall crevice may be perceived which
ferves, when opened, for the efcape of the moth; the fides are
conneaed with fo much art, that they open and Ihutas if frained
with fprings ; fo that the cone, from which the butterfly has
efc-aped, appears as clofe as that which is ftill inhabited.
Thofe which are not fumiflied with a frlky cone, fupply their
want with various materials, which they are fufficiently Ikilful to
form into a proper habitation, tofecure them while preparing foi
the perfea ftate; fome conftrua their’s with leaves and branches,