228 | THE INSECT WORLD. 
It has been calculated, let us say by the way, that forty thousand 
cocoons would suffice to surround the earth at the equator with one 
thread of silk. Folded on itself almost like a horse-shoe, its back 
within, its legs without, the worm arranges its thread all round its 
body, describing ovals with its head. It approximates gradually 
the points of attachment of the thread. As long as the cocoon 
is not very thick one can watch it through the meshes of the 
web applying and fixing its thread, still to a certain degree 
soft, in such a manner as to make it adhere closely to the parts 
already formed. 
“We can state,” says M. Robinet, ‘that the silkworm makes 
every second a movement extending over about five millimetres. 
The length of the threads being known, it follows that the worm 
moves its head three hundred thousand times in making its cocoon. 
If it employs seventy-two hours at its work, it is a hundred 
thousand movements every twenty-four hours, four thousand one 
hundred and sixty-six an hour, and sixty-nine a minute, that 
is to say, a little more than one a second.” 
About the fourth day, after having expended all its silk,* the 
worm shut up in the cocoon becomes of a waxy white colour, 
and swollen in the middle of its body. The abdominal legs 
wither away; the six fore legs approach each other and bhe- 
come black. The parts of the mouth tend downwards; the 
skin wrinkles. Very soon it is detached and pushed down towards 
the hinder part, and the chrysalis appears under the rents in 
the skin. It is at first white, but speedily becomes of a brown 
red. 
The silkworm remains in general from fifteen to seventeen 
days in the pupa state. At the moment of hatching, the moth 
begins by breaking the skin in which it is shut up, and which 
is pretty thin. But how can it come out of the silky prison 
which it has itself built ? To effect this it makes use of a peculiar 
liquid contained in a little bladder with which its head is provided, 
and which was discovered by M. Guérin-Méneville. It moistens 
the cocoon with this liquid; which soaks through and pene- 
trates the whole thickness of the silken wall which confines 
it. The threads of silk of which it is composed are moistened, 
* “ Manuel de l Hducateur du Ver a Soie,”’ p. 37. 
