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, the back 

 inside, the legs out, the worm arranges its thread all round its body, 

 describing ovals with its head. It approaches nearer the points 

 of attachment. 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 contract an intimate adherence with the parts already 

 established. 



" 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 this 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 be- 

 come black. The parts of the mouth tend downwards ; the 

 skin wrinkles. Yery 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 little 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. Gruerin-Meneville. It moistens 

 the cocoon with this liquid ; with this it soaks through and 

 penetrates the whole thickness of the silken wall which confines 

 it. The threads of silk of which it is composed are softened, 

 * " Manuel de 1'educateur du ver a sole," p. 37. 



