GENERAL CONSIDERATIONS. 5 



after the fourth, their conjoint weight is 1628 grains ; and after the fifth, when they have 

 attained a length of three inches, they weigh 9500 grains. These changes of skin, called 

 mou/tings, seem designed to accommodate the development of the body to its outer enve- 

 lope : it is a critical period ; and if any accident happens which interferes with its proper 

 performance, the animal either perishes, or is left in an enfeebled state, and is unable to 

 assume the form of the perfect insect. The number of moultings varies with the species, 

 but is always alike in the same species : the same changes are repeated in each respective- 

 ly. All these changes are best observed in the silkworm, in which their number is five, and 

 require thirty days for their complete performance. In some insects the number of moult- 

 ings extends to nine or ten, while in others it does not exceed three or four. The larva 

 ceases to eat when it is undergoing this process : an internal change takes place, however, 

 which greatly favors the casting of its envelope ; it is the absorption of the fat beneath the 

 outer skin, which becomes shrivelled, while at the same time it gives opportunity for the 

 expansion of the internal parts. The motions are strange during this period : their bodies 

 are curiously shaken and contorted, with jerkings of the head and posterior parts of the 

 abdomen ; these are designed to break up the attachments of the outer skin, and finally 

 to detach it from the new skin beneath, which is bright, fresh, and moist. A rent is finally 

 made in the old skin along the back, through which the animal now forces its way, in 

 which process it is assisted by attaching itself to some point of support : this is managed 

 so adroitly that the old skin remains whole, except the rent along the back, and so perfect 

 that it might be mistaken for the perfect larva. The change is not confined to the outer 

 envelope : the organs of respiration, though extremely delicate, as well as those of diges- 

 tion, cast off also their membranes, which are ultimately expelled from the body. It is no 

 wonder, then, that the moulting stage is one of great danger to the larva. This becomes 

 still more hazardous to the silkworm which is placed in an artificial state, and which is 

 increased by the numbers that are crowded into a small area. The last change is attended 

 with circumstances somewhat peculiar. They first seek a suitable retreat, adapted to the 

 nature of the insect itself ; a crevice in the bark of trees, a shelter under the roof of a 

 building, or a hole in the ground, in which they construct a sort of chamber, which they 

 line and otherwise prepare in a mode suitable to their necessities. They form habitations 

 with much skill, and the bestowment of much care, to defend their bodies from the ex- 

 tremes of temperature, or as a defence from their enemies. Sometimes, as in the dipterous 

 insects, the old skin is converted into a pupa case, after becoming more rigid and stiff : in 

 other instances a beautiful case is made, and a robe of silk is laboriously woven from a 

 single thread, which is formed and spun from the juices of the body. 



The silkworm is an admirable example to illustrate the instinctive powers of insects in 

 perfecting their stages of development. We admire the skill by which it suspends itself in 

 its case, and the beautiful manoeuvres by which it manages to place the band of silk around 



