TRANSFORMATIONS OF THE INSECT. 63 



it becomes hard mid dry, and too small for the growing in- 

 sect, and is then cast off' entire. 



A series of bee-larvae can be selected showing a graduation 

 in size and form from the egg and recently hatched larva up to 

 the full-grown larva. In the caterpillar and other active larvae, 

 there are usually four or five stages, each showing a sudden and 

 marked increase in size. Newport states that the caterpillar 

 of sphinx ligustri moults six times, and at the last moult be- 

 comes a third larger than at any earlier period; the larva of 

 Arctia caja moults from five to ten times. 



A few days before the assumption of the pupa state, the 

 larva becomes restless, stops eating, and deserts its food, and 

 usually spins a silken cocoon, or makes one of earth, or chips, 

 if a borer, and there prepares for the change to the pupa state. 



During this semipupa period (lasting, in many insects, only 

 for a day or several days, but in some Saw-flies through the 

 winter) the skin of the pupa grows beneath that of the quies- 

 cent larva. While the worm-like larva exhibits no trire- 

 gional distinctions, the muscles of the growing pupa contract 

 and enlarge in certain parts so as to modify the larva form, 

 until it gradually assumes the triregional form of the adult 

 insect, with the differentiation of the body into a head, thorax, 

 and abdomen. 



In a series of careful studies, abundantly illustrated with 

 excellent plates, Weismann has recently show r n that Swammer- 

 dam's idea that the pupa and imago skins were in reality 

 already concealed under that of the larva is partially founded 

 in truth. Swammerdam states, "I can point out in the larva 

 all the limbs of the future nymph, or Culex, concealed beneath 

 the skin," and he also observed beneath the skin of the larvae 

 of bees just before pupating, the antennae, mouth-parts, wings, 

 and limbs of the adult. (Weismann.) 



During its transformations the pupa skin is developed from 

 the lypodermis, or inner layer of skin. This peals off, as it 

 were, from the inner layer of the old larva skin, which soon 

 dries and hardens, and is thrown off. Meanwhile the muscles 

 of the body contract and change in form, thus causing the origi- 

 nal segments of the larva to infold and contract at certain parts, 

 gradually producing the pupa form. If, during this period, the 



