84 INTRODUCTION TO 



within the space of thirty days. From five thej 

 extend to nine or ten, the latter number having been 

 observed in the tiger moth, (Chelonia caja.) But 

 the great majority of insects do not undergo this ope- 

 ration oftener than three or four times. The cater- 

 pillars of butterflies are usually limited to the for- 

 mer of these numbers, those of moths to the latter, 

 but among moths many other exceptions occur 

 besides the one mentioned above. The proximate 

 cause, as -already intimated, of this moulting, is 

 the more rapid expansion of the body than of the 

 skin, which, in consequence, soon exceeds the capa- 

 city of its envelope: the latter, it is true, admits 

 of a certain degree of distention, but a few fixed 

 points on its surface, as well as the rigidity of the 

 part enclosing the head necessary to give support 

 to the masticating organs, prevent it keeping pace 

 with the growth of the body. The larva ceases 

 to eat when the change is approaching, and, during 

 this temporary suspension of its ordinary employment, 

 the fat lying immediately beneath the outer skin 

 is absorbed, a circumstance which greatly hastens 

 the crisis by abstracting the moisture from the skin 

 and shrivelling it, while, at the same time, it tends 

 still further to dilate the internal parts. The colours, 

 being dependent in a great measure on the fresh- 

 ness and moisture of the skin, also become dull 

 and confused. The only motions made by the larvae 

 are occasional contortions and undulatory movements 

 of the segments, which ultimately produce a complete 

 separation between the exterior skin and the new 



