CIIANaES IX LIFE AXl) FOK^f. 179 



wiiilc the clifFerencc between tlie joints furnung the future thorax and those 

 of" the abdomen can only be traced by careful study. 



This explains the voracity of the caterpillar, and shows that the main 

 end of its existence is to gormandize and groAv ; examining its interior, we 

 find that the nuiscles occu])y hardly more space than twice the thickness of 

 tlic skin to which they are attached ; and their Acry object is to move the 

 creature to a feeding spot or remove the old integument to admit of a 

 larger growth and a greater capacity for food ; those of the head are almost 

 exclusively attached to the jaws. The general cavity of the body is mostly 

 occupied by the alimentary canal and its appendages, the glands and nerves 

 and even the tracheae really requiring an insignificant amount of space ; 

 and whatever is not occupied by these organs, necessary to the assimilation 

 of food, is choked up with the fatty masses embedded in the cellular tissue. 



It is impossible, howe^•er, that it should groAv to any extent witliout 

 finding its outer integument, and especially the dense covering of the head, 

 nuich too small for its needs, since its bulk at maturity is a hundred times 

 or more that at its birth. Nature provides for this emergency, for the cat- 

 erpillar does not grow insensibly like most animals, but, as in other insects 

 l)y distinct stages ; for when the skin has become too contracted for its 

 needs it rests awhile, and then suddenly the skin bursts, and the creature, 

 in new array, Avhich had been forming beneath the old coat, crawls forth to 

 new and more vigorous activity, until similar difficulties are again encoun- 

 tered. During the resting spell, the interior head is withdrawn from the 

 old case to the first thoracic segment, which, during the moulting period, 

 is abnormally distended ; and it is here that the bursting of the old skin 

 takes place, the head being separated from the body, and the old skin 

 being gradually shoved off behind, together with the lining of the larger 

 })arts of all internal organs having considerable external openings. The 

 number of such moults varies, but never exceeds six. 



When it assumes the pupal stage its habits change completely, it being 

 now quiescent, with no })ower of eating or of any movement beyond a 

 shake or a wriggle ; and its form compacted, witli all the appendages soldered 

 firmly to it ; although the line of separation between thorjix and abdomen 

 is well marked, and tlie latter is composed of many joints movable one 

 u[)on another, nearly all special distinction between the head and thorax 

 is lost, and their segments are immovably soldered into one common tract. 

 This is an exact temporary repetition of the more important distinctive 

 external features of the crali and lobster, where the head and thorax are 

 iniited by a common shield into a cephalothorax, while the joints of the 

 abdomen are freely mo\able. This, then, is what Oken meant when he 

 pointedly calls the pupa. Crab. 



The closer we examine this " crab," the better we see how admirably 

 tlie form and projections, the position and inactivity of the chrysalis are 



