36 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I43 



on the tree and recklessly jump off to the ground, where they may 

 travel long distances to find a suitable place for pupation. Others 

 wander about in search of a secluded place, and some species pupate 

 in the ground. The caterpillars of most moths enclose themselves in 

 silken cocoons. The spinning of a cocoon is a complex piece of 

 architectural work, but the caterpillar is endowed with a sensory- 

 neuromuscular spinning mechanism that works automatically as a 

 series of coordinated acts. If its cocoon is destroyed when partly spun, 

 most caterpillars cannot begin over again, but must go on from where 

 they left off. The stimulus for spinning has been shown by Van der 

 Kloot and Williams (1953) to be of internal origin, in part hormonal, 

 but largely the condition of the silk glands. The activating nerve 

 center, however, according to these investigators (1954), is in the 

 brain. 



The caterpillars of butterflies do not enclose themselves in cocoons. 

 In preparation for pupation they spin a mat or cone of silk on the 

 underside of a twig or leaf or against some upright object, and then 

 hang themselves from it head down by means of the claws of the 

 anal prolegs and spines on the suranal plate. Some give themselves 

 extra security by spinning a loop of silk from the support that forms 

 a girdle around the thorax. When the pupa is formed it splits the 

 larval skin over the back and down the face. Then by convulsive 

 movements it pushes the larval skin up to its rear end and hangs 

 freely from the attached skin. It is commonly said that the pupa 

 holds on by pinching a fold of the skin between the movable segments 

 of its abdomen. According to Riley (1879), however, there are small 

 ridges and knobs on the end of the pupal abdomen that serve partly 

 to grasp the suspended larval skin, but the principal suspensorium is 

 the "rectal ligament," which evidently is the extracted cuticular lining 

 of the intestine held by the anal sphincter of the pupa. This "liga- 

 ment," Riley says, will support a weight 10 to 12 times that of the 

 pupa. The tenth abdominal segment of the pupa is prolonged into a 

 process known as the cremaster (or hanger) armed at the end with 

 spines and hooks. The pupa now extracts itself from the larval skin 

 by swinging the cremaster to one side and anchoring it into the silken 

 support. Here the pupa hangs independently by the cremaster until 

 the butterfly emerges. The shriveled larval skin may remain on 

 the support until it drops off or is blown away by the wind. 



When the caterpillar (fig. 16 A) sheds its last cuticular skin and 

 reveals the pupa (D) that has taken its place, the change is indeed 

 striking. The fully formed pupa, however, is the finished product of 

 a transformation process. The caterpillar itself in its final stage (B) 



