NO. 6 CATERPILLAR AND BUTTERFLY — SNODGRASS 35 



insects have been extracted from Crustacea and other invertebrates, 

 and even from the adrenal cortex of cattle. These writers suggest, 

 therefore, that something of the nature of the insect juvenile hormone 

 may be widely present in all animals, and that evolution has produced 

 specific sensitivity to it in certain tissues, as in the insects. In any 

 case, it seems certain that if the early insects did not have a juvenile 

 hormone, they could never have developed their special kind of 

 metamorphosis. 



The corpora allata arise at an early age of the embryo from the 

 ventral ectoderm of the head between the mandibular and maxillary 

 segments, and migrate to their dorsal position in most insects behind 

 the brain. Though the Thysanura do not have typical corpora allata, 

 they do have a pair of apparently glandular bodies attached to the 

 outer surfaces of certain muscles of the maxillae. These bodies are 

 termed the corps juganx by Chaudonneret (1949), who reasonably 

 contended that they are the corpora allata in a primitive position 

 between their ventral points of origin and their usual dorsal position 

 in other insects. The function of these bodies in the Thysanura, 

 however, is not determined, and, so far as known to the writer, 

 thoracic endocrine glands have not been observed in these insects. 

 Yet the thysanurans molt throughout their entire life. Perhaps the 

 corps jugaux are really moulting glands that have remained in the 

 head, where they take their origin. 



METAMORPHOSIS 



When at last the juvenile hormone that has maintained the cater- 

 pillar in its larval form gives way entirely to the thoracic-gland 

 hormone, the caterpillar comes to the end of its life, provided it has 

 not already been eaten by a bird or other predator, killed by some 

 parasite, or poisoned by an insecticidal spray. If the caterpillar were 

 a sentient being, knowing the ordeal it must now go through, it should 

 be filled with apprehensions. However, the caterpillar is a creature 

 of instincts, with no consciousness of self to be carried over into the 

 butterfly. The butterfly is a new thing entirely, having its own 

 instincts and outlook on life, and very probably no memory of ever 

 having been a caterpillar. The transformation processes are purely 

 biochemical, under the guidance of hormones and the factors of 

 inheritance. 



Yet the caterpillar seems to feel the approach of its time for pupa- 

 tion; it becomes restless and food no longer has any appeal. Tent 

 caterpillars suddenly rush out from their tents to the ends of twigs 



