NO. 6 CATERPILLAR AND BUTTERFLY — SNODGRASS 3 1 



For protection the ants take this caterpillar into their nest, and here 

 the treacherous creature eats the ants' larvae. 



Finally, there are species that have descended to parasitism. Mem- 

 bers of the Epipyropidae live as external parasites on the back of 

 fulgorids or other homopterons, and several pyralid species are ecto- 

 parasites on live sloths. Two species are known to be internal parasites 

 of the coccid Kermes. Rau (1941) records the rearing of six species 

 of moths from nests of the wasp Polistes. The larvae of these species 

 are parasitic on the wasp larvae, and spin their cocoons in the host 

 cell, where they pass the winter. Egg laying by the moths has not been 

 observed, but the females are suspected of entering the wasps' nest 

 at night when the inmates are asleep. 



All this diversity of habits on the part of the caterpillars shows that 

 the caterpillar structure, though highly specialized and standardized, 

 is still adaptable to various ways of living. It is a presumed tenet of 

 evolution that all perpetuated adaptations are beneficial to the species. 

 However, since it would seem that there is enough vegetation in the 

 world to support all the caterpillars there are, why have some species 

 departed from the ways of their ancestors? Brues (1936) points out 

 that the preference of insects for certain kinds of food is an instinct, 

 not a matter of conscious taste, and that aberrant food habits are 

 therefore due to basic hereditary changes in instinct. We are then 

 left with the question of why do instincts change. 



The caterpillar goes through several stages of increasing growth 

 between moults, but it preserves its larval form and structure up to the 

 end. Then at one final moult, after shedding its caterpillar skin, it 

 appears to have suddenly changed into a pupa. That the change of 

 form is not so sudden as it appears to be will be shown later. 



The larval status and the pupal-adult development are regulated by 

 two opposing hormones. A juvenile hormone, formed in the corpora 

 allata of the head, maintains the larval form ; a hormone discharged 

 from glands in the prothorax, under stimulus by a hormonal secretion 

 from the brain, controls moulting and permits further development. 

 The prothoracic gland hormone is known as the moulting hormone 

 because, as said by Van der Kloot (1961), it "acts on the tissues to 

 promote all the changes characteristic of a molt," but it has no influ- 

 ence on the development that follows the moult. This hormone then 

 might be likened to a green traffic light that allows traffic to cross the 

 intersection, but has nothing to do with where it goes after crossing. 

 The developmental course of the insect after the moult depends on its 

 hereditary factors, it may become another larva, a pupa, or an adult. 

 The prothoracic gland hormone has been named "ecdyson," unfor- 



