Differentiation and Morphogenesis in Insects 249 



establish the endocrine conditions which favor precocious pupation. 

 The question arises as to the behavior of the giant chromosomes under 

 these conditions. 



Experiments of this type have been performed on Chironomus by 

 Clever and Karlson (1960). Two hours after the injection of ecdyson, 

 the pattern of puffing in the salivary glands shows a changeover from 

 larval to pupal type. Clever and Karlson conclude that the primary 

 effect of ecdyson is to alter the activity of specific genes and that this 

 action is documented in the giant chromosomes. 



H V I and the synthesis of sill; 



In Nigeria and certain other parts of Africa, there occurs a lepidop- 

 teran called Epanaphe moloneyi whose mode of life bears a certain 

 resemblance to that of Rhynchosciara.^ Here again, the larval stages 

 are gregarious. They move about on the food plant as '"procession- 

 aries," spinning a thread of silk to guide themselves back to their 

 common nesting place. When the larvae are full grown, the entire 

 group of 100 or more individuals collaborate in spinning a large 

 silken chamber almost the size and shape of a football. Within this 

 chamber, each larva finally spins its own separate cocoon and pupates. 

 After about a month of adult development, the pupae are transformed 

 into moths which escape from the cocoons. 



Interest in the Epanaphe silkworm stems from the fact that its silk 

 consists essentially of two amino acids, glycine (42.5 moles %) and 

 alanine (53.1 moles % ), thereby qualifying as the simplest of natu- 

 rally occurring proteins (Lucas et al., 1958) . 



The silk is synthesized and secreted into the lumen of the silk gland 

 by a monolayer of epithelial cells which form the distal regions of the 

 two glands. These cells are extremely rich in ribosomal RNA — a fact 

 which probably accounts for their rapid synthesis of what is essen- 

 tially a single, diagrammatically simple protein. If the ribosomal RNA 

 is responsible for coding the glycine-alanine sequence in Epanaphe 

 silk, this fact should signal itself in certain predictable peculiarities 

 in the ratios of the four liases in RNA: namely, very high ratios of 

 guanine to adenine and of uracil to cytosine ( Speyer et al., 1962). 



Yeas and Vincent (1960) have tested this proposition by journey- 

 ing to Africa, collecting large numbers of Epanaphe silkworms, and 

 isolating the RNA from the silk-secreting part of the silk gland, as 

 well as from a number of other tissues and organs. 



The answer was clear-cut. The base ratios encountered in the silk- 



