252 The Nature of Biological Diversity 



thoracic glands" (Williams, 1952b, 1958). And it is this second en- 

 docrine organ which then secretes the growth hormone itself — a 

 molecule which has been isolated and crystallized and called "ecdy- 

 son." The hormone is a water-soluble, heat-stable, heterocyclic ketone 

 with the provisional formula CisH 30 O4 (Butenandt and Karlson, 1954; 

 Karlson, 1956) . 



When exposed to ecdyson, the pupal tissues, after 8 months of de- 

 velopmental standstill, are promptly stimulated to intense morpho- 

 genetic activity. The result is a predictable pattern of death and birth 

 at the cellular level as the specialized tissues of the pupa make way 

 for the equally specialized tissues of the adult moth (Williams, 1961a) . 

 Spectacular changes occur throughout all parts of the insect: in the 

 head, the formation of compound eyes and feather-like antennae; in 

 the thorax, the molding of legs, wings, and flight muscles; in the abdo- 

 men, the shaping of the genitalia and, internally, the exorbitant 

 growth of ovaries and testes. And in the newly formed skin we can 

 witness the strangest behavior of all — the extrusion and transformation 

 of tens of thousands of individual cells into the colorful but lifeless 

 scales so typical of moths and butterflies (Stossberg, 1938). 



After 3 weeks of adult development, the process is complete. The 

 full-fledged moth escapes from the cocoon and unfurls its wings. 



The genetic construction manual 



This summary of the life history of the Cecropia silkworm brings 

 us back to the concept of the genetic "construction manual" men- 

 tioned at the outset. In the case of the Cecropia silkworm, as in all 

 insects that undergo a complete metamorphosis, the construction 

 manual is obviously divided into three distinct chapters. 



The first chapter tells how to make the egg into a larva. The second 

 chapter gives directions for the destruction of the larva and the 

 reworking of the same materials into a pupa. The third and final 

 chapter tells how to reshape the pupa into an adult moth. This analogy 

 serves to emphasize that the sequential polymorphism of insect meta- 

 morphosis involves the decoding and acting out of what is little short 

 of successive batches of genetic information (Snodgrass, 1954; Wig- 

 glesworth, 1961a). Perhaps the day is not far distant when we may 

 speak of "larval operons, pupal operons, and adult operons" — collec- 

 tions of genes which are sequentially brought into play to build and 

 rebuild the insect on the unchanging foundation of the constitutive 

 enzymes prerequisite for life at all stages. 



Though these overall happenings are amazing enough, it is well to 



