332 ^"^ ANIMAL KINGDOM 



Ddvzlops 

 A DIAPAUSE : 75° F. No dcveIopme.nt 75T 



Months 



B DIAPAUSE Develops 



Chilkdal40T 75"F 



EmcrOe-nce- 



Chilkd 



Figure 1 7.7. Diapause in the pupae of the cecropia moth. A, Normal development at 

 75° F. B, Normal development with diapause broken by six weeks (or more) of chilling 

 at 40° F. C, When chilled and unchilled pupae are joined, diapause is broken in both 

 individuals. D, The brain of a chilled pupa, implanted in an unchilled pupa, induces 

 immediate development in the latter. 



pupa will end its diapause. By combining chilled and unchilled organs in 

 various ways, he showed that only the chilling of the brain is important. 

 Upon being chilled the brain releases the prothoracicotropic hormone to 

 which the prothoracic glands respond whether they were chilled or not, 

 and the released m & m hormone ends the diapause. Once the brain acts 

 upon the prothoracic glands its continued presence is not needed to 

 produce the molt. In one experiment Williams implanted a chilled brain 

 in an unchilled pupa, then moved it into a second tnichilled pupa. Both 

 pupae ended diapause promptly and developed into adults. 



151. Patterns of Muscular Innervation 



The individual motor axons of vertebrates each innervate a few 

 muscle fibers of a single muscle, forming a motor unit (p. 101). The 

 strength of muscular contraction varies according to the number of units 

 active, and its duration is controlled by the duration of stimuli from 

 the nerves. 



In arthropods the anatomic relations of nerve and muscle fibers are 

 different. A single axon not only innervates all the fibers of one muscle, 

 but may innervate those of another muscle as well. Furthermore, most 

 muscles receive two or more axons, each of which has a different effect 

 upon contraction. Usually in a three-axon system one axon produces a 

 strong, brief contraction, another a weak, sustained contraction, and the 

 third inhibits the action of the other two. By varying the frequency of 

 stimulation among the axons the muscular contraction can be varied 

 considerably. 



A study of the innervation pattern for several muscles shows how 



