PHYSIOLOGY AND BEHAVIOR OF THE ARTHROPODA 



327 



At the time of molting the new epicuticle and primary chitinous 

 layer are complete, although they are still soft and flexible. The molting 

 fluid with its digested products is completely absorbed into the body. 

 The old epicuticle and much of the primary chitinous layer remain as 

 a loose covering. At various places, especially along the back, the old 

 primary layer is thin (Fig. 17.1), so that after the secondary layer is 

 digested away a line appears along which the old skeleton will break. 



The arthropod must then swell up to burst the old exoskeleton. It 

 may contract the abdomen, forcing blood into the head and thorax, or 

 it may swallow water or air. Once the old exoskeleton has been split open 

 the organism extricates itself, shedding not only the covermg of the body 

 and legs but also the lining of the foregut, hindgut, and in the labiates 

 the linnig of the tracheal system. If the arthropod grows during the molt 

 it must swell rapidly to stretch the wrinkled new exoskeleton out to its 

 full size. Most arthropods swallow water or air to do this, and may 

 increase their volume 100 per cent. Even if the organism is not growing 

 at some particular molt, it may be necessary to compress some parts of 

 the body in order to force blood into others to achieve whatever 

 metamorphosis is taking place. A newly emerged adult moth, for ex- 



oltinO 

 line-] /EpicuticlcN 



Primary 



\ 



— chitinous — :" 

 ~ layizr 



■Secondary 







chitinous Molting 

 layer "X ' 



•bpidermis — ^ 



Moiling 

 fluid 



epicuticle 



New- 

 primary 

 chitinous 



layer 



C Hypodc-rmis 



Figure 17.1. Molting in an arthropod. A, The fully formed exoskeleton and under- 

 lying epidermis between molts. B, Separation of the epidermis and secretion of molting 

 fluid and the new epicuticle. C, Digestion of the old secondary chitinous layer and secre- 

 tion of the new primary chitinous layer. D, Just before molting. (Modified from Wiggles- 

 worth.) 



