CHAPTER 17 



Physiology and 

 Behavior of the Arthropoda 



In this large and varied phylum a number of physiologic problems have 

 been studied extensively. Some ot these have revealed mechanisms radi- 

 cally different from their analogues in the vertebrates. Some of the more 

 unique and characteristic ones— molting, hormones, innervation patterns, 

 flight, compound eye vision, behavior and social mechanisms-will be 

 discussed in this chapter. 



149. Molting 



All arthropods periodically shed their chitinous exoskeleton as a 

 part of growth and metamorphosis. The actual shedding of the old and 

 hardening of the new skeleton, which may take a few seconds (daphnia) 

 or several hours (lobster), is only the obvious culmination of the elab- 

 orate process of molting. Before shedding occurs the new skeleton is 

 preformed and materials of the old skeleton are salvaged. 



The exoskeleton is formed of three layers (Fig. 17.1). The outermost 

 is a thin, flexible, colorless epicuticle composed of wax and cuticulin, 

 a lipoprotein containing a large amount of fatty material. The middle 

 layer is the primary chitinous layer composed of chitin and cuticulin, 

 sometimes impregnated with calcium carbonate or other salts. The inner 

 secondary chitinous layer is made almost entirely of chitin and protein. 

 The epidermis lies beneath this as a single layer of cells with numerous 

 filamentous extensions into the two chitinous layers. 



The first step toward a molt (Fig. 17.1) is a separation of the epi- 

 dermis from the old skeleton by the secretion of a molting fluid. Glandu- 

 lar cells in the epidermis add enzymes to the fluid capable ot digesting 

 protein and chitin but not cuticulin. While the epidermis lays down a 

 new epicuticle the molting fluid begins to erode the old secondary 

 chitinous layer. 



Formation of a new skeleton and salvage of the old go on simul- 

 taneously. All of the secondary chitinous layer and some of the primary 

 layer are ultimately digested, although the amount of cuticulin in the 

 latter may prevent its digestion. It growth is to take place at the next 

 molt, the epidermis with its new epicuticle grows and becomes wrinkled 

 in the confines of the old skeleton. It begins to secrete a soft, pliable, 

 primary chitinous layer. 



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