328 ^H£ ANIMAL KINGDOM 



ample, contracts the abdomen to force blood into the wrinkled wings 

 and expand them to full size. After the skeleton is adjusted to its new 

 size and shape, the epidermis secretes enzymes which oxidize and harden 

 the epicuticle and primary chitinous layers. Usually the primary layer, 

 which is pale at first, darkens during this process. In the crayfish and 

 many other hard-shelled forms, calcium carbonate is deposited as an 

 additional stiffening agent. The crayfish had previously absorbed much 

 of this lime from the old skeleton and stored it on the sides of the 

 stomach between epidermis and chitinous lining as the gastroliths (p. 

 298). After the molt these concretions are exposed to the digestive fluids 

 and dissolve rapidly, providing an immediate supply for the new skele- 

 ton. 



The final event of molting occurs later. The epidermis secretes the 

 secondary chitinous layer as a permanently elastic portion of the exo- 

 skeleton. The desired flexibility of any part of the exoskeleton is 

 achieved to a considerable extent by the thicknesses of the two chitinous 

 layers. Where rigidity is required the outer layer is thick. Where a tough 

 but flexible skeleton is required the inner layer is thick, and where 

 great flexibility is wanted both layers are thin. 



1 50. Arthropod Hormones 



The molting process has been extensively studied in the crustaceans 

 and insects, and in both cases has been found to be under endocrine 

 control. Arthropods have also been shown to elaborate other hormones, 

 related to metabolism, reproduction and pigment changes. As the glands 

 secreting these hormones are discovered and studied, it is becoming 

 apparent that arthropods have an endocrine system similar in many 

 respects to that of tire vertebrates. Both are intimately related to the 

 brain. In both kinds of animals antagonistic hormones are known, and 

 in both some of the glands have reciprocal actions on each other to pro- 

 duce a controlled check-and-balance system. All evidence suggests, how- 

 ever, that the arthropod and vertebrate endocrine systems evolved 

 independently. 



Probably the most important contribution made by arthropod 

 physiologists to the field of physiology is the discovery of neurosecretion, 

 the secretion of physiologically active substances by nerve cells. In a 

 narrower and more usual sense neurosecretion refers to the production 

 of hormonal materials in the cell body of a neuron which then travel 

 the length of the axon to be stored and ultimately released at the tip 

 (Fig. 17.2). 



The primary endocrine organs of the crustaceans, for example, were 

 once believed to be the sinus glands on the optic ganglia of the eyestalks 

 (Fig. 17.3). Extracts of these glands have been shown to contain a variety 

 of hormones, including one that affects pigment distribution in the 

 compound eyes, two more that control pigmentation of the body, one 

 that induces molting, and several others influencing metabolism and 

 reproduction. In contrast to this complexity of their secretions, the sinus 

 glands present a puzzling anatomic simplicity. Each one appears to be 



