610 



EVOLUTION OF ANIMALS 



Part V 



to increase in size before they mature. In adults, its secretion partially con- 

 trols the growth of the eggs. This has been discovered by removing the gland 

 from young females in various stages of maturity. Its removal prevents the eggs 

 from ripening. Evidently sex does not affect the corpus allatum since a trans- 

 plant of one from an adult male into an adult female deprived of her own 

 gland will bring on the maturity of her eggs. 



Coordination and Sense Organs. The nervous system is highly developed 

 and serves to coordinate the activities of the body with whatever is going on 

 inside and outside it. The central nervous system consists of a pair of dorsal 

 ganglia, the brain, and a series of pairs of ventral ganglia and nerves connect- 

 ing and branching out from all of them (Fig. 30.18). From the subeso- 

 phageal ganglia the ventral nerve cord extends posteriorly formed by a series 

 of paired ganglia and connecting nerves. Each division of the thorax contains a 

 pair of ganglia from which nerves extend to the legs, wings, and internal 

 organs. There are only five pairs of abdominal ganglia, some of the once larger 

 number having been fused during the evolution of grasshoppers. In addition to 

 the central nervous system, insects have a visceral nervous system, ganglia and 

 nerves concerned with the control of the purely involuntary activity of the 

 salivary glands and parts of the digestive canal. 



The Sensitivity of Insects. In spite of their armor, grasshoppers are highly 

 sensitive to their surroundings. They have sense organs for the reception of 

 tactile stimuli, hearing, taste, smell, and sight, all of these connected with the 

 central nervous system. 



TACTILE HAIRS. Their delicate sense of touch is due to many protruding hairs 

 that are in contact with sensory nerve cells. In a simple type of such an organ 

 three kinds of cells are concerned, the hair cell which secretes the hair, the 



Subesophageal 

 ganglion 



Thoracic 

 ganglia 



Cut end 

 ol. canal 



lobe 



Brain 



Fig. 30.18. Nervous systems of grasshopper. View after alimentary canal re- 

 moved. The largest ganglia are those associated with greatest activity, e.g., with 

 wings and legs. (After Hegner: Invertebrate Zoology. New York, The Macmillan 

 Co., 1933.) 



