122 BRANCH ARTHROPODA 



takes up the food supply and then re-enters the heart through 

 the side openings. It does not supply the tissues with oxygen, 

 since it receives only enough for its own use. 



Respiration is carried on by a series of air-tubes called 

 tracheae. These tubes are interbranching and penetrate to 

 every portion of the body. The air enters them through a 

 pair of stigmata or pores, one on either side of each segment. 

 The functions of these tracheae are to take up oxygen from the 

 air and to distribute it to the tissues of the body, since this is 

 not done by the circulation of the blood, and to collect and carry 

 off the carbon dioxid. 



Insects which live in water either come up to the surface to 

 breathe and, in some cases, to take down a supply of air held 

 on the outside of the body by a fine pubescence, or they are 

 provided with tracheal gills which will enable them to breathe 

 air mixed with water. Gilled insects, of course, do not have to 

 come to the surface to breathe. 



The Nervous System. — Besides the central or ventral (Fig. 

 93) nervous system (see Branch Arthropoda), insects have 

 a small and varying sympathetic nervous system (Fig. 93), 

 consisting of a few small ganglia sending nerves to the automatic- 

 acting visceral organs. Commissures connect the sympathetic 

 system with the brain just at the origin of the subesophageal 

 commissures. 



Touch. — The sense of touch is located in the " hairs " dis- 

 tributed over the various parts of the body, but most numerous 

 on the feelers. 



Taste is located on small papillae or in ])its on the mouth- 

 parts, particularly on the tips of the palpi and on the upper wall 

 of the mouth. 



Smell is probaljly the most used sense of insects. The 

 organs of this sense are minute papillae and " microscopic pits " 

 on the antennae and mouth ]:)arts. It has been proved that 

 most insects find their food l)y this sense. " It is believed that 

 ants find their way back to their nests by the sense of smell and 

 that they can recognize by scent, among hundreds of individuals 

 taken from various communities, members of their own com- 

 munity."' 



' Kellogg's " American Insects." 



