24 • Journal op the Mitchell Society [December 



gans or trachea. The insects differ from man in that their circulatory 

 organs are very poorly developed and their respiratory organs are well 

 developed to counterbalance this. The insects have no lungs but a 

 system of trachea or tubes which open to the exterior through minute 

 pores called spiracles. From the spiracles the trachea branch and re- 

 branch until they reach all parts of the body. The oxygen is carried 

 to the cells through these tubes and the carbon dioxide carried away 

 through the same set. 



The nervous system and sense organs of insects are so different from 

 those of man that we are often at a loss to account for the sensory hfe 

 of insects. There is in the insects no brain as we find it in the verte- 

 brates but the nervous functions are distributed to a chain of ganglia 

 along the ventral wall of the body. There is therefore more or less of 

 local control for each region of the body. The psychic life of such 

 animals is therefore rather low and in no way to be compared with 

 that of man. 



The sense organs of insects may be grouped into three classes 

 (Comstock) : 



Mechanical sense organs. — ^Touch and hearing. 



Chemical sense organs. — Taste and smell. 



Organs of sight. 



Touch organs are generally distributed over the body and need no 

 special discussion. Organs of hearing are apparently not universally 

 present. They occur among the singing orthoptera and in mosquitoes 

 and perhaps in bees, but whether they occur in other forms is by no 

 means clear. In the grasshopper the ears are located on the first seg- 

 ment of the abdomen, but in the katydids and crickets they occur on 

 the tibia of the fore legs. In the mosquitoes they undoubtedly occur 

 on the antennae, as the antennae are provided with whorls of setae 

 which gradually decrease in length from the base of the antennae out- 

 ward. It has been demonstrated that these setae vibrate to notes of 

 different pitch and it is believed that the vibrations are transferred to 

 the nerve endings. Most beekeepers believe that bees can hear be- 

 cause they make such different hums under different circumstances, 

 and even an amateur can tell the difference between the ])usy hum, 

 the swarming hum, and the angry hum of an outraged bee. 



The chemical senses of insects are very poorly understood. This 

 is due in part to the wide distribution of these organs over the body 

 and to the fact that several different types of sense organs are fre- 

 quently closely associated. We say that insects taste because we 



