44 THE SIX-FOOTED INSECTS 



of the body, se])arating at the anterior end, so as to pass 

 around the esophagus, after which there is a large mass, 

 frequently termed the cerebrum, as it occupies the upper 

 part of the head. This gangUonic mass contains three 

 pairs of gangHa, while the subesophageal is composed of 

 three or four, and the primitive distribution of thorax and 

 abdomen is one pair of ganglia to each segment. This con- 

 dition is modified, especially in the higher insects, so that 

 the ganglia may be fused, causing a single ganglion in the 

 thorax, and five, three, or one in the abdomen. 



The various ganglia of this system act with considerable 

 independence and even dismembered parts of an insect may 

 maintain their movements if the ganglia are not destroyed. 



The special senses present many diverse features as com- 

 pared with higher animals, but insects give good evidence 

 of possessing sight, hearing, smell, taste, and touch, although 

 the exact range of these functions may differ considerably 

 from the same senses of vertebrates. 



The reproduction in insects agrees for the most part with 

 that of other arthropods, and except for certain remarkable 

 dcN'iations, such as are found on the aphids and bees, a 

 general statement will suffice. Insects have separate sexes, 

 and in a great majority of cases the males' and females are 

 distinct and usually may easily be distinguished by external 

 characters. The reproductive organs are located in the 

 abdomen, the ovaries in the central anterior part, and are 

 composed of a number of ovarioles or tubular structures, 

 within which the ova are developed and from M'hich they 

 pass into the oviducts. These oviducts combine near the 

 end of the abdomen into a common duct leading to the 

 external opening. Frequently connected with this common 

 duct is a sac-like structure, the spermatheca, which serves 

 for the retention of the spermatozoa. In the males the testes 

 are located about as the ovaries, and lead by rather slender, 

 more or less curved vasa deferentia to a common duct which 

 leads to the external opening at the posterior end of the 

 abdomen. The external organs are modified widely in 

 different groups of insects, and in many cases furnish most 



