THE RAT- FLEA, CERATOPIIYLLUS F ASCI ATI'S BOSC. 449 



nervous system, in the large terminal ganglion-mass, which 

 represents a fusion of the most posterior ganglia. The difference 

 in the number of the abdominal ganglia in the two sexes of the 

 flea shows, therefore, that in the female the concentration has 

 gone one step farther than in the male, since only six abdominal 

 ganglia remain free in the female, but seven in the male. The 

 nervous system of the female has therefore reached one stage in 

 evolution higher in the female than in the male. Similar 

 differences between the sexes are known to occur also in other 

 insects, especially in the Hymenoptera (the order which includes 

 the bees, ants, and wasps), an order in which the superiority in 

 intelligence and in the social virtues of the female over the male 

 is very marked. 



Besides the difference in the number of ganglia, the nervous 

 systems of the male and female flea differ also in the arrangement 

 of the nerve-stems given off from the hindmost ganglion-mass. In 

 the male two stout nerves are given off, which run on either side 

 of the " corkscrew-organ'' (see p. 454), and are distributed mainly 

 to the powerful muscles which work the penis. In the female, 

 however, three pairs of moderately stout nerves are given off, 

 which go to the genitalia, but I have not been able to trace their 

 exact distribution. 



Comparing the two figures, it is seen that the male and female 

 nervous systems are approximately of the same absolute length. 

 Since, however, the female flea is considerably larger than the 

 male, the nervous system of the female is relatively much 

 the shorter, and does not extend so far into the abdomen as that 

 of the male. Consequently the nervous system of the male is 

 the easier to dissect out. 



As regards minuter details, the nerve-ganglia are seen to 

 contain a number of nuclei, representing the ganglion-cells, 

 which have a bilaterally symmetrical arrangement, showing that 

 each ganglion-mass is a fusion of a pair of ganglia. The nerves 

 which come off from the ganglia right and left contain small, 

 elongated nuclei, which are the nuclei of the connective tissue- 

 sheaths of the nerves. The connectives running between the 

 successive abdominal ganglia contain no nuclei, but the stout 

 connectives passing forwards from the metathoracic ganglion- 

 mass contain elongated nuclei similar to those of the peripheral 

 nerves. 



