448 E. A. MINCHIN ON SOME DETAILS IN THE ANATOMY OF 



dissections if one is content to get out only the large meta- 

 thoracic ganglion-mass, in the thoracic series, and not to worry 

 about the ganglia of the first two thoracic segments, which 

 require very careful dissection. 



A remarkable feature of the abdominal nervous system is that 

 it presents very marked differences in the two sexes of the flea. 

 These sexual differences are seen at a glance in the two figures 

 on PI. 26, which are drawn from two preparations to the same 

 scale by means of a camera lucida. At the upper end of each 

 figure we see the large metathoracic ganglion-mass, and at the 

 lower end the large hindmost or terminal ganglion-complex from 

 which nerves are given off to the genitalia. Between these two 

 larger nerve-centres at the two extremities there is a series of 

 smaller ganglia ; and it is easy to see that this series comprises 

 seven ganglia in the male and only six in the female.* 



It is seen, then, that the male flea has one pair of ganglia 

 more in its abdominal nervous system than the female. Is this 

 an indication of superiority on the part of the male sex ? By 

 no means, rather the contrary ! In the embryonic development 

 of insects there are some ten or eleven pairs of abdominal ganglia, 

 and in the ontogenetic development, or in the phylogenetic 

 ovolution, of insects the tendency is for these ganglia to be 

 concentrated by fusion which takes place progressively from 

 behind forwards. In some of the Diptera the tsetse-fly, for 

 example, and I believe in the common house-fly also the 

 concentration of the nerve-ganglia has reached its maximum 

 possible, since the whole ventral chain is concentrated into one 

 large mass situated in the thorax, a mass which represents the 

 three pairs of thoracic ganglia plus the whole abdominal chain, 

 all telescoped forwards into one large ganglion-complex. In the 

 flea, however, the process of concentration and specialisation has 

 not gone so far, and is seen only at the hindmost end of the 



* This curious point was discovered by Major Christophers, in his 

 dissections of fleas made in my laboratory. Previous to his work, I had 

 counted the ganglia of a female flea that I was dissecting, and had noted 

 that there were six small ganglia. Subsequently I made a mounted pre- 

 paration of the abdominal chain of a male flea, and was surprised to 

 observe seven small ganglia ; thinking I had made a mistake in my former 

 observation, I looked up my old notes and altered " six " to " seven, ' 

 never suspecting the sexual differences which were subsequently shown to 

 exist. 



