472 MEDICAL ENTOMOLOGY 



writers have never succeeded in dissecting out the duct which leads from 

 the spermatheca to the oviduct. 



The ovaries are of a different type to those of the Diptera. Each 

 ovariole, (Plate LVIII, fig. 7) when mature, consists of a long tube 

 which tapers gradually from base to apex. The lower portion is divided 

 into a series of follicles, three or four being generally distinguishable. 

 Of these the lowest contains a mature egg, while the next two or three 

 contain nurse cells and ova of approximately the same size as one another. 

 The rest of the tube is filled up with a large number of young ova, dimin- 

 ishing in size and degree of maturation from the base upwards in a 

 gradual manner. The lowest seven to ten of these are slightly less in width 

 than the tube in which they lie, and are partially separated from one 

 another at the sides, though the marginal epithelium is not developed. 

 Higher up the ova are smaller and are more crowded together, two 

 often lying side by side in the transverse axis of the tube, and higher 

 still they diminish rapidly in size until they form a crowded mass of 

 cells at the upper part of the tube. As many as twenty ova can be 

 distinguished in some of the ovarioles. 



All the ovarian tubes in the same flea are not in the same degree of 

 development, and even when the abdomen is obviously distended and 

 the flea about to oviposit one finds some of the ovarian tubes in an 

 immature condition. In many fleas, even in those which have some ova 

 almost mature, one finds ovarian tubes which are almost entirely 

 undifferentiated, consisting only of a mass of large cells (Plate LVIII, 

 fig. 5) attached to the oviduct by a fine canal. Probably only a small 

 number of eggs is deposited at one time, oviposition being repeated a 

 large number of times at short intervals. 



The respiratory system is simple, each spiracle giving off a trachea 

 which breaks up at once into a number of branches for the supply of the 

 adjacent tissues. There are no air sacs. 



The nervous system (Plate LVIII, fig. 1) shows a remarkably low 

 degree of differentiation. The brain consists of two lateral masses, cor- 

 responding to the supra-oesophageal ganglia, which are 

 The nervous system . . , 



united to one another by a broad commissure, and two 



almost equally large ganglia below and behind them, the sub-oesophageal 

 ganglia ; the two pairs of ganglia are united by commissures which 

 enclose the oesophagus. The three thoracic .ganglia are distinct from 

 one another and are of approximately equal size. There are seven abdo- 

 minal ganglia, all equal in size except the last, which is larger than the 

 rest. Both the thoracic and abdominal ganglia show evidence of being 



