64 FLEAS, FLUKES AND CUCKOOS 



The abdomen of the flea consists of a series of segments each of 

 which is protected, externally, by a dorsal and ventral overlapping 

 chitinous plate known respectively as thGtergum dindsternum(?\2itGXVIl). 

 The first sternum is missing, and the three last segments are highly 

 modified in connection wdth the sexual organs and form a series of flaps 

 and levers and struts of peculiar complexity. 



The abdomen contains the digestive and excretory organs, the heart 

 and circulatory system and also the ovaries and testes. The main 

 nerve cord runs along the ventral side of the body and has, in addition 

 to the brain, a series of swelHngs at intervals along its length. These 

 swellings (see Plate XVII) are nerve centres known as ganglia. In the 

 male flea there are eight such nerve centres and in the female only 

 seven. This fact does not suggest that the male is the more gifted of the 

 two — on the contrary, a fusion of the gangha indicates a more highly 

 specialised or "advanced" condition. Apart from this curious form of 

 sexual dimorphism the internal structure of a flea is rather generalised 

 and presents no very unusual features. For further details the reader 

 is referred to the excellent descriptions of the anatomy of insects which 

 can be found in Ford's and Imms' books in the Mw Naturalist 



series. 



In many insects the male can be described as the weaker sex and 

 this is certainly true in the case of fleas. The female is larger, lives longer, 

 weighs twice as much and is hardier and more resistant if conditions 

 deteriorate. It is also said to assume the active role in mating. This last 

 point is difficult to prove and indeed is probably a subjective impression 

 due to the greater size of the female which in mating adopts a position 

 covering the male. When a male bird flea approaches a female it can 

 be seen to cock its antennae out of their grooves, and in view of the 

 important role these organs play in mating this might well be described 

 as taking the initiative. The ancient writers thought the antennae of 

 fleas were ears and it is of course quite possible that with them the flea 

 perceives vibrations. They may also serve as organs of smell and touch. 

 In bird fleas and their aUies they are much larger in the male sex and 

 when cocked are held aloft like a pair of horns (Plate Xld). During 

 copulation the male takes up a position beneath the female and uses 

 the antennae to grasp her firmly from below. At other times they 

 are folded back neatly into the grooves along the sides of the 

 head, thus adding to the general streamHned effect of the body 

 (Plate XI). 



