78 FLEAS, FLUKES AND CUCKOOS 



impressed by their enormous strength. He considered the feat equiva- 

 lent to a man dragging two full sized elephants round a cricket ground. 

 The modern physiologists, as Imms has pointed out, take another view. 

 They hold that as the body of an animal becomes smaller so the relative 

 (not the absolute) power of a muscle increases. The great strength of a 

 flea is therefore more apparent than real, and according to present day 

 calculations the feat of the performing fleas would be compared with 

 greater accuracy to a man pulling two sheep round a cricket ground. 



The structure of the legs has been especially studied in fleas from 

 the sand-martin. There is a broad flat coxa (hip) joined by a small 

 joint called the trochanter, to a short stout femur (thigh) and tibia (shin), 

 and an elongated five-jointed tarsus (foot). The pair of claws on the 

 fifth tarsal segment do not, at first sight, appear to be particularly 

 powerful, but they are nevertheless marvellously well adapted to 

 clinging. 



As we have already pointed out, the bristles and spines lie close to 

 the flea's body, almost like scales on a fish. Those on the feet, however, 

 stick out at an acute angle (Plate XXXIIIc) and act as grappling 

 irons. Anyone who has attempted to transfer fleas from one tube of 

 alcohol to another with the aid of a paint brush, cannot fail to be 

 impressed by the manner in which even a dead flea hangs on to hair 

 or bristles. 



Fleas can also climb up a vertical glass plate as long as the surface 

 is moist. On a dry, clean, glass surface they cannot keep a foothold and 

 fall off after reaching a height of a few inches. All fleas seem to show a 

 desire to climb upwards, away from the ground. This reaction, which 

 is called negative geotropism, may also help them in finding a host. 



The human eye is not sufficiently quick to see the action of a flea's 

 leg when it actually jumps. Some writers believe that the only move- 

 ment is a sudden straightening of the leg, and its extraordinary force is 

 due to the simultaneous extension of both the femora and tibia. Possibly 

 owing to the fact that the hind end of the body is heavier than the head, 

 a flea turns over in mid-air and lands facing the way from which it 

 came. It is the back legs which touch the ground first and take the 

 impact of landing. 



Anyone who has bred fleas and watched them, knows that they 

 react to various stimuli, such as air currents, vibrations or touch, by 

 apparently random leaps. In this way they no doubt escape from 

 certain enemies and also in other cases may reach a passing host. As 



