FLEAS 77 



on either side of the head near the anterior edge of the antennal 

 grooves. Although the eyes of the fleas are situated laterally, they are 

 in fact displaced dorsal ocelli. These relatively simple organs probably 

 do httle more than enable their owners to perceive the difference 

 between Hght and darkness and thus would only assist them in finding 

 a host if it were in their immediate vicinity. Bird fleas are photonegative, 

 that is to say, other things being equal, they move away from light. 

 If, however, a collecting box or container is opened suddenly, the fleas 

 inside often begin to crawl towards the aperture, for their attraction 

 to the source of air currents is very marked and is apparently stronger 

 than their aversion to light. As Strickland pointed out they become 

 greatly agitated if blown upon. This reaction no doubt considerably 

 assists them in finding a host which may be moving about or breathing 

 in their vicinity. It has been suggested that the pygidium (Plate XIX) 

 or sensilium, is an organ connected with this particular reaction. This 

 is an extraordinary saddle-shaped structure which is present in both 

 sexes, situated on the dorsal surface near the terminal end of the 

 body. Its surface is densely clothed with spicules, and honeycombed 

 with widely spaced pits, from the centre of which arises a single long 

 sensory brisde. From above these pits appear like small rosettes 

 (Plate XIX). A few experiments have been performed in Russia and 

 Germany to test whether the pygidium is in fact connected with any 

 particular sense. It is claimed that if this organ is painted over or 

 cauterised the fleas cannot perceive air currents and consequently fail 

 to find a host. An American entomologist once tried tickling the 

 pygidium of feeding fleas and he recorded that they showed no visible 

 response. This, however, is scarcely a fair test, since fleas, Hke some of 

 their mammalian hosts, seem marvellously insensitive to outside 

 stimuH once their attention is focused upon a hearty meal. 



It has already been pointed out that the adult flea is totally devoid 

 of wings. Therefore unUke certain other parasites, for example the 

 mosquito, it cannot fly in pursuit of its host. The jumping legs of the 

 flea are, however, very powerful and to a certain extent are good 

 substitutes for wings. 



In 1910, Mitzmain, an American naturahst, measured the leap of a 

 flea and found it could cover a distance of thirteen inches horizontally. 

 This was believed to equal a 300 yard jump by a six foot man. At the 

 end of the last century Rothschild observed "performing" fleas at a 

 circus moving objects many times their own weight, and was greatly 



