50 THE FLEA [CH. 



The preference which fleas show for certain animals, 

 and the repulsion which they manifest on being allowed 

 to suck blood from an unaccustomed host, lead one 

 to believe that they have a sense of taste. This sense 

 in other insects is apparently seated in certain micro- 

 scopic pits and hairs which form the ends of nerves 

 and are distributed round the mouth. Whether fleas 

 can hear is not, it seems, definitely known. 



A large number of fleas possess what is called 

 a frontal tubercle. It is a notch in the centre of the 

 forehead but nearer to the mouth than to the antenna. 

 Sometimes the tubercle projects from a groove. This 

 is most marked in the genus of African fleas Lis- 

 tropsylla. The real nature of this organ is unknown. 

 Some regard it as an organ of sense. Its homology 

 is also uncertain. To some it suggests the egg- 

 breaker of the larva and they regard it as a relic of 

 the larval stage. To others it suggests an eye and 

 they regard it as the remnant of an unpaired ocellus 

 possessed by the ancestral flea. 



An exceedingly remarkable organ of sense, which 

 is found in all fleas, is called the pygidium. It is 

 a sensory-plate plentifully supplied with hairs and 

 nerves and always placed on the back of the ninth 

 abdominal segment. Of all its uses we are still some- 

 what uncertain but some observers declare that at 

 the season of love the male flea bestows caresses on 

 the pygidium of the female. 



