FLEAS 73 



are pure silk, very strong, densely woven yet soft, and pale brown in 

 colour. 



The pupa itself vaguely resembles the adult in shape. Its head, 

 body and legs, held close to its sides, can be made out, and in fact it 

 suggests a wax model of a flea made by a rather indifferent artist. 

 One of its most fascinating features, clearly visible in the pupa of the 

 common hen flea though not in all species, is the vestigial wing buds on 

 the thorax. They represent the only concrete evidence that fleas are 

 descended from winged ancestors — a fact most entomologists inferred 

 years before these structures were demonstrated by Sharif. 



This stage of the life-cycle may last two weeks or more than a year. 

 Long after the flea is fully developed it can lie dormant within the 

 cocoon (Plate XVIII) waiting for some outside stimulus to precipitate 

 hatching. It then bursts out within a spHt second. The vibration 

 caused by the footfall of a passing animal may be sufficient. 



After emerging from its cocoon a flea can live for a considerable 

 time without feeding. In this stage both sexes survive for about the 

 same period. Providing they are kept in rubbish, adults of the common 

 rat flea can live without food for seventeen months in captivity. In the 

 case of well fed fleas which are subsequently starved the females live 

 nearly twice as long as the males. On a full diet of human blood the 

 human flea has survived 513 days and the common hen flea 345 days. 

 A Russian Ceratophyllid, however — a true hero of the Soviet Union — 

 is said to have lived 1,487 days ! In captivity, however, fleas, Hke their 

 hosts, probably survive much longer than in nature. It is not unusual, 

 for example, for an adult robin to attain 10-12 years in an aviary, 

 whereas only about one year is the average expectation of life in the 

 wild. Hirst found that in soHtary confinement a flea lived twice as 

 long as in the company of others and it seems probable that in nature 

 they have a short life and a merry one. 



The food of an adult flea is blood. Accordingly the mouth-parts 

 have been transformed into a piercing and sucking apparatus (Plate 

 XII). In Ford's book in the New Naturalist series there is a very fine 

 description of the mouth-parts of a relatively primitive insect (a cock- 

 roach) which shows how these have been modified in butterflies to form 

 an apparatus for imbibing nectar from the centres of flowers. In fleas, 

 however, it is no easy matter to decide which portions are homologous 

 with those of the primitive biting types, and the experts are by no means 

 agreed upon this question. 



