The Plague Fleas 



60 1 



five to ten days, a minute, active, caterpillar-like larva emerges from the 

 egg to feed upon such organic matter as it may find for the six to eight weeks of 

 this stage. During the larval period the skin is shed three or four times. When 

 full grown, the larva empties its alimentary canal, spins itself a tiny silken cocoon, 

 sometimes including minute bits of rubbish or grains of sand in its structure, 

 sheds its skin for the last time, and becomes a pupa. As such it is inactive for 

 from two to eight weeks, according to external conditions of temperature and 

 moisture, then opens the cocoon and emerges from the pupa shell, a perfect in- 

 sect — the flea proper. 



The adult fleas, both males and females, have soft exoskeletons at first, but 

 soon they harden, through the formation of chitin, to the well-known tough and 

 brittle armor. 



Fig. 239. — Various fleas, magnified about 30 diameters. The specimens are 

 treated with hot 20 per cent, caustic potash for a few minutes, dehydrated in 

 alcohol, cleared in xylol and mounted in balsam, a, Ctenocephalus canis, cf; 6, 

 Ctenocephalus canis, 9 ; c, Ctenocephalus felis, & ; d, Ctenocephalus f elis, 9 

 (Bacot, in Journal of Hygiene, "Plague Supplement in, 1914"). 



The male differs from the female in being smaller and in its shorter abdomen. 

 Both insects hop about in search of the appropriate warm-blooded hosts upon 

 whose blood they are to live. Each kind of flea has a preferred host, but the 

 tastes of all are more or less cosmopolitan, so that in the absence of the preferred 

 host, another kind of warm-blooded creature wiU do. Adult fleas live solely by 

 sucking blood. 



The longevity of a flea varies according to conditions of temperature and mois- 

 ture. Life is longest when the temperature is high and the ground not too dry. 

 They may live for months without feeding; when regularly fed they can live at 

 least a year and a half. The longevity of the fleas in the adult stage, the long 

 periods of abstention from food that they may suffer without dying, and the ac- 

 cessions to their numbers that may occur through the maturation of their embry- 

 onal fellows in the same place, explain why families returning to their closed city 

 houses, or going to their closed country houses, sometimes fimd them after months 



