LIFE HISTORY 409 



lairs or rubbish piles in which they hatched, avoiding light and 

 feeding upon what bits of organic matter they can find, such as 

 mouse pills, crumbs, hairs, epidermal scales from their hosts and 

 the excrement of adult fleas. Some species, if not all, devour their 

 shed skins after moulting. According to Bacot and Ridewood, 

 who have recently made observations on the larvae of a number of 

 species of fleas, the larvae become very excited and impatient 

 when disturbed. They sometimes lie quiet, coiled like a watch 

 spring, for repose or concealment, but when about to moult they 

 stretch out at full length. They crawl by alternately expanding 

 and contracting the body like an earthworm, first securing a hold 

 with the hooks at the posterior tip of the body, then with the 

 head which is bent under to hook over some irregularity on the 

 surface. The duration of the larval stage varies with the tem- 

 perature and humidity and to some extent also with the species. 

 Under favorable conditions, i.e., at rela- 

 tively low temperatures and high humid- 

 ity and with plenty of food, the larvae of 

 some species pass through their two 

 moults and enter the pupal stage in a 

 week, whereas under unfavorable condi- FIG. isi. Cocoon of 

 tions the duration of the larval existence h ^ 2 an flea ' Pulex irritans ' 

 may be drawn out to over three months. 



When ready to undergo their transformation into adults, the 

 larvae spin little silken cocoons which are somewhat viscid, so 

 that particles of dust and lint readily adhere to them and give 

 them a dirty, dingy appearance (Fig. 181). According to Lyon 

 the adult insects may emerge from the cocoons of the cat flea, 

 Ctenocephalus felis, in from two to 14 days, but in most species 

 at least a week is required for the transformation to take place, 

 and this time may be greatly increased by unfavorable climatic 

 conditions. Strickland, in his work on the rat flea of England, 

 Ceratophyllus fasdatus, found that the average pupal existence 

 was 17 days and was extended to four months or more by low 

 temperatures, the fully formed adult insect remaining dormant 

 within the cocoon until exposed to a temperature of about 70 F. 

 There is much probability that the winter in temperate climates 

 and the hot dry season in tropical climates is tided over by fleas 

 in the cocoon, the emergence of the adults coinciding with the 

 advent of moderately high temperatures and humidity. 



