6 BULLETIN 248, U. S. DEPARTMENT OF AGRICULTURE. 



and a comb of fine hairs. When full grown the maggots are usually 

 less than one-fifth of an inch in length. The larva of a common 

 species is illustrated in figure 3 (p. 14) . 



The food of the larvae appears to vary somewhat with the species, 

 since some seem to thrive on a considerable variety of foods, while 

 others are more restricted in their diet. In nearly all species it 

 seems certain that the partially digested blood voided by the adult 

 flea in feeding constitutes an important part of the diet of larvae, 

 especially when newly hatched. The remainder of the food consists 

 of particles of organic matter, either of animal or vegetable origin, 

 which are found in the cracks of floors, in the nests of the host, or 

 even mingled with the sand near the habitations of the host. 



THE COCOON AND PUPA. 



When the larvae have attained full size they spin cocoons of more 

 or less oval shape (fig. 1, b; fig. 4). These vary from almost white 

 to brownish, but owing to the particles of sand and dust usually at- 

 tached the color is often dark. The insect in this stage thus is 

 rendered inconspicuous. In structure the cocoons range from rather 

 light, flimsy silken coverings to very thick tough or even thick brittle 

 encasements. Within the cocoon the larva molts its skin and enters 

 the pupal, or resting, stage (fig. 1, c), which somewhat resembles the 

 adult insect. At first the pupa is very pale in color, but it grad- 

 ually darkens as the time for the appearance of the adult approaches. 

 The length of time spent in the cocoon varies with climatic condi- 

 tions. At Washington, D. C, Mr. Pergande found that the dog flea 

 would emerge as an adult within from 7 to 9 days after spinning the 

 cocoon. In his experiments in England Mr. Bacot found the period 

 from spinning of cocoons to the emergence of adults to range as fol- 

 lows: European rat flea, from 8 days to over a year; human flea, 

 from 7 to 239 days; dog flea, from 7 to 354 days; and Indian rat flea, 

 from 7 to 182 days. 



In these experiments Mr. Bacot found that the period within the 

 cocoon varied markedly with the temperature. This was particularly 

 true with the Indian rat flea, which had its cocoon stage greatly 

 lengthened when the daily mean temperature fell below 65° F. These 

 long resting periods were generally not produced in the case of the 

 human flea until the mean temperature fell to 50° F., and to 40° or 45° 

 F. with the European rat flea. The work of this investigator sug- 

 gests that the winter is passed in this stage, and that fleas may thus 

 tide over dry hot periods. It is certain that the cocoon offers much 

 protection from adverse weather conditions. The larva may remain 

 quiescent for long periods within the cocoon before actually pupating. 

 and another resting period may occur within the cocoon after the 



