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fascia' Ks, Pulex irritans, Ctenocephalus ccmis, Leptopsylla niusculi 

 and Xenopsylla cheopis. The different stages were subjected to various 

 conditions, though chiefly carried out in four incubators, two of which 

 were maintained at 75° F., but with different degrees of humidity, 

 and a similar pair at 8^ F. In addition a cellar and cupboards were 

 used. The methods employed in rearing fleas and experimenting 

 with them at the various stages are described. 



The eggs were placed on paper or cloth in the receptacle in which 

 the larvae were to be reared, sand and food being added before the 

 eggs were put in, and subjected to varying conditions. The results 

 show that, compared with the later stages in the life-history, eggs are 

 relatively insusceptible to external conditions. The upper limit of 

 temperature which is fatal to eggs has not been determined. 

 C.fasciatus hatched at 85° F.; of P. irritans, 9 per cent, hatched at 93° F. 

 and once 27 per cent, of the eggs of X. cheopis hatched at 93° F. At 

 low temperatures the numbers which hatch are reduced ; at 40" 9° F. 

 eggs of C.fasciatus hatched, while those of A', cheopis and P. irritans 

 failed. As regards the influence of humidity, it is safe to say that a 

 temperature of 65° to 80° F. with a humidity of "70 or over is most 

 favourable and that if the temperature be above 60° F. humidities 

 below 50 to -55 are harmful. In the case of P. irritans there is possi- 

 bility of complete failure at a humidity of '50, but 70 per cent, of the- 

 eggs of C.fasciatus hatched at a temperature of 75° F. with a humidity 

 of only '48. Low temperatures check or prevent oviposition, and in 

 the case of C.fasciatus and P. irritans warmth (75° F.) combined with 

 low humidity favours the fertility of eggs laid. 



The larval stage may be subdivided into two periods, (1) an 

 active and (2) a quiescent or resting phase, passed within the cocoon. 

 It is shown that in the dry incubators (75° F. and 84° F. and 

 humidity '60) and warm cupboard (67°-69'^ F. and humidity 

 •65-'71) active larvae of P. irritans, as well as of C. fasciatus 

 and X. cheopis, died, while in the cocoon stage some survived. In the 

 case of all the species investigated, the newly hatched larvae were able 

 to live from several days to over a month without food, provided 

 the conditions were not otherwise unfavourable. Besides food 

 and temperature, a certain amount of moisture is necessary. 

 Local moistening by the urination of animals or sweat from their 

 bodies may convert what would otherwise be an impossible place into 

 a favourable situation ; while draughty conditions, with a compara- 

 tively high humidity, may be less favourable than a drier situation 

 with a nearly still atmosphere. Investigation as to the nature of 

 the food of larvae was also made. It was proved that the faeces of 

 the adult fleas are a possible diet and favourable for the larvae of all 

 three species. In the case of C . fasciatus this food seems a necessity. 

 Low temperature is responsible for very protracted active larval life, 

 but temperature is not the sole factor determining the length of the 

 period elapsing between the hatching of the egg and the spinning of 

 the cocoon. In the experiments on the cocoons it was proved that 

 the length of the cocoon period is largely determined by conditions 

 of temperature, a fall in temperature causing A', cheopis and P. irritans 

 to lengthen their cocoon period, an effect not seen in C. fasciatus. 

 C. fasciatus frequently passes the cold weather in the cocoon, and a 

 similar disposition is show)i by Ctenocephalus canis and P. irritans. 



