122 Parasitic Arthropoda 



the batch was then placed at 70° F. for one night, near a white' 

 rat. The next day all the cocoons were empty and the fleas were 

 found on the white rat. 



Thus, temperature greatly influences the duration of the pupal 

 period, which in Ceratophyllus fasciatus averages seventeen days. 

 Moreover, when metamorphosis is complete a low temperature will 

 cause the imago to remain within the cocoon. 



Sexually mature and ovipositing fleas, he fed at intervals and kept, 

 alive for two months, when the experiment was discontinued. In 

 the presence of rubbish in which they could bury themselves, unfed 

 rat fleas were kept alive for many months, whereas in the absence of 

 any such substratum they rarely lived a month. In the former case, 

 it was found that the length of life is influenced to some degree by the- 

 temperature and humidity. In an experiment carried out at 70° F. 

 and 45 per cent humidity, the fleas did not live for more than four- 

 months, while in an experiment at 60° F. and 70 per cent humidity 

 they lived for at least seventeen months. There was no indication 

 that fleas kept under these conditions sucked moisture from surround- 

 ing objects, and those kept in bell jars, with an extract of flea-rubbish 

 on fllter paper, did not live any longer than those which were not so- 

 supplied. 



Curiously enough, although the rat is the normal host of Cerato- 

 phyllus fasciatus, it was found that when given the choice these fleas 

 would feed upon man in preference to rats. However, none of the 

 fleas laid eggs unless they fed on rat blood. 



The experiments of Strickland on copulation and oviposition in 

 the rat flea showed that fleas do not copulate until they are sexually 

 mature and that, at least in the case of Ceratophyllus fasciatus, the 

 reproductive organs are imperfectly developed for some time (more 

 than a week) after emerging from the pupa. When mature, copula- 

 tion takes place soon after the fleas have fed on their true host — the 

 rat — but not if they have fed on a facultative host only, such as man. 

 Copulation is always followed by oviposition within a very short 

 time. 



The effect of the rat's blood on the female with regard to egg- 

 laying, Strickland concludes, is stimulating rather than nutritive, 

 as fleas that were without food for many months were observed to 

 lay eggs immediately after one feed. Similarly, the male requires 

 the stimulus of a meal of rat's blood before it displays any copvilatory 

 activity. 



