Strickland's (1914) studies on the biology of the rat flea, Ceratophyllus fasciatus, have so important a general bearing that we shall cite them in considerable detail.
He found, to begin with, that there is a marked inherent range in the rate of development. Thus, of a batch of seventy-three eggs, all laid in the same day and kept together under the same conditions, one hatched in ten days; four in eleven days; twenty-five in twelve days; thirty-one in thirteen days; ten in fourteen days; one in fifteen days; and one in sixteen days. Within these limits the duration of the egg period seems to depend mainly on the degree of humidity. The incubation period is never abnormally prolonged as in the case of lice, (Warburton) and varying conditions of temperature and humidity have practically no effect on the percentage of eggs which ultimately hatch.
The same investigator found that the most favorable condition for the larva is a low temperature, combined with a high degree of humidity; and that the presence of rubbish in which the larva may bury itself is essential to its successful development. When larvæ are placed in a bottle containing either wood-wool soiled by excrement, or with feathers or filter paper covered with dried blood they will thrive readily and pupate. They seem to have no choice between dried blood and powdered rat feces for food, and also feed readily on flea excrement. They possess the curious habit of always devouring their molted skins.
An important part of Strickland's experiments dealt with the question of duration of the pupal stage under the influence of temperature and with the longevity and habits of the adult. In October, he placed a batch of freshly formed cocoons in a small dish that was kept near a white rat in a deep glass jar in the laboratory. Two months later one small and feeble flea had emerged, but no more until February, four months after the beginning of the experiment. Eight cocoons were then dissected and seven more found to contain the imago fully formed but in a resting state. The remainder of 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 surrounding objects, and those kept in bell jars, with an extract of flea-rubbish on filter paper, did not live any longer than those which were not so supplied.