466 MEDICAL ENTOMOLOGY 



temperatures (0-8 C.) they may be kept alive for two months. He also 

 found that under certain circumstances the larval and pupal stages 

 might be prolonged for as much as a year, so that even after the 

 removal of the adult fleas and the host from any place which has 

 been infected, imagines may continue to appear at intervals for a 

 considerable time, as long as fourteen months in one case. Strickland, 

 who has worked on similar lines, is of the opinion that the persistence 

 of the imagines is not due to a succession of individuals emerging 

 from the pupae, but to the actual survival of the original fleas for 

 very long periods without food in one case for at least eighteen 

 months. Bacot, on the other hand, under the most favourable con- 

 ditions which he could devise, could not keep adult fleas alive for 

 longer than ninety-five days, and attributes the persistence of the 

 insects in breeding cages after the removal of the host to the long 

 period which may elapse between the time at which the larva becomes 

 full fed and emergence from the pupal case. He found that, after 

 spinning its cocoon, the larva may rest within it for a varying length 

 of time, as much as four hundred days ; the pupal stage is about ten 

 or twelve days, but even after the imago has shed its pupal skin it 

 may still rest for a time within the cocoon. He states that the eggs 

 hatch after four to twelve days, according to the temperature, and 

 that the active life of the larva may be anything from ten to a 

 hundred and fourteen days or more. 



Unfortunately only preliminary notes * of the work of these observers 

 have as yet appeared. The detailed accounts will be awaited with 

 interest. 



The experiments of the Indian Plague Commission with X. cheopis 

 have not revealed any evidence of a similar longevity. They found 

 that this species, if removed from the host, will live for several days 

 if kept in bran, sacking, rice, etc., and from eight to fourteen days 

 if kept in sand which is slightly moist. When in association with 

 their normal host, the rat, six per cent were found alive on the forty- 

 first day, suitable precautions having been taken to prevent reproduc- 

 tion from vitiating the experiment. When fed on human blood some 

 were found alive after twenty-seven days, and after twenty days when 

 fed on the blood of a guinea pig. A similar shortening of life has 

 been observed in Pulex irritans when removed from its normal host. 



As a general rule excessive light, dryness, and heat are inimical to 

 fleas, and a moderate degree of humidity advantageous. Fleas which 

 * British Medical Journal, October 26, 1912, May 31 and June 14, 19"13. 



