70 FLEAS, FLUKES AND CUCKOOS 



true, and if the eggs are laid on the host it is preferable that they 

 subsequently roll off. 



Like birds themselves, different species of fleas lay differently 

 shaped eggs. The human flea [Pulex irritans) has a nearly spherical 

 ^gg, but those of the common hen flea are elliptical. The eggs of the 

 tropical rat flea {Xenopsylla cheopis) are midway between the two. In 

 colour they are pearly white and relatively smooth with rather soft 

 shells which are easily dented. They are devoid of the elaborate 

 sculpture and ornamentation which adorn the eggs of butterflies and 

 many feather flee but under a high power magnification the surface is 

 seen to be finely pitted. When the host scratches and preens itself they 

 roll off and are conveniently scattered. A keen entomologist once 

 collected a spoonful off the lap of a visitor who, during tea, was affec- 

 tionately fondling his kitten. Often the eggs are deposited directly in 

 the nest of the host. They are just visible to the naked eye and are 

 faintly reminiscent of a fine dusting of castor sugar. A female flea, at 

 intervals, deposits a total of between 300 and 500 eggs. Except in the 

 case of a sedentary species they are laid singly or in small batches — 

 during the day and night — either on the host or on dried twigs and 

 leaves in the nest. The sedentary species sometimes lay small tgg 

 masses, in which the individual eggs are glued together. 



The hen stick-tight flea expels her eggs forcibly and they fall well 

 clear of the head of the host on which this species congregates. Accord- 

 ing to the temperature and humidity they hatch in 2 to 14 days. 



All but five of our sixteen bird fleas belong to the genus Cerato- 

 phyllus and these are essentially fleas of temperate cHmates. Especially 

 in the early stages of their life-cycle they require a cool humid atmo- 

 sphere. 



The eggs of the common rat flea, which is fairly closely related to 

 the bird fleas and until recently was included in the genus Ceratophyllus, 

 hatch at a temperature of 41 °F. The eggs of the tropical rat flea, 

 however, require temperatures above 54°F. before the larvae can 

 emerge. Hirst, discussing the problem of bubonic plague and its flea 

 vectors, remarked that in all probabihty each species of rat flea has 

 adapted itself to some particular range of climatic conditions. This is 

 also undoubtedly true of bird fleas and it of course includes adaptation 

 to the ranges of humidity and temperature found in the various types 

 of nest they infest. It is obvious that conditions in a gannet's nest, a 

 sand-martin's nest and a starling's nest are very diflferent. The 



