68 FLEAS, FLUKES AND CUCKOOS 



enough to carry these differences in one's head and, once under the 

 microscope, there is Httle more difficulty in "spotting" species of fleas 

 than species of butterflies. 



In nature bird fleas probably copulate when the host is incubating 

 eggs or when the young are in the nest or on the host itself, for they 

 seem to require not only a blood meal but a certain degree of warmth 

 to stimulate their interest in the opposite sex. In a glass tube they 

 remain completely indifferent until the tube is heated in the palm of 

 the hand. They will then mate readily enough. 



Some fleas seem to require a meal from their true host before they 

 will copulate. In the laboratory the common rat flea [Nosopsyllus 

 fasciatus) will feed more readily on man than on rats but according to 

 Strickland this does not provide an adequate stimulus. The common 

 hen flea [Ceratophyllus gallinae) however, which is very hardy and less 

 particular in more ways than one, will breed successfully on mammalian 

 blood, man or rat. Some of the house-martin fleas copulate without 

 a feed at all — a fact which we have observed ourselves. 



Many male fleas die soon after mating, but the female survives, not 

 only to deposit her eggs but to supply an important element in the diet 

 of her offspring. It has been proved that she requires a blood meal 

 before laying fertile eggs and therefore in the absence of the host 

 breeding is impossible. Blood appears to have a stimulating effect upon 

 the reproductive organs of the female, for she generally lays within 

 twenty-four hours of feeding even if she has been starved for weeks 

 previously. 



In countries with well defined seasons, most phases of an insect's 

 life-cycle are restricted to certain periods of the year. For example 

 the purple emperor butterfly is only seen on the wing in June and July, 

 and wasps are not troublesome round the Christmas tree. Fleas are also 

 more in evidence at certain seasons. For example, in temperate 

 climates such as our own, the number of fleas per rat rises in the summer 

 and falls off sharply in the winter, whereas in parts of tropical India 

 the opposite is true and the flea population is at its height in the so 

 called cold season. In Texas the hen stick-tight flea almost vanishes 

 after the spring rains, but is again plentiful in dry cool periods in the 

 autumn. Pliny, many hundreds of years ago, remarked upon these 

 seasonal fluctuations and drew attention to the "fleas which skip 

 merrily in summer time in victualling houses and inns, and bite so 

 shrewdly." 



