. Transactions of the Entomological Society of London. ip of their food-plantsonce considered to belong to widely different orders. It must not, however, be concluded that the speciesof Epizoa are constant. They too are liable to variation,especially those which, like the fleas, do not spend all theirlife on the host. We find among them many instances ofconspicuous variation, individual as well as flea of the hedgehog, for instance, is different in thewestern Mediterranean countries from the form found inCentral Europe and Great Britain. The rodent flea, Ctenoph-thalmus agyrte
. Transactions of the Entomological Society of London. ip of their food-plantsonce considered to belong to widely different orders. It must not, however, be concluded that the speciesof Epizoa are constant. They too are liable to variation,especially those which, like the fleas, do not spend all theirlife on the host. We find among them many instances ofconspicuous variation, individual as well as flea of the hedgehog, for instance, is different in thewestern Mediterranean countries from the form found inCentral Europe and Great Britain. The rodent flea, Ctenoph-thalmus agyrtes (very common in Great Britain on volesand mice), has developed into a number of geographical raceson the Continent, and even the Scottish and British specimens,taken as a whole, show some distinctions. Text-figs. 1-5represent a portion of male genital organs of five fleas repre-senting Ctenophthalmus agyrtes, in various parts of Europe :agyrtoides Wahlgr. (1911) from Scandinavia, eurous Roths. (1912) from Hungary and Russia, agyrtes Hiller. Figs. 1-5. Portion of $ -genitalia of Ctenophthalmus agyrtes and itsgeographical representatives: 1. agyrtoides; 2. agyrtes;3. eurous; 4. provincialis; 5. baeticus. (1896) from Central Europe, Northern France and the BritishIsles, provincialis Roths. (1910) from the French Alps andSouthern France, and baeticus Roths. (1910) from America, North of Mexico, we know a number of specieswhich also consist of four or five geographical varieties, andthe same can be said of certain species inhabiting othercontinents. So far as our knowledge goes at present, thisgeographical variation of the fleas is not dependent on differ- ( cxlv ) ences in the host of hosts, but must, at least in the main,be attributed to those factors, whatever they may be, whichare the cause of the modification into geographical races ofnon-parasitic insects and other members of the animal world. Epizoic life is, on the whole, one of ease and affluence,atte
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