. Transactions of the Entomological Society of London. of many-combed species, while the bubonic plague flea,Xenopsylla cheopis, belongs to a combless genus. The humanflea is allied to the rabbit-flea and the hedgehog-flea. Thetwo latter species have a comb on the genal portion of thehead and pronotum, while in the human flea the pronotalcomb is entirely lost and the genal one at most representedby one tooth. The question why the combs are lost in many species willbe difficult to answer. I may mention, however, one instancewhich throws a little light on points of this kind. Bat-fleasappear ori
. Transactions of the Entomological Society of London. of many-combed species, while the bubonic plague flea,Xenopsylla cheopis, belongs to a combless genus. The humanflea is allied to the rabbit-flea and the hedgehog-flea. Thetwo latter species have a comb on the genal portion of thehead and pronotum, while in the human flea the pronotalcomb is entirely lost and the genal one at most representedby one tooth. The question why the combs are lost in many species willbe difficult to answer. I may mention, however, one instancewhich throws a little light on points of this kind. Bat-fleasappear originally to have had a comb on the pronotum,metanotum and abdominal tergites 1-6, i. e. 8 dorsal are preserved in a number of species. In others someof the combs are lost or reduced to a few very short teeth. ( civ ) We have also species in which the lost combs are replacedby what we call false combs in Siphonaptera. The abdo-minal combs of the North American Myodopsylla palposus,for instance, are represented by small teeth (text-fig. 24).. Fig. 24. Metanotum and first abdominal tergite of Myodopsylla -pal-posus Baths. (1904).„ 25. Metanotum and first abdominal tergite of Myodopsylla in-signis Roths. (1903). In an allied species (M. insignis) Nature has thought betterof it and replaced the lost combs by combs developed fromthe row of long bristles which is present in all fleas at somedistance from the apical margin of the segments (text-fig. 25).There is apparently a tendency towards the loss of organsafter they have persisted, perhaps, a long time; if thisloss is antagonistic to the welfare of the insect, the speciesis doomed to destruction unless another organ can undertakethe function of the lost one. At any rate, this seems to bethe explanation of the development of such false combs inspecies which we must assume from all we know of themand their allies to have possessed originally normal combson the segments now provided with false ones. Bat-parasites provide
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