( civ ) 



We have also species in which the lost combs are replaced 

 by 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 better 

 of it and replaced the lost combs by combs developed from 

 the row of long bristles which is present in all fleas at some 

 distance from the apical margin of the segments (text-fig. 25). 

 There is apparently a tendency towards the loss of organs 

 after they have persisted, perhaps, a long time ; if this 

 loss is antagonistic to the welfare of the insect, the species 

 is doomed to destruction unless another organ can undertake 

 the function of the lost one. At any rate, this seems to be 

 the explanation of the development of such false combs in 

 species which we must assume from all we know of them 

 and their allies to have possessed originally normal combs 

 on the segments now provided with false ones. 



Bat-parasites provide us further with an interesting case 

 of the evolution on parallel fines which occurs in these differ- 

 ent groups of Epizoic insects. The legs of Polyctenidae have 

 so-called pseudojoints varying in number according to in- 

 dividuals, species or genera. The pseudojoints are pale rings 

 where the chitin is less thick and hard, rendering the 

 legs more flexible. Similar pseudojoints are found in Nycteri- 

 biidae, and in the bat-infesting genus of bed-bugs, Loxaspis 

 (text-figs. 26-30). The bat-fleas, however, although long- 

 legged, have no pseudojoints. The occurrence of such a 

 curious structure in parasites living on the same family of 

 hosts, or even on the same individual, is certainly not 



