g6 FLEAS, FLUKES AND CUCKOOS 



despite their varied origins they all show a marked increase in the 

 number of teeth in the pronotal comb (see Plate X). The mammal 

 CeratophyUi in question rarely have more than 22 teeth in these 

 combs, but the bird fleas all have from 26 to 40. Thus in all bird 

 Ceratophyllidae we find without exception a combination of two 

 facts (a) the environment "bird" (b) a larger number of spines in 

 the pronotol comb. This structural modification, therefore, appears to 

 be connected with, or possibly is the direct consequence of the change of 

 host, and thus presents an example of convergent evolution. 



In the family Pulicidae there is a general tendency towards a 

 reduction and final loss of combs. The change to bird host has apparent- 

 ly accelerated the process. This phenomenon can be observed in the 

 two genera of bird fleas derived from rabbit fleas (Spilopsyllinae). The 

 rabbit fleas (Plate X) still possess pronotal and genal combs or at 

 least a pronotal comb, but the shearwater and auklet fleas have neither 

 one nor the otlier (Plate XI). 



Thus the change of hosts has produced convergent evolution in 

 both famflies, but leading in opposite directions. When a CeratophylUd 

 flea leaves a mammal and becomes permanently parasitic on birds the 

 result is additional teeth in the pronotal comb, but if a PuUcid flea 

 takes this same step, the reverse appears to happen and one must expect 

 an increased rate of reduction and final loss of the combs. Therefore, 

 when studying this type of evolution it must always be remembered that 

 the eflfect of the change depends on the derivation and nature of the 

 flea, as well as on the nature of the host and other environmental factors. 



There is another morphological modification which can be observed 

 in certain bird fleas, which is almost certainly associated with the change 

 of host. This is a tendency towards a shift in position and ultimate 

 reduction of the plantar bristles of the feet. The normal number of 

 plantar bristles is five and this primitive condition is stiU found in the 

 Ceratophyllidae. In the whole family PuHcidae there is a tendency 

 towards reduction, and all the rabbit fleas for example, have only 

 four pairs. Once again the condition is accentuated in the bird fleas, 

 for both in the two species derived from rabbit fleas and the two 

 derived from the tropical rat fleas, this reduction has developed 

 farther than in any other allied fleas. The third pair of plantar bristles 

 shows either a change of position — a shift upwards and inwards 

 which Jordan describes as "crowding towards the apex" — or is lost 

 altogether. 



