62 FLEAS, FLUKES AND CUCKOOS 



if to 4 mm., the largest being the rock-dove flea {Ceratophyllus columbae) , 

 although both the sand-martin flea {Ceratophyllus styx) and the moorhen 

 flea [Dasypsyllus gallinulae) come very near it in size. At the other end of 

 the scale we have one of the house-martin fleas [Ceratophyllus rusticus) 

 and the house-sparrow flea [Ceratophyllus fringillae) . 



Fleas vary in colour from almost black to pale brown. Their integu- 

 ment is extremely tough and slippery, as anyone knows who has tried 

 to squash a flea in his fingers. The insect generally manages to squeeze 

 under a nail and make good its escape with a disconcertingly sudden 

 jump which the eye cannot follow. 



It is, of course, well known that the hard part of an insect is external. 

 In other words the skeleton consists of a chitinous outer covering to the 

 body similar to that of a crab or a lobster instead of an internal scaffold- 

 ing like the bones of mammals and birds. This hardening and toughen- 

 ing of the cuticle is most pronounced, on the whole, in parasitic insects. 



If a flea is examined under the microscope it is found to be covered 

 with strong, rather widely spaced bristles, arranged in definite rows or 

 groups, varying in length and thickness, and lying close to the body. 

 This greatly adds to the general streamlined effect. Each of these bristles 

 is set in a socket (Plate XXXIIIc) and articulates with the cuticle. They 

 are very valuable characters when it comes to classifying the fleas. 



On certain segments the hardened exo-skeleton is produced into a 

 series of spine-like backwardly projecting teeth, which form combs 

 (Plate X). These combs greatly facilitate the animal's progress 

 through fur and feathers and protect vulnerable areas of their bodies. 

 Parasitic insects from quite unrelated groups which live in the fur of 

 mammals, such as modified flies (Nycteribiidae) and bugs (Polyctenidae) 

 parasitising bats, and the beetle [Platypsyllus castoris) from the beaver 

 have also developed comb-like structures. A few of the feather lice have 

 somewhat similar devices formed from expansions of the cuticle. This 

 type of comb is only found on parasitic insects. The genal comb 

 (Plate Xb) which protects the mouth, and the pronotal comb situated 

 on the first segment of the thorax are generally the most conspicuous 

 in fleas. 



The head of the flea, as in all insects, encloses the brain and bears 

 the mouth parts, eyes and antennae. It varies considerably in shape, 

 and in the fleas which have become "fixed" such as the hen stick- 

 tight flea [Echidnophaga gallinaceus) and the parrot stick-tight flea 

 [Hectopsylla psittaci) the front of the head is sharply angled. This 



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