FLEAS 63 



facilitates their close, and in the case of the female, permanent attach- 

 ment to the host by their mouth-parts. A similar modification (Plate 

 XI), but less pronounced, can also be observed in the common 

 rabbit flea {Spilopsyllus cuniculi) which is partly sedentary, and in the 

 related shearwater flea {Ornithopsylla laetitiae). 



Fleas are descended from winged ancestors (see p. 73) and the 

 thorax still bears a strong resemblance to the thorax of flying insects. 

 It has become secondarily adapted to support the jumping legs of the 

 flea. It consists of three segments which are broken up externally into 

 different sized chitinous plates thus giving it the appearance of a sort of 

 crazy pavement. In the hen stick-tight flea, and the parrot stick-tight 

 flea, the thorax is greatly reduced and the three segments are narrow 

 and crowded together. These fleas have lost the power of jumping and 

 consequently there is a corresponding reduction in the huge muscles of 

 the thorax. 



The respiratory organs of a flea consist of a network of tubes, which 

 end blindly, known as the tracheal system. The air enters through the 

 external openings, the spiracles, which are conspicuous features along 

 the sides of the body, and is carried to all the tissues by the ramifications 

 of the tracheae. 



In certain sedentary fleas, the spiracles on the thorax have been 

 lost and are now represented by mere pin-point depressions in the 

 cuticle, while those on the abdomen are greatly enlarged. 



The chitinous external portions of the spiracles are, of course, rigid, 

 but just below the surface is a complicated apparatus whereby the 

 tubes may be shut. A rhythmical opening and closing of the spiracles 

 can generally be observed, which is associated with the inflation 01 

 deflation of the main trunks of the tracheal system. Sometimes when the 

 flea takes a lot of exercise, or is ripening eggs, the first and eighth 

 abdominal spiracles, which are much larger than the rest, remain open 

 continuously. 



This type of respiration is obviously quite different from that of 

 vertebrate animals such as birds or mammals. Human beings are apt 

 to regard their own personal structure as "normal" and everything 

 that differs from it as distinctly humorous. It is difficult for them to 

 realise that fleas breathe through holes in their sides, have a nerve cord 

 below their stomachs and a heart in their backs; or that certain other 

 arthropods lay eggs through their elbows, urinate through their heads 

 and regularly practise virgin birth. 



