CHAPTER XXIV 

 FLEAS 



DAVID Harum says, " A reasonable amount of fleas is good 

 for a dog. They keep him from broodin' on bein' a dog." A 

 goodly supply of fleas might likewise keep man from brooding over 

 anything deeper than the presence of these fleas, but in many 

 cases this in itself is a rather serious thing to brood over. Not 

 only are fleas very annoying pests and a common cause of in- 

 somnia, but they may also serve as the disseminators of a number 

 of serious human diseases, among which the terrible bubonic 

 plague stands foremost. 



General Structure. Fleas are insects which are more or less 

 distantly related to the Diptera or two-winged flies, but which 

 have become so specialized by their particular mode of life as 

 external parasites as to necessitate their segregation into a dis- 

 tinct order of their own, the Siphonaptera. Their bodies are 

 ordinarily much compressed to facilitate gliding between the 

 hairs or feathers of their hosts. The head is broadly joined to 

 the thorax, which is relatively small. The abdomen is large and 

 much compressed from side to side; it consists of ten segments, 

 the first seven of which are simple rings, each protected by two 

 horny plates, a dorsal " tergum " and a ventral " sternum " 

 (Fig. 177). The last three segments are modified differently in 

 the male and female in connection with the sexual organs. In 

 both sexes the " tergum " of the ninth segment has a pitted area 

 covered with little bristles which is called the pygidium, and is 

 probably sensory in function. All parts of the body are furnished 

 with backward-projecting bristles and spines which aid the flea 

 in forcing his way between dense hairs and in preventing him from 

 slipping backward. The efficiency of these spines is apparent 

 when one attempts to hold a flea between his fingers. Many 

 fleas have specially developed, thick, heavy spines arranged 

 in rows suggestive of the teeth of combs and therefore known as 

 ctenidia or " combs " (Fig. 179). Such a comb may be present 



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