FLEAS 65 



In the case of the hen stick-tight flea, which is sedentary, copulation 

 often takes place between adjacent individuals, without either of them 

 detaching themselves from the skin of their host — although the male is 

 not permanently fixed hke the female. The process may last twenty- 

 five minutes or longer. 



It is easy to tell the sexes apart even without the aid of a hand lens, 

 as the end of the body of the male has a rather rakish upward tilt — 

 somewhat reminiscent of a drake's tail — whereas the female's body 

 merely narrows terminally. The external sexual organs or genitalia 

 are of primary importance in distinguishing between closely related 

 kinds, or species, of fleas. In fact among the bird fleas it is sometimes 

 the only practical way of telling them apart. These organs in the 

 male are fantastically complicated. The terminal segments are 

 modified for grasping the female, and the penis with its guiding 

 rods is itself a structure of extraordinary complexity — in fact it is the 

 most complex genital organ to be found in any insect. The more 

 one considers it, the more difficult it is to understand how such a 

 structure can have been evolved either by a series of mutations or by 

 natural selection, or by means of both. We have tried to understand the 

 way in which this apparatus worked from studying permanent prepara- 

 tions of copulating martin fleas, and we have puzzled over the sUdes for 

 hours. An American morphologist attempted the same study with 

 another species of bird flea. Although he elucidated many obscure 

 points much remained a mystery; he concluded his description with a 

 sentence which exactly expresses our views. " Truly," he wrote, "the 

 thing does not make sense." 



The genitalia of the females are much less compHcated but they 

 also afford a most important clue to classification and relationship. 

 The female flea has the capacity of storing the male sperm, and 

 releasing it at intervals as her eggs ripen. This enables her to lay 

 fertilised eggs as long as two months after copulation. The internal 

 organ which receives and stores the sperm from the male, the recepta- 

 culum seminis or spermatheca (unpaired in all British bird fleas) is 

 chitinised and thus visible from the outside in cleared and mounted 

 specimens (Plate XVII). In outhne it roughly resembles a barrel- 

 shaped flask with a thick neck. The subtle diff'erence in the proportions 

 of these two parts affords the simplest character for distinguishing 

 between the females of closely related species (Plates XIII and XIV). 

 In Britain where one is concerned with relatively few fleas, it is easy 



