Dr. Karl Krapelln on the Pdiclclae. 37 



among later investigators, by Landois and Tasclienberg. 

 But although the last two authors especially pronounced most 

 decidedly in favour of the independent position of the Fleas in 

 the system, and although the most accepted special works 

 upon the Diptera exclude the Fleas as not belonging to the 

 series of forms in that order *, we find that even in the most 

 recent manuals of zoology the group of insects in question is 

 almost without exception cited as a suborder of the Diptera. 

 This may pass in the first place as a proof that really stringent 

 arguments have not yet been brought forward in favour of 

 either view ; but we might also derive the hesitation felt by 

 many zoologists to raise the rank of the Fleas (even under 

 otherwise sufficient grounds) from the circumstance that they 

 lead a parasitic existence, and by this means have possibly 

 undergone profound and peculiar morphological changes by 

 " adaptation," as is sufficiently established for other groups of 

 parasitic forms. In opposition to this, however, it must be 

 remembered that with only isolated exceptions (the females of 

 the Sarcopsyllidffi) the Pulicidee are not stationary, but only 

 temporary parasites, that their whole development is completed 

 without parasitism, and that therefore we cannot well assume 

 any considerable adaptation to a parasitic mode of life. But 

 if this be so, if we succeed in proving that the Pulicidai possess 

 a series of morphological characters which cannot be regarded 

 as acquired by parasitism, we must necessarily, in judging of 

 their position in the system, consider the same points of view 

 to be prescriptive that have been generally adopted for the 

 establishment of orders, suborders, and families in the class of 

 insects. 



These general points of view, however, do not offer us a 

 very brilliant prospect. The Linnean principium divisionisj 

 the form, number, and texture of the wings, having proved to 

 be untenable, we find on the one hand the kind of transforma- 

 tion and its various stages, and on the othei the structure of the 

 organs of the mouth, raised into the most important criteria 

 of the nearer or more distant relationship of the groups of 

 insects. But, as is always the case, when a single character 

 is thrown too much into the foreground, and the general 

 morphological relations of the two series of forms are not 

 allowed to be prescriptive, difficulties make their appearance 

 even with these apparently so thorough-going principles of 

 division, which considerably diminish their value. The 



* It is interesting that the well-known work on the Diptera of the 

 ' Fauna Austriaca ' by Schiuer certainly expresses itself decidedly enough 

 in the above sense, but then gives a detinition of the true Diptera, which 

 might very well embrace the Pulicidae. 



