40 Dr. Karl Kriipelin on the Pulicidae. 



nevertheless it is easy even here to recognize the typical 

 position of the pieces forming the sucking-tube (dorsallythe 

 labrum and ventrally the hypopharynx) , and the labium 

 which encloses these as a sheath. _ Further, the latter bears 

 at the end that enlarged portion which is so characteristic of 

 all Diptera, and which is probably to be interpreted as formed 

 by uniarticulate labial palpi. 



The same unity in the Diptera appears also in the special 

 structure of the thorax and its appendages. That this appears 

 always separated from the head by a deep incision is cer- 

 tainly not without significance ; but it can furnish no decisive 

 datum for the collocation of the Diptera. Of more importance, 

 no doubt, is the fusion of the thoracic segments into a compact 

 thoracic mass, which occurs in all the forms referred to this 

 group. It is indeed true that in orders of insects (I refer 

 particularly to the llhynchota) the formation of the thorax 

 as regards the separation or fusion of the segments composing 

 it shows manifold differences, without its being necessary that 

 we should separate forms which are united for other reasons, 

 seeing that the fusion or separation of the thoracic segments 

 has to do essentially with a function of the mechanism of 

 flight, and the free segmentation of the thorax in a wingless 

 form may very well be explained as a correlative phenomenon 

 of adaptation. But the conditions are different if, on the 

 contrary, a wingless form exhibits complete amalgamation of 

 the tlioracic segments. In my judgment it thereby demon- 

 strates most unmistakably its descent from winged insects, 

 and in this sense the compact structure of the thorax, with 

 the characteristic process of the mesothorax described as the 

 *• scutellum," in Aklophagus^ the Nycteribiidag, and the Brau- 

 lida3, decidedly acquires the significance of a still uneffaced 

 relationship with the winged groups standing next to them. 

 And just as on account of this character tlie assumption is 

 justified that the forms just mentioned stand in close phylo- 

 genetic r.-lationship with winged insects, so does the ex- 

 amination of the dorsal a])pendages of the thorax lead to tlie 

 same conclusion. All Diptera do not possess a pair of 

 wings and a pair of halteres ; but the two organs which, 

 because special, are certainly of such great importance in 

 characterizing the Diptera, disappear so gradually in the 

 continuous series of forms, that we may trace their progress 

 to the rudimentary state, as it were, step by step. An 

 Ornithohia pallida which, as Lipoptena cervi, follows a per- 

 fectly different mode of life, enables us at once to understand 

 the case, when we see Melophngus, which is never parasitic 

 upon birds, entirely destitute of wings. But as regards the 



