500 C. E. Osten-Sacken's essay on 



mesopleural perform the same office in front ; the meta- 

 pleural fan of bristles (in certain families) acts as a 

 screen in front of the halteres, &c. Hence the persistency 

 of the certain bristles in the same places, not only- 

 through the immense divisions of the Calijj^trata and 

 Acalyptrata, but even among more distant families, like 

 Asilidce and DoUchoindidce ; hence also the possibility of 

 a uniform terminology. 



Still, this hypothesis of Macquart's does not explain 

 how certain families can exist without any macrochaetse 

 at all. The integuments of a Tahanid are apparently 

 not harder than those of a Tachina, and yet they are 

 unprotected by bristles. We may perhaps get over this 

 difficulty by observing the different mode of locomotion 

 of different groups of Diptera. There is a well-marked 

 contrast in this respect between the aerial Diptera, which 

 are most of the time on the wing and use their legs 

 merely for alighting (Tahanidce, Bomlnjlidce, Si/rphidce), 

 and the pedestrian Diptera, which use their legs for 

 running or seizing their prey, and fly only at intervals 

 (most of the Cyclorhapha, except the SyrphidcB ; among 

 the Orthorhapha, the Asilida and Doliclwpodidce) . The 

 pedestrian Diptera are those principally provided with 

 macrochsetaB, probably because in their mode of life — 

 running upon leaves, struggling with their prey, or 

 ovipositing on caterpillars — they are more exposed to 

 contacts and collisions than the aerial Diptera. The 

 latter are not only in this respect less exposed, but most 

 of them possess the power of regulating the momentum 

 of their flight, which involves the faculty of poising 

 themselves in the air. Observe the flight of a Syrphus, 

 the cautious way in which he turns round a solid object 

 and repeatedly touches it with the tip of his tarsi, with- 

 out alighting, and compare it to the headlong flight of a 

 Calliphora. The most bristly of all the Diptera and the 

 least cautious in their flight are the Calyptrata, those 

 very flies which C. C. Sprengel, in his 'Das entdeckte 

 Geheimniss der Natur, 1793,' called the "stupid flies" 

 (die dumme Fliegen), for their clumsiness, their inability 

 to discover honey in flowers, and the ease with which they 

 are deceived by odours and appearances. Stratiomyidce, 

 TahankUe, BomhyUda, SyrpludcE, all have the power of 

 poising, and all are absolutely or nearly eremochaeta. 

 Thereikhe and Empidce, which also have that power, are 

 provided with only very few macrochastae. I assume. 



