importation or non-importation of Diptera. 493 



distant regions, except S. carnaria. The single instance 

 in Macq., Dipt. Exot., ii., 3, p. 95, that this species had 

 been received from Haiti requires further confirmation). 



It would seem, therefore, that there are some obstacles 

 to the introduction of this species by means of shipping 

 into distant countries. It may be that the chances of 

 transportation are diminished in this case owing to the 

 circumstance that S. carnaria, although living about 

 human dwellings, does not often appear within them ; 

 and for this reason, even if it made its appearance on 

 board ship, would be less apt to remain within it than 

 the other house-flies. But as this reasoning is not 

 applicable to the larvae, the case remains doubtful. 



Similar remarkable cases of non-importation may be 

 quoted in the vegetable kingdom. A great many of the 

 common European weeds have been imported into the 

 United States, and have to a considerable extent, espe- 

 cially in the environs of towns and villages, almost 

 crowded out the native flora. One might naturally have 

 expected that the poppy and the blue-bottle {Centaur ea 

 cyaniis) would be, with the cereals, among the first 

 arrivals. Nevertheless these two weeds have never 

 spread in America. 



3. PSILOPUS FALLENS. 



This is an instance of importation under the most 

 improbable circumstances. The dolichopodid PsHopus 

 pallens, Wied., although it occurs along the Atlantic 

 coast of North America, has the general appearance 

 (colouring, &c.) of a European Psilopus, quite different 

 from the numerous American representatives of that 

 genus, which have the more brilliant colouring of 

 tropical species. Wiedemann received it from North 

 America more than half a century ago. When I sent 

 Dr. Loew the first specimens which I found, he wrote 

 me that they were absolutely identical with a species 

 which he had found on the island of Rhodes {Psilopus 

 alhonotatus, Loew). This was remarkable, but I have 

 gathered some facts since which bridge over the distance 

 between Ehodes and New York. In the collection of 

 Mr. Bergenstamm, in Vienna, I have seen specimens of 

 the same species from Barcelona ; and Mr. Van der 

 Wulp informed me that it has been found in Amsterdam 

 and in Haarlem (c/. Tijdschr. voor Ent., xi., p. 20). 



