- 214 — 



long ago. It did not spread Eastward at once, because the ne- 

 cessary conditions for its existence were wanting on tlie im- 

 mense plains it had to cross, just as the Colorado beetle lived 

 in the Rocky Mountains on Solanum 7'ostratum, and did not 

 spread Eastwards until civilization brought the potato-plant 

 (Solanum tuberosum) with it, and thiis bridged over for that 

 beetle the distance between its native mountains and the 

 Atlantic coast. The condition which civilization brought, and 

 which favoured the rapid Eastward progress of E. tenax, 

 consisted in the drains, sewers and cesspools, those necessary 

 concomitants of crowded centresj and the usuai abodes of the 

 larva of Eristalis (1) 



The immigration of E. tenax into New-Zealand is of a 

 stili more recent date than that in North -America. The Cata- 

 logues of the N. Z. Diptera by Nowicky (1876) and Professor 

 J. W. Hutton (1881) do not mention it. It was first noticed 

 in AVellington (North Island) in October and November 1888. 

 In June 1890, Mr. W. W. Smith (Ashburton, South Island) 

 writes: « it is now widely dispersed and very plentiful in the 

 South Island » (Notes on Eristalis tenax in New Zealand by 

 W. W. Smith, in the Entom. M. Mag. London, 1890, p. 240-242). 

 From this article I borrow the foUowing passages: 

 « How long before the spring 1888 the speoies was intro- 

 duced into these islands I cannot say; it certainly could not 

 ha ve been many years; but the two previous seasons were 

 most unfavourable for its increase, being for the most part 

 very wet and windy; since tlien we have been favoured with 

 exceptionally good seasons.... » « The two hot, dry summers 

 in succession, with the intervening mild winter, produced, no 



(1) AH the details and references about the geographical distribution of .B;-j"sì«?ìs 

 tenax will be found in my two articles: 



1. Facts concerning the importation or non-importation of Diptera into distant 

 countries {Trans. Ent. Soc. London, 1884, p. 489-496); 



2. Some new facts concerning Eristalis tenax (Entom. 3Tonthly Mag. London, 

 XXIII, p. 97-99; 1886). 



