IS90.] 241 



new facts concerning Eristalis tennx,''' by Baron Osten-Sacken (Ent. 

 Mo. Mag.,xxiii,97),in which the distinguished Dipterologist discusses 

 the rapid dispersion of the species over the North American continent. 

 After discussing the causes favouring the dispersion of the species in 

 North America, he remarks : " it will be interesting in this respect to 

 watch whether it will ever be imported into distant islands, like New 

 Zealand, St. Helena, &c." On November 10th, 1888, I took two 

 specimens ( ^^ ? ) on the flowers of Veronica Andersoni. Last year I 

 was in Australia throughout the summer, but my boy has five speci- 

 mens he took in January, 1889 ! and saw several others, while all 

 through the last season, from November, 1889, until a fortnight ago, 

 the species has appeared in great numbers over the whole extent of 

 country on the east coast of the south island. 



As I have shown the fly to have appeared simultaneously in the 

 north and south islands, or, at least, to- have been observed in both 

 about the same date for the first time, it will be well to consider the 

 causes favouring its sudden appearance in, and rapid invasion of, New 

 Zealand ; and, at the present time, when so much stress is laid by some 

 entomologists on climatolog}^ in producing much of the varied pheno- 

 mena of insect life, it seems to me that the speedy diffusion of E. 

 tenax over New Zealand is a case strongly favouring this theory ; of 

 course, all insect life mainly depends on certain climatological con- 

 ditions for its existence, but exceptional conditions of climate, whether 

 long continued, or changing, cause the diminution or increase of 

 certain species ; by the former cause I propose to account to some 

 extent for the sudden irruption of E. tenax in New Zealand in 1890. 



How long before the spring of 1888 the species was introduced 

 into these islands I cannot say ; it certainly could not have been many 

 years ; but the two previous seasons were most unfavourable for its 

 increase, being for the most part very wet and windy ; since then we 

 have been favoured with exceptionally good seasons. The summer of 

 1889 was hot and dry, and was followed by an exceedingly dry, mild 

 winter, while the long season now closing has been a continuance of 

 hot, dry weather, from early spring until the present time ; both the 

 introduced and indigenous flora have bloomed profusely throughout 

 the season, and have been visited by vast numbers of insects, indeed, 

 it has been phenomenally an entomological year in New Zealand, and 

 one of great swarms of Diptera ; it has been the best season for col- 

 lecting for the last decade, as insects of all Orders have appeared in 

 great numbers. 



Baron Osten-Sacken, in referring to the sudden appearance of 



