242 i September 



this Dipterou in all parts o£ the United States (I. c, p. 98), says: 

 " The conditions which civilization brought and which favoured the 

 rapid eastward progress of HJ. tenax consisted in the drains, sewers, 

 and cesspools, those necessary concomitants of crowded centres, and 

 the usual abode of the larvse of Ei'istalis.'''' These facts supply an 

 important feature of the case, but another cause must have operated 

 to produce the sudden appearance of the species in 1884 over the 

 vast continent of North America ; whether the hypothesis I am about 

 to offer to account for the irruption of Eristalis this year in New 

 Zealand will explain the American case I cannot say ; possibly the 

 American Dipterologists will refer to their notes on the seasons pre- 

 ceding it, and enlighten us on the subject. 



Now, it is well known that hot, dry weather is peculiarly adapted 

 to the development and life habits of Diptera ; the two hot, dry 

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

 doubt, the natural conditions for its complete and rapid establishment 

 in N. Z. ; I am supported in my views by the unpleasantly numerous 

 cases of typhoid, dysentery, diarrhoea, and, of course, the inevitable 

 La Grippe ; the presence of these diseases betokens the presence also 

 of the natural conditions requisite for the larvae of E. tenax, viz. : 

 stagnant pools, ponds, drains, and sewers, &c., caused by the abnormally 

 dry season, while the creeks and rivers have not been so low for 

 twenty-seven years. 



The introduction of foreign species into the fauna of any region 

 requires careful attention, especially the causes and conditions 

 favouring their development, as such cannot fail to be of interest and 

 importance to future workers in the group to which they belong. 



I have endeavoured to show what appears to me to have been the 



chief cause of the irruption of E. tenax in N. Z. in 1890 ; it, however, 



leaves unexplained how and when the species was introduced ; but, in 



the absence of positive proof, I think it probable that it came to N. 



Z. from the Pacific coast, the numerous intervening groups of islands 



would provide a ready means of dispersion if the necessary conditions 



for its larval stage existed in the islands ; nevertheless, I think that 



in all probability it was imported direct by the mail steamers which 



have plied monthl}' between San Francisco and Auckland for the last 



twenty years ; from this source unquestionably came Carpocapsa 



fomonella in imported apples to New Zealand ; possibly we may soon 



have records of the occurrence of E. tenax in other islands in the 



Pacific. 



Ashburton, N. Z. : 



June 12t/i, 1890. 



