188 Transactions. — Zoology. 



It might perhaps be well to mention here that the universal 

 superiority which the northern animals exhibit when com- 

 peting with the natives of southern latitudes in the struggle 

 for existence is attributed by Darwin to the severe competition 

 which has so long existed amongst the organisms in the 

 northern hemisphere, chiefly owing to the great extent of the 

 land in those regions compared with isolated areas such as 

 New Zealand and other oceanic islands. It is needless to say 

 that we see the words of that great naturalist verified daily, as 

 both the native plants and animals are constantly being sup- 

 planted by northern forms. 



2. MUSCA VOMITORIA. 



During last June (1889) I observed on my sitting-room 

 window a specimen of this familiar European insect. Formerly 

 I have often been astonished at its absence from New Zealand, 

 seeing that the ubiquitous Musca domcstica so long ago ob- 

 tained a footing, and is now so exceedingly common every- 

 where. It is consequently somewhat strange that this almost 

 equally abundant species has until so very recently been 

 unable to procure a passage. However, now that it has arrived, 

 the process of naturalisation appears to be progressing rapidly, 

 as I have seen upwards of five specimens during September 

 in various locahties, including three in the Botanical Gardens. 

 In habits and appearance this insect closely resembles the 

 larger of our native species of flesh-flies, but may be easily 

 recognised by its pale-blue abdomen striped with black. It 

 will no doubt increase enormously' in numbers, and perhaps 

 largely, if not completely, supplant the New Zealand flies. 

 As all these insects have many generations in one summer, 

 the process of competition between the several species should 

 not be a difficult matter to observe. 



For those desirous of knowing the economic influence these 

 two new dipterous are Hkely to exert in New Zealand, it may 

 be satisfactory for them to hear that Eristalis tenax is quite 

 harmless in all its stages, the larvse feeding on the putrid mud 

 at the bottoms of ditches, where it obtains air by means of 

 a remarkable telescopic tube at its posterior extremity, capable 

 of being adjusted to the exact depth at which it happens to 

 be situated from the sm-face. As to Musca voviitoria, it is 

 only an additional scavenger who will industriously remove 

 effete animal matter from the surface of the earth, and as 

 such we need not feel at all sorry it has come here, for, 

 although the flesh-flies are no doubt often very inconvenient, 

 the immense sanitary benefits they confer on mankind can 

 hardly be overestimated. 



