drains, canalizations, cesspools and dung-heaps in which it can 
wallow in abundance, and perhaps better protected against 
possible enemies. Different in this from other kinds of insects, 
which disappear with the culture of the land, /. tenax thus 
gained a new impulse, and spread in new countries with an 
astounding rapidity; it entered into a kind of commensalism 
with man, like the Musca domestica, Teichomyza fusca and some 
other dipterous insects, which are at present hardly found any- 
where, except among human habitations. It is very rare now 
to come across a carcase, and to see H. tenax hovering about 
it; the only instance I have found in the literature consulted 
by me concerns another species of Hristalis, EH. anthophorinus 
Zett., and that case occurred in a distant and primitive country. 
Zetterstedt, (Dipt. Scand. II, 666) being in Lapland, observed 
a small swarm of flies of this species round the carcase of a 
sheep. I shall give the detail of this observation in the Supple- 
ment VII. 
In an article entitled: ,Facts concerning the importation 
or non-importation of Diptera into distant countries“ (Trans. 
Ent. Soc. London 1884, p. 489—496), I have shown that we 
know very little about the laws which govern this class of 
phenomena; that importations will occur against all probability, 
and that, on the contrary, other importations which appear to 
us easy and probable, do not take place during centuries of 
intercourse. Since publishing this paper, I found in Darwin’s 
Life and Letters“ vol. HU, p. 248, in a letter of his to J. D. Hooker, 
the following passage which expresses the same idea: ,Now I 
maintain against all the world that no man knows anything 
about the transoceanic power of migration“ etc. Without enter- 
ing into the detail of this question, which is foreign to my 
present purpose, I will merely show the very interesting cir- 
cumstances of the immigration of Hristalis tenar into the New 
World. 
Before 1875 LV. tenax was known to occur in the temperate 
regions of the old world only. With the exception of the East- 
Indies, the Austro-Malayan Archipelago, and the greater part 
