538 On Dipter a as Spreaders of Disease. [September, 
Metschnikoff, who is experimenting upon it, hopes to extir- 
pate the Phylloxera , the Colorado beetle, &c., by its agency. 
Coming to better-known and still under-valued fly- 
destroyers, we have interfered most unwisely with the 
balance of Nature. The substitution of wire and railings 
for live fences in so many fields has greatly lessened the 
cover both for insectivorous birds and for spiders. The war 
waged against the latter in our houses is plainly carried too 
far. Whatever may be the case at the Cape, in Australia, 
or even in Southern Europe, no British species is venomous 
enough to cause danger to human beings. Though cobwebs 
are not ornamental, save to the eye of the naturalist, there 
are parts of our houses where they might be judiciously 
tolerated : their scarcity in large towns, even where their prey 
abounds, is somewhat remarkable. 
But perhaps the most effectual phase of man’s war against 
the flies will be negative rather than positive, turning not so 
much on putting to death the mature individuals as in 
destroying the matter in which the larvae are nourished. Or 
if, from other considerations, we cannot destroy all organic 
refuse, we may and should render it unfit for the multiplica- 
tion of these vermin. We have, indeed, in most of our 
large towns and in their suburbs, abolished cesspools, which 
are admirable breeding-places for many kinds of Diptera, 
and which sometimes presented one wriggling mass of larvae. 
We have drained many marshes, ditches, and unclean pools, 
rich in decomposing vegetable matter, and have thus nota- 
bly checked the propagation of gnats and midges. I know 
an instance of a country mansion, situate in one of the best 
wooded parts of the home-counties, which twenty years ago 
was almost uninhabitable, owing to the swarms of gnats 
which penetrated into every room. But the present propri- 
etor, being the reverse of pachydermatous, has substituted 
covered drains for stagnant ditches, filled up a number of 
slimy ponds as neither useful nor ornamental, and now in 
most seasons the gnats no longer occasion any annoyance. 
But if we have to some extent done away with cesspools 
and ditches, and have reaped very distinct benefit by so 
doing, there is still a grievous amount of organic matter 
allowed to putrefy in the very heart of our cities. The dust- 
bins — a necessary accompaniment of the water-carriage 
system of disposing of sewage — are theoretically supposed 
to be receptacles mainly for inorganic refuse, such as coal- 
ashes, broken crockery, and at worst the sweepings from the 
floors. In sober fadt they are largely mixed with the rinds, 
shells, &c., of fruits and vegetables, the bones and heads of 
