Nature's Scavengers . 
175 
1876.] 
of the bat or the swallow, and as decidedly like the flight of 
the pigeon, undertaken merely for exercise or amusement. 
We have also to point out the very peculiar statement 
that the flies in well-ventilated rooms are found to be lean. 
We have yet to learn that flies attach themselves perma- 
nently to certain rooms or houses. We see them flying in 
and out at doors and windows, always, if provisions are not 
plentiful, ready to wing their way to <£ fresh fields and pas- 
tures new.” It must, likewise, not be forgotten that the 
house-fly is not the only insert given to hover about with no 
ostensible purpose save amusement. Who has not seen the 
gnats dancing on a fine evening, and generally in a very 
analogous manner, taking their centre from the top of some 
projecting objeCt, such as the summit of a tree, the top of a 
chimney, or even a gate-post. Are these, too, capturing 
microscopic cells and germs, either with their mouths or by 
adhesion to their limbs ? If so, they go to work in a very 
foolish manner. Were they to disperse, instead of collecting 
in thousands in one small spot of air, their probability of 
meeting with good sport would be much greater. The cock- 
chaffer, also (Melolontha maialis s. vulgaris), hovers — or, as 
the Germans more happily say, <£ schwarmt ” — in hundreds 
over the tops of trees in fine spring evenings. Is it cell- 
hunting ? Alas 1 the farmers and gardeners know too well 
what is its diet 1 
From all these considerations we feel bound to declare 
that the claim of the house-fly to be admitted as an aerial 
scavenger is not made out, and that — regard being had to 
its known habits as a propagator of disease and carrier of 
septic matter — the advice to spare or protect it is most inju- 
dicious. The pious adage that everything is of some use may, 
perhaps, be accepted if qualified by the equally true state- 
ment that everything is of some detriment, and that the evil 
in many cases largely overbalances the good. So far as we 
are then aware, there is no animal engaged in the task of 
removing aeriform or volatile nuisances, or in clearing the 
air of minute suspended solids. 
There is still a further objection to the view that flies be- 
come lean in well-ventilated rooms for lack of floating germs 
to feed upon. Prof. Tyndall finds that in air kept perfectly 
motionless all suspended solid matter is totally absent, and 
that in such air putrefaction does not occur. The more 
quiescent, therefore, the air of any apartment, the fewer 
suspended germs, cells, or spores will be present, and the 
leaner, instead of plumper, will be the flies ! 
Summing up the faCts which we have thus briefly recorded, 
