1876.] Nature's Scavengers. 169 
districts in tropical South America exceptionally healthy, 
and, by a curious coincidence, free from mosquitoes and 
other insedts of similar habits. Yet more, there are places 
— now and always healthy— where mosquitoes were formerly 
unknown, but where they have now been introduced, proba- 
bly by shipping, and appear to be gaining ground. Surely 
these considerations go far to disprove the notion of mos- 
quitoes being important sanitary agents, specially destined 
for the purification of pools and rivers. But there is yet a 
further objection, applying more or less to all natural sca- 
vengers which remove nuisances by using them as food. 
Granting that the mosquito in its larva state consumes a 
certain quantity of putrid or putrescent matter, there is a 
very considerable set-off. Its excrements, its cast-off skins, 
and finally its body when dead, are matter no less offensive 
and dangerous than the food which it has eaten. An insedt 
when dead is, size for size, as decided a nuisance as the 
remains of a larger animal. That pestilence has followed 
the death and decomposition of large armies of locusts is a 
well-known fadt. We cannot believe that the millions of 
dead mosquitoes can have any much more beneficial effedt. 
We are not aware of any Coleopterous, Hemipterous, 
Homopterous, Orthopterous, Hymenopterous, or Lepidopte- 
rous insedt that — either in the larva state or after reaching 
maturity — takes part in the purification of waters. Many 
species of the two former orders are aquatic, but they seledt 
living prey. 
Water-snails and other Mollusca may be ranked among 
the scavengers of the water, subjedt always to the limitation 
that they, in turn, produce a certain amount of pollution. 
They have, at any rate, this advantage over the mosquito 
and its Dipterous allies, that they do not issue from the 
waters and devote the rest of their lives to the annoyance 
of mankind. 
But besides solid suspended nuisances, in a coarser or 
finer state of subdivision, polluted water contains a certain 
amount of organic matter in a state of adtual solution, such 
as the extradtive matter of plants and the fluid secretions 
and excretions of animals. This soluble impurity cannot be 
eaten out of the water like suspended matter, and the bulk 
of the above-mentioned agencies are, therefore, useless for 
its removal. Fresh-water Mollusca may, perhaps, purify 
such waters to a small extent, but the oxygen liberated from 
aquatic plants is far more widely efficacious. 
But the worst remains to be told. So long as a stream of 
water is only moderately impure the plants and animals 
