on the larvae of Muscidae. 23 
M. corvina lays only 24 large eggs, of a peculiar structure, 
and the larvae undergo only two stages of development, the first and 
the third, omitting the second. This shortened mode of develop- 
ment enables larvae to reach their maturity quieker and this rapidity 
of growth, in the struggle for existence, is a compensation for the 
small number of eggs laid by the imago. 
We have seen that the carnivorous larvae of diptera are very 
prolific and that the struggle for existenee among them depends prin- 
eipally on the number of the eggs laid; it remains to point out the 
causes which prevent the coprophagous larvae from enjoying the 
same fecundity. — The number of species of carnivorous flies in 
our regions (that is of flies living on the soft and semiliquid parts 
of carrion) is not large, some nine species, if we take some rare 
ones into account (Oalliphora, several Lueiliae and Pyrelliae, two 
or three viviparous Sarcophagae and Oynomyia mortuorum). They 
have it all to themselves; their action must be rapid; food is plenti- 
ful; all there conditions favor multiplication. The numerous Co- 
leoptera and coleopterous larvae (Staphilinidae, Histri, Silphae ete.), 
oceurring about carcasses, compete very little with the dipterous 
larvae, but rather prey upon them, destroying multitudes and thus 
making room for new broods. The large number of eges laid in- 
volves their small size, and plentiful food favors a comparatively long 
period of development. 
Quite different eircumstances attend the development of the 
dipterous larvae living in dung; here, instead of plenty of elbow- 
room, there is a severe competition, and instead of abundance, scarcity. 
Numerous coleoptera (belonging at least to 14 genera) live on dung, 
but equally numerous are the genera and species of dung-flies 
(31 genera at least). This, from the very beginning, was a check 
to feeundity which, in the course of time, must have kept down and 
erowded out the too prolific species, while those with a less numerous 
progeny survived, because with the diminution of the number of eggs 
there was an increase of their size and a shortening in the period 
of development. Other conditions being equal a large egg gave a 
large larva, which required less food for its full development. This 
was an advantage which enabled the less prolifie flies to crowd out 
the more prolific ones. „The coprophagous flies offer us an instruc- 
„tive lesson of natural selection and of the law of mutual succour, 
„in virtue of which living animals of various species and genera, as 
„if by common consent, diminish their fecundity, to prevent a common 
„ruin and render cohabitation possible." 
The coprophagous diptera do not form a single, compact group; 
