624 
DIPTERA. 
order to see them come hovering round it. The species of Drosophila 
which is commonest in Nothern India is shown in all stages on PL LXVI. 
The fly is interesting as having what is, as far as I know, the shortest 
life-history yet recorded for any insect with a complete metamorphosis. 
Though most abundant in the rains, it appears to breed all the year 
round. In one instance under observation the flies paired at 8 a.m. on 
August 31st, remaining coupled for eleven minutes, and the same even¬ 
ing the female laid thirty-nine eggs on some half-decayed bananas. On 
the afternoon of September 2nd the male fly died, and at 11*30 a.m. on 
the following day the female died also. The eggs had hatched during 
the evening of September 1st, the larvae all pupated on the 4th and 5th, 
and on September 7th the flies emerged : the whole life-history may thus 
be passed through within a week. Such rapid multiplication not only 
quite explains the abundance of the flies, but affords a very instructive 
example of the effective working of those conditions and influences 
which together make up what is called the £t balance of nature ’ 5 and 
tend to prevent the too great increase of any one species of animal. We 
will assume that these flies have thirty broods a year, each brood 
consisting of forty eggs, of which all develop to maturity, half males and 
half females ; a simple calculation will show that at this rate the 
progeny of a single pair would at the end of a year reach an appalling 
number. If they were packed tightly together so that each cubic inch 
of space contained a thousand of them, they would very easily cover the 
whole of India, from Kashmir to Cape Comorin, from Karachi to Cal¬ 
cutta, with a solid cake of flies a hundred million miles thick, or would 
coat the whole world with a layer of insects a million miles in depth. 
And yet as it is we do not particularly notice them. 
Owing to their fondness for fruit they are sometimes taken to be 
harmful fruit-flies by those who do not know their habits, but they are 
in reality beneficial, as they seem never to lay eggs in any but bruised 
or over-ripe fruit. The eggs themselves are beautiful objects, and the 
curious horns or processes which they possess appear to act either as 
breathing-tubes or as supports to prevent the egg from sinking too deep 
in the decaying pulp in which it is usually laid. The Indian species have 
not been studied ; the genera known to me at present are Drosophila , 
Curtonotum , and Asteia, the first two including most of the common 
forms. Curtonotum is very common in the hills. The fact that some 
