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egg is first hatched and a maggot is produced, which is called in
technical language a larva. This hatching of the egg takes place in
warm weather in from eight to twenty-four hours after the eggs are laid.
The larva, which prefers dark, moist places, continues in this state
about five days, during which time it moults or casts its skin twice. At
the end of this time, it is full grown, having reached the size of ½-inch.
At the end of the larval stage it seeks a dry place, and encases itself
with a dark reddish-brown covering, which is called a puparium, in
which condition it remains at rest a few days, gradually undergoing the
change from the pupa to the developed fly. This period is known as
the pupal stage.
If the temperature is high, the perfect fly emerges from its sac
between the 3rd and 4th day after pupation, by pushing off the anterior
end of the pupal case. Flies become sexually mature in from 10 to
14 days after emergence from the pupal state, and four days after
mating they are able to deposit eggs.
Provided the conditions are favourable, the whole development from
the egg to the perfect fly may be accomplished in about 9 or 10 days,
and 14 days later the second generation may be depositing eggs.
The fly is constantly feeding, and it seems to be indifferent as to
whether it feeds on the putrid contents of the dust-bin or the food on
our tables; and it will pass from one to the other with the utmost
impartiality. It is this special feature of its life which renders it so
dangerous.
The fly cannot take solid food. It must dissolve all solids before taking
them into its digestive system ; and this it does by means of its saliva,
which is produced in abundance by its salivary glands, of which it has
two. The ducts of these glands are connected to a single tube which
opens into the mouth. On a suitable stimulus, the saliva is secreted,
and poured out upon the particle the fly desires to dissolve and absorb.
The fluid food is then sucked up by a powerful sucking arrangement,
with which it is provided, first into its crop, then at leisure into the
proventriculus, and afterwards into the stomach, where it undergoes
digestion. Another prominent habit of the fly is that of returning its
food from the crop, in the form of large drops of liquid. Some of these
drops are deposited on the surface upon which the fly is resting. It
matters not where that may be, whether on the rim of the milk jug or
the dust-bin. These drops are known as "vomit spots." In most cases
they are re-absorbed by the fly. The "vomit spots" are distinguishable
from the excreta spots by being of light colour, the latter being brown.