3° 2 
The Two-winged Flies 
Fig. 41 i. — Head, antennae, 
and beak of mosquito, lat¬ 
eral aspect. 
called balancers, or halteres, whose use seems to be chiefly that of orienting 
or directing the fly in its flight. The possession of these balancers 
is a certain diagnostic character in distinguishing Diptera from all 
other insects. The wings are membranous and usually clear, and 
supported by a few strong veins. No flies can bite in the sense 
of the chewing or crushing biting common to beetles, grasshoppers, and 
other insects with jaw-like mandibles, but some have mandibles elongate, 
slender, and sharp-pointed, so that they act as needles or stylets to make 
punctures in the flesh of animals or tissues of plants. The great majority 
of flies, however, have no mandibles at all and no piercing beak, but lap up 
liquid food with a curious folding fleshy proboscis, 
which is the highly modified labium or under-lip. 
They feed on flower-nectar, or any exposed sweet¬ 
ish liquid, or the juices of decaying animal or 
plant substance. To take solid food as the 
house-fly does from a lump of sugar, the solid 
has to be rasped off as small particles which are 
either dissolved or mixed in a salivary fluid 
that issues from the fleshy tip of the proboscis. 
All the Diptera have a complete metamorphosis, the young hatching 
from the egg as footless and often headless larvae (maggots, grubs), usually 
soft and white, and in many cases ob¬ 
taining food osmotically through the 
skin. The life-history is usually rapid, 
so that generation after generation suc¬ 
ceed one another quickly. Thus it may 
be true, as an old proverb says, that 
a single pair of flesh-flies (and their 
progeny) will consume the carcass of 
an ox more rapidly than a lion. The 
pupae of the more specialized flies are 
concealed in the thickened and darkened 
last larval moult, the whole puparium 
looking much like a large elliptical brown 
seed. 
The Diptera include the familiar 
house-flies, flesh-flies, and bluebottles 
of the dwelling and stables; the horse-flies and greenheads, that make 
summer life sometimes a burden for horses and their drivers; the buzzing 
flower- and bee-flies of the gardens; the beautiful little pomace-flies with 
their brilliant colors and mottled wings that swarm like midges about 
the cider-press and fallen and fermenting fruit; the bot-flies, those disgust- 
Fig. 412.—The blow-fly, Calliphora ery- 
throcephala. Larva, pupa, and adult. 
