4 8 
THE TASMANIAN NATURALIST. 
When present the wings are four in number, and are membranous, 
the finer parts being strengthened by comparatively strong veins or 
nerves. On each side there are small processes that enable the wings 
to interlock, so that the two wings on each side beat as one. 
Probably no order of insects has excited greater interest than the 
Hymenoptera, owing to the social habits of many of the bees, ants, and 
wasps. And it is well known that in their nests ants frequently 
harbour other insects, which are useful to them, such as aphides and 
mealy bugs, who give off sweet secretions that are palatable to the ants. 
Diptera, or True Flies. 
This order is readily distinguished from the others by its members 
possessing only two wings when adult; these in structure somewhat 
resemble those of the Hymenoptera. The missing pair is represented, 
however, by two small processes known as halteres, that project from 
the side of the body behind the wings. When adult they are provided 
with a sucking apparatus or tongue, but some species have a whole 
armoury of lancets, spears, etc. 
The metamorphoses or changes are complete, the larva usually 
being in the form of a legless maggot; and the pupa as an elliptic 
object, protected on the outside by the hardened skin of the maggot. 
A large proportion of 
the species live in decaying 
vegetables or animal matter 
during their larval stages, 
which are frequently of short 
duration. Many others are 
parasitic within the bodies 
of other insects, and some¬ 
times of Irogs, birds, and 
even of domestic animals. 
Some live within the tissues 
of living plants, and a few 
are openly predacious. 
During comparatively re¬ 
cent times it has been proved 
that some tropical diseases, such as malaria and yellow fever, are almost or 
entirely caused by the attacks of flies, especially mosquitoes ; and great 
parts of Africa are caused heavy losses by the tsetse flies, which attack 
men, horses, and other animals, who are killed by microbes injected into 
their systems by the bites of flies. It is now known also that the 
common house fly has been the cause of much of the typhoid, con¬ 
sumption, diphtheria, and other diseases that kill thousands of people 
every year. Bubonic plague has been often directly traced to the bites 
of fleas (an aberrant group of flies), and lately fleas have even been 
blamed for the spread of leprosy. 
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