The chief characteristic of the Diptera is expressed in the name of the Order, since, with the exception of certain aberrant and apterous forms, flies possess but a single pair of membranous wings, which are attached to the meso-thorax.
Eaton discovered on the desolate shores of Kerguelen's 'Island apterous and semi-apterous Diptera (Tipulidae and jEph_ydridae) of a degraded type adapted to the climatic peculiarities of the locality.
All fruit and forest trees suffer from these curious insects, which in the female sex always remain apterous and apodal and live attached to the bark, leaf and fruit, hidden beneath variously formed scale-like coverings.
Wingless females of many tropical species present a close superficial resemblance to woodlice; and one interesting apterous form known as Pseudoglomeris, from the East Indies, is able to roll up like a millipede.
Lichtenstein has established the fact that from the egg of the Aphis of Pistachio galls, Anopleura lentisci, is hatched an apterous insect (the gall-founder), which gives birth to young Aphides (emigrants), and that these, having acquired wings, fly to the roots of certain grasses (Bromus sterilis and Hordeum vulgare), and by budding underground give rise to several generations of apterous insects, whence finally comes a winged brood (the pupifera).
The female is apterous, and has a dark-brown plano-convex body; it is found in the proportion of 150 to 200 to one of the male insect.
Some Hexapods which are very primitive in other respects happen to be also Apterous, but this cannot be held to prove that the possession of wings is not a primitive character of Hexapods (compare the case of the Struthious Birds).
Thus the apple blight (Aphis mali) after producing many generations of apterous females on its typical food-plant gives rise to winged forms which fly away and settle upon grass or corn-stalks.
In the autumn a single fertile egg is laid by apterous females in a crevice of the bark of the vine where it is protected during the winter.
From this egg in the spring emerges an apterous female who makes a gall in the new leaf and lays therein a large number of eggs.
Some of the apterous young that are hatched from these form fresh galls and continue to multiply in the leaves, others descend to the root of the plant, becoming what are known as root-forms. These, like the parent form of spring, reproduce parthenogenetically, giving rise to generation after generation of egg-laying individuals.
Perhaps, however, the name may only signify a large terrestrial biting apterous insect, surpassing the ant in size and predatory habits.