calypter

From Greek kalypter (καλυπτη'ρ), covering, sheath.

calypter noun - a small membranous flap at the base of the hind edge of the wing in some flies; it covers the halteres. Also called the alula, though an alula is only a calypter if it covers the haltere.

Calyptratae (singular Calyptrata) are a group of flies (diptera) that have calypters. The Acalyptratae are a group that lack calypters.

"The posterobasal portion of the axillary membrane, joining the hind margin of the wing to the thorax, forms two basal lobes called the calypteres (sing. calypter; squamae, squamulae). The proximal lobe, called the lower calypter (basicalypter, squamula thoracica), begins as a narrow, membranous ligament arising from the furrow between the scutellum and the postnotum and ends where the more distal lobe, the upper calypter (disticalypter, squamula alaris), folds sharply over it. The upper calypter is believed to be homologous with the jugal region or neala of higher insects, e.g. Neoptera, including the jugal lobe of the Mecoptera (Hennig 1973). It is usually larger than the lower calypter, but in some groups, e.g. Tabanidae, Acroceridae, and many Calyptratae, the lower calypter is larger than the upper one. The fringe of hairs along the posterior margin of each calypter is called the calyptral fringe, and the fold between the two lobes is the calyptral fold." McAlpine et al 1981 page 29

See page 30 of Manual of Nearctic Diptera (various authors)--available as PDF's from the Entomological Society of Canada