Revision of How to tell if its a Lonchaeid from Tue, 2021-04-06 19:02

In simple terms – how to tell a Lonchaeid
This is a simple guide to the family developed to help with some common mistakes made by contributors of photos to to Dipterainfo and iNaturalist.

Lonchaeids are small, stout-bodied, hairy acalyptrate flies – (that's without a large lower calyptere that covers the halter!)  with a body length oif 3–6 mm. They have broad wings, realtively large head , a broad thorax and a broad wide abdomen. The body colour is usually shiny blue-black except in genera Lamprolonchaea & Fulgenta where its metallic blue-green.  The wing is broad,  usually clear, only occasionally light brorown or with apical darkening, but never patterned; halter always black.

HEAD:  with eyes large, usually bare, occasionally hairy but never touching on top of head as in some other families. The frons is without without lateral setae; face broad usually without strong facial ridge or antennal grooves. Antennae with first flagellomere short and prominent, to long and decumbent, mainly black, but often orange-brown basally and / or ventrally; pedicel and scape usually darkened. arista bare to pubescent, or plumose.

THORAX:  with the top (mesonotum) rather strongly arched, slightly dusted  to highly polished; without a complete transverse suture, covered in fairly dense setulae, almost without setae as the dorsocentral and acrostichal setae both reduced to 2 pairs on rear of mesonotum. Scutellum with 4 marginal setae, with or without, additional setulae at margin and disc sometimes without anterodorsal setae, but these always present on posterior margin; katepisternum with 1–2 strong dorsocentral setae and numerous setulae..

WING:  Broad -  anal lobe and alula well-developed; costa (C) extended to vein M1, constricted, but not completely broken at position of humeral and subcostal breaks; Costal cell noticeably broad often causing the costa to bulge to accomodate it. Vein Sc complete and free from vein R1; pterostigmal section (between insertions of veins Sc and R1), short to long; cells bm and dm separated; cell cup present; A1 continued to, or near to, wing margin as a fold or not; apical section of vein CuA1 shorter than crossvein dm-cu. Wing membrane usually clear, but occasionally yellowish to dark fumose, rarely with apical darkening; upper calypter well-developed, with whitish to blackish margins and fringes, halter invariably entirely blackish.

Legs stout, coxae, femora, and tibiae always blackish; tarsi often with basal tarsomeres yellowish; femora rather swollen, with numerous setae and setulae; tibiae with numerous rows of setulae, but without setae on the shaft.

Confusion with other families
Lonchaeidae superficially resemble some small Muscidae but as well as not having calypteres Lonchaeidae are separable by the absence of a complete transverse suture on the mesonotum and other characters such as having more setae on the mesonotum and legs.

Agromyzidae, lonchaeids differ in having a longer cell dm (shorter than distal section of vein M in Agromyzidae) and usually strong lateral setae on the frons. Females with often with an oviscape, a nonretractable basal segment of the ovipositor which is different to the usually fully hidden one in Lonchaeidae, Anthomyiidae do not have a darkened costal section on the wing between Sc and R1, they have strong setae on hind tibia, almost always including two or more posterodorsals and they almost always have distinct vibrissae on the genae. A general caution should be taken towards certain dark Agromyzidae, with dark halteres. 

Lauxanidae – Lonchaeids can often be confused with some of the entirely black, shiny species of the family Lauxanidae Macquart, 1835 especially those in the genera Calliopum Strand, 1928, Lauxania Latreille, 1804, Minettia Robineau-Desvoidy, 1830 and Xenochaetina Malloch, 1923 in addition to the 2 pairs frontal of vertical setae found on the head of all Lauxanidae but these all have white halteres and often entirely orange antennae both of which features are not found in Lonchaeidae.

The metallic blue-green species in the genus Lamprolonchaea can resemble members of several other acalyptrate families, but the black halter and lack of setae on the tibiae are useful diagnostic features.

Scratchpads developed and conceived by (alphabetical): Ed Baker, Katherine Bouton Alice Heaton Dimitris Koureas, Laurence Livermore, Dave Roberts, Simon Rycroft, Ben Scott, Vince Smith