DIPTERA > BRACHYCERA | Higher Diptera

MUSCOMORPHA > SCHIZOPHORA > Acalyptratae > Ephydroidea > Diastatidae (6 species)

(subfamilies: none)

This family includes six very small, greyish or brownish-grey flies in the Diastata genus, which are closely allied with two, not dissimilar Campichoeta species. Although some authors treat Campichoeta as a subfamily, they are now separated and placed in their own family, Campichoetidae. The recognition of having two families was principally based on characters in the genital structure, but the genera also differ in that, whilst having similar wing venation, Diastata has two distinct costal breaks, whereas Campichoeta lacks the humeral break.

The biology of the family is still questionable, although it is likely that the larvae are saprophagous in decaying vegetation and possibly deadwood as the adults are typically found in associated habitat.

Although I'm sure that these species are very much overlooked and under-recorded simply due to their small size, the available distribution data strongly suggests that only four of the six species are likely to be found locally. The most frequently recorded species, both nationwide and across my general area, is Diastata adusta, closely followed by Diastata fuscula. Thereafter the number of confirmed sightings drop off quite significantly, but with Diastata costata and Diastata nebulosa also requiring consideration.

The original 'key' confirms that all six species have very similar appearance; face with a silvery or greyish sheen; frons and antennae are yellow; the third antennal segment may be darkened dorsally and apically in fuscula and nebulosa or largely brownish in adusta; occiput mainly grey dusted; thorax thickly grey dusted with darker shades in adusta, and indistinct brownish stripes in nebulosa; legs are pale yellow, with darker shades in nebulosa or entirely brownish in adusta.

The current family description further confirms that all these species are very small (2.5-4mm); the arista is short to medium plumose; ocellar bristles present; postvertical bristles converging; two pairs of frontal bristles, the outer pair curving forward, the inner pair backward; interfrontal bristles absent; vibrissae present; tibiae with dorsal preapical bristle; wings with various patterns of dark markings, although in some species clear, but cell C always darkened; costa with both humeral and subcostal breaks; vein Sc incomplete; cross-vein dm-cu present; cell cup closed.

More specifically, courtesy of Steven Falk's accounts:

Diastata adusta - readily distinguished by the small dark wing blotch centred on cross-vein dm-cu, and lack of any darkening of the subcostal area beyond the tip of R1. The femora are blackish and the tibiae paler brown. Could only be confused with the very scarce Diasta vagans, which has the entire costal area darkened and the legs yellowish. Widespread and common in damp habitats, especially where rushes are numerous - locally that would probably include wet heathland, valley mire, damp and/or marshy woodland, general wetland areas.

Diastata fuscula - the only one of these species without blotches on the wings or a distinctly darkened subcostal area; although the wings are slightly darkened between the costa and R1 and the cross-vein rm-cu is very faintly darkened. Common in dry woodland, especially on low foliage.

Diastata costata - in this species the wings are clear like fuscula, but the subcostal area is darkened; the antennae are entirely yellow, whereas in fuscula the third segment is darkened dorsally as can be seen in the following photos. Described as widespread, but local, occurring in both broadleaved and coniferous woodland or within areas of well- established scrub. There have only been ten confirmed sightings of this particular species in Dorset (VC9) at the time of writing, and only one in South Hampshire (VC11).

Diastata nebulosa - this species has strongly marked wings and could only be confused with Diastata ornata, which seems to only occur in northern and western areas. That said, Diastata nebulosa also shows a more northern and western bias and, although there are a few records from west Dorset, I'm aware that at least one has been observed towards the outer range of my general area, so I guess it's always a possibility. It favours similar wet and boggy habitat as Diastata adusta.

New family, added Nov.24