DIPTERA > BRACHYCERA | Higher Diptera

MUSCOMORPHA > SCHIZOPHORA > Acalypratae > Sciomyzoidea > Coelopidae (3 species)

(subfamilies: none)

A small family of fewer than forty species worldwide which, up to a few years ago, was represented in Britain by just two closely related species Coelopa frigida and Coelopa pilipes as the latest member of the family Malacomyia sciomyzinae was formerly placed in the Helcomyzidae (Dryomyzidae > Helecomyzinae) - see the Dryomyzidae family account.

In the strict sense, these three coelopid species are commonly known as kelp or wrack flies, although they're often referred to in general terms as 'seaweed flies'. And, so they are, because although the formal taxonomic classification of species needs to be followed to maintain consistency, they are part of a community of strandline flies that live and breed on the seashore in close association with various species from other families.

The strandline habitat is described in detail a couple of pages further on for Heterocheila buccata (Heterocheilidae) so I won't repeat it here, preferring to utilise this space to describe the coelopid species. I'm also using this family as a convenient place to add a separate page where I can feature as many of the individual 'seaweed flies' that I can together, as it will provide a useful reference point for direct comparison rather than having to flit back and forth between families.    

Coelopa frigida - a dark-grey, generally bristly fly with a somewhat flattened body and particularly bristly brownish- orange legs. Although rather variable in size, Coelopa frigida averages slightly larger than Coelopa pilipes with males varying from 3-9mm and females 3-7mm. Described as the most widely distributed coelopid but, for some reason, with relatively few confirmed sightings from Dorset (VC9) and no records at all from South Hampshire (VC11), which is strange.

Coelopa pilipes - not dissimilar to the previous species, but easily distinguished as they have a covering of fine black hairs rather than bristles, especially noticeable on the legs. They also have three, rather than two, frontal bristles. On average smaller than Coelopa frigida with both sexes ranging from 4.5-7.5mm body length. Again, this species only has a handful of records from Dorset and none from South Hampshire according to both the NBN Atlas and iRecord databases.

Malacomyia sciomyzina - the sole member of its genus and far more likely to be mistaken for a small Heterocheila buccata rather than either of the much darker Coelopa species. It has an orange-yellow, rather than dark-grey, abdomen, and similarly coloured orange legs. The thorax has a grey or brownish-grey mottled appearance, which can be variable from one individual to another. Unfortunately, there are very few photos of live specimens, so it's hard to make comparisons, but the consensus of opinion is that the photos featured below are of this species. Almost certainly under-recorded, as there are currently only around eighty confirmed observations nationwide.

Although easier said than done, it's useful to try to get a nicely detailed lateral photo of the head of all these species as they have a distinctive jutting out chin, which in the case of Coelopa frigida is studded with coarse bristles, and in Coleopa pilipes with a profusion of fuzzy hairs. In many situations those species can be distinguished with confidence simply by the bristles or hairs on the legs, but with some individuals it may be less obvious, so you'd need a clear shot of the head to be sure. Malacomyia sciomyzina has a similar profile that is a good distinguishing feature in comparison to the much flatter face of Heterocheila buccata.

Not surprisingly, kelp flies are most commonly found on beaches with a steady supply of washed-up kelp as they are very much dependent on the decaying green and brown algae for both feeding and breeding. As with most strandline species they seek out Laminaria and Fucus seaweed, but the best beaches will change as the age, density and spread of this matter is strongly influenced by the weather and resultant sea conditions and tides. Similarly with the temperature, as lower temperatures allow larger accumulations of beached seaweed to remain fresh, whereas higher temperatures cause an acceleration in the decomposition of algae and increase in egg development. I guess that the temperature is relative as the adults can be found throughout the year, although I'm pretty sure you'd have trouble finding many during cold winter spells. I don't know how many trips down to the coast I'll make to try to photograph these species, but it will be interesting to keep a log of the conditions.

As previously noted, the following page acts as a gallery for the various 'seaweed flies' I've been able to photograph.

Updated, Dec.24 (v.2)