Chironomidae

Non-biting Midges (1)

DIPTERA > NEMATOCERA | Lower Diptera

CULICOMORPHA > Chironomoidea > Chironomidae (628 species)

(subfamilies: Buchonomyiinae, Chironominae, Diamesinae, Orthocladiinae, Podonominae, Prodiamesinae, Tanypodinae and Telmatogetoninae)

Although chironomids superficially resemble some closely related species, such as the phantom midges (Chaoboridae) and biting midges (Ceratopogonidae) or even mosquitoes (Culicidae), they can be distinguished by a combination of characters. The wing venation is a good place to start as chironomids have a maximum of seven veins reaching the margin, together with a simple M vein and a costal vein ending before or at the wing tip. Another good visual feature is that the postnotum usually has a longitudinal median groove. The wings lack scales and, being non-biting species, the mouthparts are reduced, which clearly separates them from mosquitoes and, to a lesser degree, from phantom midges. It's the larger biting midges that are more likely to be confused but, in the Ceratoponidae, the M vein always divides forming a long fork.

The adults can generally be described as minute to medium-sized (1-10mm) delicate, gnat-like midges, typically with slender wings, abdomen and legs; a yellowish to greenish, brownish or black body, which invariably has contrasting longitudinal dark stripes on the scutum; a head that is often overhung by the thorax; ocelli absent; mouthparts poorly developed; antenna generally with 13-17 segments with conspicuously long whorls in the male; postnotum (subscutellum) usually with a distinct longitudinal groove; wings usually clear, although in some species (eg. Tanypus, on the next page) with markings or (eg. Anatopynia, Pentaneura or Stictochironomus) with dark staining; vein M1+2 simple, never forked (not to be confused to the junction with crossvein RM).

The larval habitats are varied, mainly aquatic, but there also some that are terrestrial living in decaying vegetation, roots, rotting wood or dung. Most of the aquatic larvae occur in stagnant or sluggish waters, although with the Orthocladiinae species and some Tanypodinae they are far more likely to be found in streams or fast-flowing rivers. The adults tend to favour damp habitats, usually in close proximity to the larval breeding grounds, where they will seek out a sheltered spot, happily resting for long periods on foliage or, in the case of some species, on flowers where they may take nectar or pollen - most though, don't appear to need any nourishment during this stage of their life-cycle. When seen at rest they can usually be recognised by the characteristic forward-pointing position of their front legs, and by the way they hold their wings at an angle over the body, somewhat tent-like, rather than overlapping.

During the warmer months of the year, it's not unusual to see large swarms gathering just before dusk. Usually this is near water, where the males will fly up and down rhythmically trying to attract a female. If mating occurs, the pair will drop to the ground or onto adjacent vegetation. Some species will actually swarm over the water close to the surface, whilst others may only be seen in large gatherings at higher altitudes, possibly around tree tops. It's also not unusual to see smaller clouds of some species during the day in shady spots in woodland and other sheltered habitats.

Given the large number of species, it's no surprise to learn that identification from photos is notoriously difficult unless you're very familiar with the group and, even then, it may only be to the subfamily or, in respect of Chironominae, to one of the two major tribes. And, like a number of other Diptera groups, whilst the males of most species can be determined from external characters, the females generally can't. When considering the subfamilies, it's worth noting that although there are now eight on the current British list, virtually all of the species you're likely to find are going to be within either the Chironominae, Orthocladiinae or Tanypodinae.

Prior to updating this page I'd accumulated quite a few notes that, given sufficient photographic detail, could help when trying to determine the correct subfamily. There are also a few pointers that could lead to a tribe, possibly the genus or, very rarely, the actual species. It's useful to have them written down in one place in some form of logical order, which is what I've attempted to do on the following page - a personal aide-memoire that I can adapt or add to as I gain more knowledge. That said, I do need to clarify that some of this information has been from feedback given by various experts on the UK Diptera FB group, mostly from Dr Tony Irwin who is one of the country's leading authorities on these species. He also provided the confirmed identity of all the named species featured below.

A quick look at the NBN Atlas database really highlights the problems with identification because, although there are currently in excess of 60,000 confirmed sightings, over 45,000 at the time of writing are at family level, which is an almost pointless statistic. Furthermore, the following 5000 or so records are only for one of the primary subfamilies or tribes, which in order were Tanytarsini (Chironominae), Orthocladiinae, Chironomini (Chironominae) and Tandypodinae. From there it doesn't take much working out to realise that the numbers of remaining records for defined species are actually very low. The vast majority of species have less than 50 confirmed sightings and hundreds of those have less than 20, which are almost certainly from an expert who has identified a specimen under a microscope. The most frequently observed, with just over 400 records, is Chironomus plumosus commonly known as the Buzzer Midge, which is the largest species in the family with a length of up to 12mm, so obviously one that's going to be noticed. After that it's the Chironomus genus for specimens that can be reliably identified to the genus, but not beyond. The numbers then drop off quite quickly, but I found it interesting to note that most of the species I've had identified by Tony Irwin were close to the top of the list.