DIPTERA > BRACHYCERA | Higher Diptera

MUSCOMORPHA > SCHIZOPHORA > Calyptratae > Oestroidea > Polleniidae (8 species)

(subfamilies: none)

The Pollenia clusterflies, so named due to their habit of forming 'clusters' when hibernating, were formerly part of the Calliphoridae, but are now separated and placed within their own family. Whilst these particular flies are found outside during the warmer months of the year, they usually migrate indoors as soon as the temperature drops. They find their way in through small gaps in walls or under tiles favouring lofts and roof spaces where they may be found in large numbers. Once they've located a suitable place to overwinter, they omit a pheromone that attracts more individuals. In some situations, the numbers can increase to the point of becoming an infestation, which often remains undetected until there's a warm sunny day when they suddenly become active. Obviously, this makes them a pest species, but unlike the Calliphoridae blowflies, they do not pose a health risk as their eggs are laid outdoors in soil or leaf-litter, not in exposed human food or in uncovered pet food bowls.

They are distinctive, broadly-built, rather flattened flies that, when fresh, have a characteristic covering of crinkly, golden yellowy-brown hairs over the top and sides of the thorax. However, these hairs can quickly rub off giving them a rather different look, which can make quick visual identification in the field less obvious, although it's rare to find an individual without some of these hairs still being visible. In all other respects, the general features appertaining to the calliphorid species apply.

There are eight described species that are all very similar in appearance, such that proper and reliable identification is only possible with microscopic examination. Nevertheless, there are a few useful features to be aware of that may be visible in photos, which can help distinguish certain species.

Pollenia labialis is a small to medium (4-9mm), rather variably-sized species that resembles the far more common Pollenia rudis. It can be distinguished by the unique combination of having dark, coal-black basicostae together with brown, rather than yellowish, posterior thoracic spiracles. Also, a relatively short and weak fascial ridge and third antennal segments that are only red at their extreme base and slightly more than twice as long as the second segment. I'm not sure how many of these features can truly be seen in either the featured photograph or others I took of this particular individual, but I think it's a 'likely' assessment.

Pollenia angustigena also resembles Pollenia rudis, but differs in having just one anterodorsal seta on the mid-tibia whereas Pollenia rudis has two or occasionally three. The following photo is reasonably clear in that respect, but probably not reliably so, plus there are other details such as the short hairs on the hind femora being pale rather than dark that certainly can't be determined. The single anterodorsal though is an important feature, which is why I've tentatively suggested that the individual could be this species.

My thinking here is the same as usual, in that if I took the safe approach of labelling them all as Pollenia sp., I'd never get to study the descriptions and features to appreciate the subtle differences. A case in point is that Pollenia angustigena males can pretty much be distinguished by their narrowly separated eyes, which is why I keep going back to the fifth photo featured here, especially as the mid-tibia again appears to have just one anterodorsal. However, both of those features are also present in Pollenia griseotomentosa and when I look at the description of that species, such as the male having a noticeably large and globose head and a barely discernable fascial ridge, I could be convinced that it's also a possible candidate. And, to support that thinking, this species is further described as having comparatively short legs with somewhat shortened front tarsi that are only about the same length as the tibia. That seems to be true looking at the first photo, but I'm mindful that Pollenia griseotomentosa is one of two species that have never been officially recorded in Dorset.

It becomes a bit more difficult from there except for Pollenia amentaria, a particularly broad-bodied species with males that have rather large heads and correspondingly large eyes that almost touch. But, the most characteristic feature are the black tergites that lack any obvious dusting. It is said to be more frequent in the west and north rather than locally but, according to the NBN Atlas, it's the second most recorded clusterfly in South Hampshire sharing that position with Pollenia angustigena.

The most frequently recorded in both South Hampshire (VC11) and Dorset (VC9) is Pollenia rudis, which unfortunately is almost impossible to identify from photos as most of the important features are associated with various leg bristles. Some of the other species can be eliminated to narrow down the options, but it's too easy to make assumptions that an individual is probably Pollenia rudis just because it's the most likely species in most areas. That's certainly the situation with females, but the larger males can sometimes be distinguished by their broader build in conjunction with eyes that are almost but not quite touching, a comparatively broader frons about as wide as the third antennal segment, two or three anterodorsals on the mid tibia and, from a side shot at the right angle, numerous long erect hairs on the ventral surface of the hind tibia. During a trip to Garston Wood I found quite a few males that were congregating and looking carefully at the various photos taken on the day I think that Pollenia rudis is the most likely species.   

Two reasonably common species I haven't mentioned are Pollenia pediculata and Pollenia viatica. The former should be considered if there's enough detail to see a swelling at the junction of the humeral cross-vein and the subcosta with a small tuft of pale hairs underneath, coupled with a rather broadened distinct facial ridge that is somewhat flattened on top. Other features apply, but those are the important ones to look out for. Pollenia viatica is trickier because, apart from a very broad and low facial ridge and palps that are mostly yellow rather than black, it's back to leg bristles that, in my opinion, are rarely going to be clear in photos.

For completeness, the final species is Pollenia vagabunda, which appears to be extremely rare as it has only been officially recorded a handful of times from a few scattered locations over the past fifty or so years, and never from Dorset or any neighbouring counties.      

The following photos refer, including a late season male photographed in the garden which is a good example of an individual where the golden yellow-brown hairs have almost completely worn away. I could have included more images of different individuals for comparison but, in truth, they all look so similar that there would have been little point.

Updated July 24 (v.2)