Sunday, 10 September 2017

Understanding Eristalis: some simple rules

Eristalis is one of the most frequently encountered hoverfly genera, with two species (E. tenax & E. pertinax) occurring almost entirely throughout the year. Four further species are common from April until October (E. arbustorum, E. hortiicola, E. intricaria & E. nemorum. The remainder are scarcer (E. abusiva & E. rupium) or vanishingly rare (E. cryptarum & E. similis).

Although widespread and often abundant, they are far from easy to identify but a few simple rules help to arrive at a diagnosis on many occasions. Nevertheless, even the most experienced specialist will stop short of making a firm diagnosis on those occasions where critical characters are unclear. It is important, therefore, to be aware of the limitations of what can be done and to err on the side of caution if critical characters are not adequately depicted. So, where should one start?

The first rule is to avoid making a diagnosis based solely on abdominal patterns or wing shades. Eristalis are infinitely variable in both respects and so these characters are rarely reliable diagnostic characters on their own. The complications arise because there is seasonal and sexual polymorphism in many species. Attention needs to be paid to the colour and shape of the legs, the extent of dusting on the face and the shape of the stigma on the wing. The other useful feature is the size of the animal, but judging this from photographs is always problematic so it is not terribly helpful in many cases where a photograph or series of photographs is involved.

The one exception is Eristalis intricaria which is a bumblebee mimic and is most frequently confused with Volucella bombylans and perhaps Merodon equestris. Unlike the latter two, E. intricaria has partially yellow hind tibiae and the scutellum is paler (the colour under the hairs and not the hairs themselves).

Leaving aside Eristalis intricaria, we need to think about a logical sequence of diagnosis. So, where to start?

There are two species with nice obvious characters that are usually visible if a sequence of photographs is taken from various angles: the colour of the front feet and the shape and colour of the hind tibia. So, to get started, we can say:

1a. Hind tibia completely dark (may be slightly paler towards the junction with the femur). Hind tibiae slightly curved and with feathering. Eyes with a strong band of dark hairs (visible in most good photos). ……………………………….Eristalis tenax
1b. Hind tibia partially yellow towards the junction with the femur (usually about 25% of the tibia). Eyes lack dark hair band …………….couplet 2

2a. Front and mid tarsi yellow or orange (beware occasionally muddy but never fully darkened on any segments). ……………………………….Eristalis pertinax
2b. Front and mid tarsi with some segments dark…….. couplet 3

3a. Wing with stigma tightly quadrate – not diffusing or extending towards the junction of vein R1 with the Costa. Face with a distinct un-dusted central stripe …………Eristalis nemorum
3b. Wing with more extended stigma …………………….Couplet 4

4a. Face completely dusted, hind metatarsus enlarged and stigma diffusing towards the junction of vein R1 with the costa …………………………….Eristalis arbustorum
4b. Face with obscure undusted stripe or with a distinct un-dusted stripe ………. Couplet 5

5a. Wing with a strong area of infuscation (clouding) that is often blackish in places …… Couplet 6
5b. Wing may be somewhat clouded, especially around the veins but lacking distinct clouds. This leads into the problem area where we often cannot be sure what we have.

6a. Hind metatarsus dark. Wing cloud distinct but not extensive …….. Eristalis horticola
6b. Hind metatarsus pale yellowish. Wing cloud often extensive and dark …….. Eristalis rupium

From this point on, life gets very complicated.

Some Eristalis arbustorum have partially rubbed faces and making sure this is rubbing and not a facial stripe is often a problem. Separating E. arbustorum from E. abusiva is very tricky and is dependent upon the length of the hairs on the arista and the colour of the middle tibia (partially darkened in E. arbustorum and almost clear yellow in E. abusiva). We do see exceptional photographs that capture these characters nicely, but they are the exception and so many of these examples only get as far as Eristalis sp.

Very occasionally, we see specimens of what appear to be quite large animals with dark feet but no band of eye hairs and a very distinct pale base to the tibiae. If the thoracic pleurae are well depicted, and the hind femur is clearly shown, then sometimes we can detect ashy dusting of the pleurae and of the hind femur – Eristalis similis; but beware – confusion with (especially) E. nemorum is possible and I have seen quite eminent Diptersts make this mistake (and have done so myself).

Finally, there is Eristalis cryptarum. In theory this should be easy to identify from photographs because it has completely orange hind tibiae. In practice, I'm not sure that it would be recognised as an Eristalis from some angles and on the one occasion I encountered it in the wild I did not recognise it immediately! This one needs careful checking because it is confined to a very small part of southern Dartmoor!

In making a diagnosis, it is always worth bearing in mind that there are further species in Europe that complicate matters. Although we do not expect them to occur in the UK, they might just be here and undetected; that would complicate matters very seriously!

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