Heptageniidae Family - ID Tips
Class: Insecta; Order: Ephemoptera; Family: Heptageniidae
|
Vital Stats |
Rating |
Scale |
|
BMWP Score |
10/10 |
1 = Filthy to 10 = pristine |
|
LIFE Flow Group |
1/6 |
1= Rapid velocity (>100cm/sec) to 6= drought resistant |
|
Family Level Difficulty Rating |
2/10 |
1= Easy to 10= very difficult |
|
Species Level Difficulty Rating |
5/10 |
1= Easy to 10= very difficult |
The Heptageniidae belong to the group commonly known as mayflies . They do not tolerate polluted water so their presence in a waterbody would indicate a relatively pristine habitat. Despite their placing in the family level LIFE Flow Group which suggests the Heptageniidae do not tolerate water that is not fast flowing, some species can be found in lower reaches of rivers and in lakes; they do however have a strong preference for stony substrata.
Identification
Identification to family level for this group is fairly straightforward. All British mayflies are characterized by the presence of three cerci or ‘tails’. The Heptageniidae are made distinctive by their strongly dorso-ventrally flattened bodies and broad heads; the seven pairs of gills consist of a flat plate and a separate tuft of filaments.
Ecdyonurus spp.
The most recent edition of the FBA key for the Ephemoptera has some useful additional features for separating the four British Ecdyonurus involving the number of fine teeth on the tarsal claws and the pigmentation pattern on the tarsus . The traditional reliance on the relative shapes of the pronotum is subjective and fraught with difficulty. In fully mature larvae there are distinctive patterns on the underside of the abdominal segments and these can help to separate the species. If the patterning is not clear according to the descriptions below then it is likely the specimen is not fully mature and may not be identifiable to species. Figure 1 below shows the pattern present on Ecdyonurus torrentis. Note the pair of pale diagonal bars and dots on a dark background. E. Insignis has the same pattern with an additional lateral bar but the pigment is the negative of E.torrentis (dark bars and spots on a pale background). Ecdyonurus dispar has a horizontal dark line at the top of the segment but is otherwise pale throughout. The remaining member of the genus, Ecdyonurus venosus may be separated by inspection of the tarsus and claw as stated in the FBA key. It is the only species with two fine teeth on each claw (see Figure 2) and a single dark band located distally on the tarsus.
Figure 1. Ecdyonurus torrentis, ventral view
Figure 2. Tarsal claw of Ecdyonurus venosus showing spines
Heptagenia and Electrogena spp.
Heptagenia sulphurea is probably the most distinctive of this grouping by virtue of the high contrast black and white patterning on its legs and body (Figure 3 below). The much rarer (at least in England) H. fuscogrisea is distinguished from H. sulphurea by noting the reddish brown bands on the femora and the lack of a continuous fringe of setae on the femora (see diagrams in the FBA key). The patterning on Electrogena lateralis is much less conspicuous than on the two aforementioned species, however a light cross-shaped area separating four dark patches on the femur is easily visible (Figure 4 below). A further Electrogena species, E. affinis, has only been found in the River Derwent (Yorkshire), and according to the FBA key can be separated by the presence of white spots along the front of the head in E. affinis and by the number of spines on the tarsal claws (at least two in E. affinis, only one in E. lateralis)
Figure 3. Heptagenia sulphurea
Figure 4. Electrogena lateralis
Rhithrogena spp
The two Rhithrogena species are easily separated from the other genera by the presence of a central dot on the femora and the large, overlapping first pair of gills. Separation of the two species is more difficult and relies on the shape of the gills and mouthpart structures as detailed in the FBA key and not discussed further here.
Juvenile Heptageniidae
Some of the Heptageniidae genera are more difficult to split when the specimens are small due to characters being under developed. For example, juvenile Ecdyonurus lack the backward pointing projections on the pronotum and may be confused with Heptagenia or Electrogena. However, an angled notch on the thorax will still be present in Ecdyonurus (Fig ure 5) in comparison with a smooth S-shaped outline present on the other genera. A good additional character for separation of these three genera can be made by inspection of the ventral faces of the abdominal segments. In Ecdyonurus, a single ganglion clump is present on one of the distal segments (Figure 6). This is a visible as a darkened patch in juvenile specimens but fades as the specimen grows. In Electrogena lateralis a sequence of (usually fainter) patches can be seen on additional segments. (Figure 7) Heptagenia sulphurea lacks the patches entirely. Care should be taken not to misinterpret gut contents when looking for this feature.
Figure 5. Juvenile Ecdyonurus (right specimen), dorsal view, shown with juvenile Rhithrogena sp.
Figure 6. Juvenile Ecdyonurus, ventral view
Figure 7. Juvenile Electrogena lateralis , ventral view
Figure 8. Juvenile Heptagenia sulphurea, ventral view
References
Elliott J.M. & Humpesch U.H. (2010) Mayfly Larvae (Ephemoptera) of Britain and Ireland: Keys and a Review of their Ecology. Freshwater Biological Association. Scientific Publication 66
