Nemouridae Family - ID Tips
Class: Insecta; Order: Plecoptera; Suborder: Arctoperlia; Family: Nemouridae
|
Vital Stats |
Rating |
Scale |
|
BMWP Score |
7/10 |
1 = Filthy to 10 = pristine |
|
LIFE Flow Group |
4/6 |
1= Rapid velocity (>100cm/sec) to 6= drought resistant |
|
Family Level Difficulty Rating |
4/10 |
1= Easy to 10= very difficult |
|
Species Level Difficulty Rating |
7/10 |
1= Easy to 10= very difficult |
The genus Nemoura is in the Nemouridae family, a type of British stonefly (Plecoptera) which is found in flowing freshwater across Britain. Nemouridae nymphs are separated from the other British stonefly nymphs by first checking that the glossae are as long as paraglossae, then checking that the second tarsal segment is shorter than the first. The hind legs can be stretched back beyond the tip of the abdomen. Here we concentrate on the identification of the Nemoura genus.
Identification
First the Nemoura genus must be separated from the rest of the Nemouridae family, the Nemoura don’t have any prosternal gills, see image below.
This separates Nemoura from Protonemura and Amphinemura, but not from Nemurella. To separate Nemoura from Nemurella you will need to look at segments 1 and 3 of the hind tarsus and the bristles on the upper side of the hind femora. Nemoura have segment 1 of the hind tarsus about 1/3 to 1/2 the length of segment 3, see image below, whereas they are about equal length in Nemurella. Small Nemourella picteti can be difficult to distinguish using this feature so it is best to look for the traverse row of bristles on the hind femora.
Nemurella have a well defined transverse row of long stout bristles on the hind femora which should be easily visible. Make sure you don’t confuse this with the transverse row at the top of the femur of Nemoura dubitans, in Nemurella the row is just below half way down the femur.
To separate Nemoura cambrica group (Nemoura cambrica and Nemoura erratica) from the other species can be difficult, especially with smaller specimens. The key gives a number of features to look at and it is best to look at all of these, although the hairs of the cercal segments can be misleading so don’t rely on this feature as a few hairs are sometimes visible on the cercal segments of N. cambrica group.
Nemoura cambrica group are a stout bodied species with relatively short legs whereas the other species are more slender bodied with longer legs. The photo below shows the stout bodied, relatively short leg species (below) and a slender longer legged species (above). Again this can be misleading due to the size of specimens and how stretched out they are in there preserved state.
Probably the most reliable feature to use in splitting Nemoura cambrica group from the other Nemoura species is the bristles on the upper side of the hind femora. Nemoura cambrica group have many long stout curved bristles, see below.
The other species in the Nemoura genus have fine straight bristles, some of which may be long, see photo. However although Nemoura dubitans is in the fine straight bristle direction at this couplet in the key, the species description says the bristles are stout and short! There are more notes on this later.
The intermediate hairs on the cercal segments are not a particularly reliable feature to go on, as some may be present, especially towards the distal segments on all species. The intermediate hairs are shown below.
Separating Nemoura avicularis, cinerea and dubitans.
Nemoura avicularis is easily identified by the antennal segments, the second antennal segment being a lot darker than the first.
Nemoura cinerea has the ocelli clearly visible as dark spots, and the second antennal segment it not darker than the first.
Nemoura dubitans is more difficult to identify as it doesn’t have the obvious distinguishing features of avicularis and cinerea, however once cambrica group has been ruled out, check the second antennal segment is not darker than the first and the ocelli should be visible but not darkened. The bristles of the upper part of the hind femora are stout and short in this species, however they should not be curved like Nemoura cambrica group, the key also says there should be a few which form a single row across the top of the femur. There should be long intermediate hairs on the cercal segments, and, apart from on the distal segments, this does not occur in cambrica group.
References:
Hynes, H.B.N (1993) Adult and Nymphs of British Stoneflies (Plecoptera) A Key. Freshwater Biological Association. Scientific Publication 17.
