Chloroperlidae Family - ID Tips

Class: Insecta; Order: Plecoptera; Suborder: Arctoperlia; Family: Chloroperlidae

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 1/10 1= Easy to 10= very difficult 
Species Level Difficulty Rating 3/10 1= Easy to 10= very difficult 

Chloroperlidae are a family of British stonefly (Plecoptera). The nymphs are found in very clean and fast flowing river habitats.  They can easily be separated from other stonefly nymphs by inspection of the last segment of the maxillary palp, which is reduced and only about ¼ as wide as the preceding segment. Chloroperlidae also have reduced glossae and no gills present on the thorax.

chloroperlidae 1.JPG

 

In the UK there are only two representatives of the Chloroperlidae family, previously these were both in the genus Chloroperla but have now been split into Chloroperla and Siphonoperla. There are a number of features given in the key and again it is a good idea to look at a combination of these to ensure correct identification, especially as it can be hard to distinguish features in younger specimens where some features may not have developed or be difficult to see.

In fully grown (7-10mm long) Chloroperlidae the area on top of the head behind the labrum can be used. The M-line (see page 9 in the FBA key for a diagram) in Chloroperla tripunctata is clearly visible as a boundary between a pale area at the front, which makes an ‘M’, and a darker area behind. This can be seen in the image below, it is easiest to look where the lighter and darker areas meet.

chloroperlidae 2.JPG

 

In Siphonoperla torrentium, this M-line is not visible, the front of the head behind the labrum generally uniform in colour.  See image below. 

chloroperlidae 3.JPG

 

Note in small or very pale specimens these head pattern differences may not be apparent..

Another, perhaps more reliable feature to check is the length of the posterior arm of the epicranial suture (where the sutures on the top of the head meet the back of the head capsule, see p9 in the FBA key).  In Siphonoperla torrentium the posterior arm of the suture is very short or absent, see image above.  It is quite hard to see in the photo, but under the microscope should be easily visible.  It may be necessary to push the head forwards from the thorax so that the full length of the suture is revealed.

In Chloroperla tripunctata, the posterior arm of the suture is longer, the key states ‘about as long as the distance from the fork to each lateral ocellus’, this is visible in image 2. Unless the specimen is very small, determining if this suture is small/absent or longer is generally quite easy and does not require measurement.

A final feature to check is the long fine bristles along the side of the pronotum.  This should be continuous in Siphonoperla torrentium and arranged in anterior and posterior groups on each side in Chloroperla tripunctata.  Unfortunately this feature is sometimes very unclear, probably because the setae have a tendency to fall out after preservation, while other times it works well. In the mature specimens used here, the feature is not clear, while the two groups can be seen in the Chloroperla tripunctata, the sides of the pronotum of the S. torrentium are mostly without bristles.

References:

Hynes, H.B.N (1993) Adult and Nymphs of British Stoneflies (Plecoptera) A Key. Freshwater Biological Association. Scientific Publication 17.