Cryptic Critter - ID Tips

Different species of Hydropsychidae are often found dominating different reaches of a river catchment. This spatial partitioning knowledge can often help seasoned field scientists to identify what likely species are to be found in the upper, middle and lower reaches of a river.

However, species of Hydropsychidae need further analysis in the laboratory for confirmation.

The main key available for use by taxonomists is; ‘A revised key to the caseless caddis larvae of the British isles’, J.M. Edington & A.G.Hildrew (2005), which provides excellent feature characters to follow in the identification of Hydropsychidae species. However, as with many taxa, the features that need to be identified are not always easy to see on Hydropsychidae. There are some additional features or tips which are not included in Edington & Hildrew, but can also assist in identifying the species found.

Hydropsychidae can be identified to species level as follows:

• Looking at the front edge of the pronotum for the presence of long bristles. If these are present, the species is:

Cheumatopsyche lepida.

One aspect not covered by Edington & Hildrew is that the region of this character often has lots of debris attached to the long hairs (Figure 1), which is usually more apparent than the hairs themselves. The species is also often fairly pale and quite small in comparison to other Hydropsychidae species.

Figure 1

• If no long bristles are present on the front edge of the pronotum, but transverse sutures are present on the thoracic plates, then the species is:

Diplectrona felix

This species is often found in the highest reaches of river catchment. It does not have light markings on the thoracic plates or head, but is darker and larger than C.lepida.

• If none of the features above are present, but the 7th abdominal segment is devoid of gills (Figure 2), then the species is:

Hydropsyche siltalai

Figure 2

This species is fairly common, and often has pale head markings on the dorsal surface.

• If the species has none of the above features, having gills on the 7th abdominal segment, and pale posterior prosternites**, then the species is either:

Hydropsyche contubernalis or Hydropsyche bulgaromanorum

The species always found is H. contubernalis, and H. bulgaromanorum is now unlikely to occur in the UK. Hydropsyche contubernalis usually has distinct pale areas on the head and ornate black and yellow markings on the thoracic plates, making it easy to separate out from other Hydropsychidae species in the initial sort. Hydropsyche bulgaromanorum has a bump at the front of the submentum, (found at the front of the underside of the head).

**The Posterior prosternites are found on the underside of the first thoracic section to the rear of the first set of legs (Figure 3). This feature is often hidden by a fold of skin or debris, and so the area may need brushing off and the body segment stretched out carefully to get a good view. The posterior prosternites are often not fully formed in shape when the specimen is not fully grown and may be difficult to identify when pale. However, the prosternites are shiny and can be seen more clearly by adjusting the angle of the light source.

The Edington & Hildrew key splits into two areas at this point. Species that have:

1. Posterior and anterior pale patterns separated.

And

2. Posterior and anterior pale patterns joined.

However, pale patterning on the head can be highly variable, well developed, over developed or undeveloped in some species. This can make speciation difficult from this point!

Following 1

• If the posterior prosternites are slender, entirely dark in the central and lateral areas (Figure 3), then the species is:

Hydropsyche angustipennis

Figure 3

The prosternites are very clear and uniformly dark, but caution needs to be followed with smaller specimens where the prosternites may not be as clear.

• If the prosternites are bicoloured, then the species are:

Hydropsyche pellucidula & Hydropsyche instablis

H. pellucidula sometimes, (but not always!) has a Y or bar shaped pale posterior pattern on the dorsal surface of the head, and the frontoclypeus has a V-shaped outline. This second feature is widest at the front of the head. The head patterning can be very variable in this species and requires caution in identification and avoiding confusion with species further in the Edington & Hildrew key!

H. instablis, does not appear to be as common, and sometimes, (but not always!) has a U shaped pale posterior pattern on the dorsal surface of the head. The frontoclypeus has a U-shaped outline, is shiny and devoid of hair. This feature appears widest at the centre of the head. As a result the head can appear enlarged and slightly flattened in comparison.

Following 2

The species found are;

Hydropsyche exocellata, Hydropsyche fulvipes and Hydropsyche saxonica

These species are very rarely found and the first may no longer be found in the UK. However, using the Edington & Hildrew key, the most reliable differentiation between these species is likely to be a combination of the extent of dark and pale areas found on the underside of the head and the shape of the submentum.

Reference

Edington, J.M., Hildrew, A.G. (2005) ‘A revised key to the caseless caddis larvae of the British isles – with notes on their ecology’ Freshwater Biological association. Scientific Publication 53.