Stoneflies (Plecoptera)

The bodies of stoneflies are soft, cylindrical and even flattened. The adults have a pair of wings which are equally large although the males have wings which are smaller than the females. When at rest the wings of larger species, e.g. Dinocras, are folded over the back but in smaller species, e.g. Leuctra fusca (the needle-fly), they are sometimes slightly rolled up. The antennae are long and there are usually 2 tails at the end of the body although these can be absent. The larvae are aquatic and may have a pair of separate wing buds (photo above) and, in comparison with the mayflies, they have only 2 tails and no gills along the length of the body. Gills maybe present in large species, e.g. Perla, under the base of the legs (photo below). Flight is not particularly strong or directional.

The 150 species found in Europe are very widespread wherever water is plentiful. They can be found during most months in spring, summer and even autumn. A common group of insects found in and around most ponds, lakes, streams and rivers. However, their main preference is for fast flowing rivers in uplands.

Adult stonefly after flying, the wings are still note back in place over the body

ECOLOGY: As a group they are little known by people and easily over looked. The adults live for around 2 - 3 weeks and are common in vegetation around water. The larvae take a year to develop although in some cases this may extend to 2 or even 3 years. They are strongly developed larvae able to cope with the fast flowing currents of upland rivers. The larger species are predators feeding on mayflies, mites and other small invertebrates. The smaller species may be herbivorous feeding on algae. The adults lay the eggs on the water washing them off the body. Like mayflies they ecdyse many times (up to 35 instars) and when ready to emerge as an adult the larva climbs out of the water on to a rock nearby. From here it sheds its exoskeleton for the last time.

Stonefly exuvia (shed exoskeleton) on a rock by the side of a river. The adult has emerged and left the skin lying stuck to the rock.