Gyrinus substriatus Stephens, 1828

This native Palaearctic species is generally common throughout Europe extending east through Siberia and Mongolia, it is also widespread across North Africa and Asia Minor and extends north to the Arctic Circle; in the UK it is locally common or abundant throughout, including all the islands, north to Shetland and is by far our most common member of the family. Adults are active from March until October and occur in most permanent and still or slow-moving water bodies; fresh as well as brackish and both shaded and fully exposed e.g. woodland and moorland ponds, but they usually avoid large wave washed environments and we have occasionally found them during the summer in temporary woodland ponds and among dense vegetation beside fast-running streams, they usually occur in numbers and from late summer very large aggregations may be observed. They are diurnal and predatory, spending much of their time gyrating rapidly on the water surface, and dive readily when alarmed, they carry a small bubble of air on the abdomen which is replenished continuously as they swim and so they can remain submerged for long periods, prey items include insects and carrion on the water surface; we have often observed them around a decaying fish etc. and they will attend in numbers to feed. Mating occurs in the spring and is easily observed on the water, oviposition occurs in late spring; eggs are laid singly on submerged vegetation or stones etc. and larvae emerge after about 2 weeks. Larvae develop through the summer and are predatory on other insect larvae etc. which they find by crawling among aquatic vegetation or swimming although they are poor swimmers and generally remain among stems and foliage, they pass through 3 instars and are fully developed after about a month. The fully-grown final instar larva leaves the water during the summer and constructs a cocoon from plant debris and substrate, usually beneath the soil but sometimes against plant stems etc. and the adult emerges after a week of two. Larvae are distinctive, elongate with a narrow head displaying large curved mandibles, and an elongate prothorax which is narrower than the meso- or metathorax, long legs and long filamentous gills to each abdominal segment, they are thus fully aquatic and do not need to surface in order to breath. Dispersal occurs mostly in the autumn when the adults leave the water and fly during the evening and at night. The vast majority of gyrinids sampled will be of this species.