Gyrinus substriatus Stephens, 1828
ADEPHAGA Clairville, 1806
GYRININAE Latreille, 1810
GYRININI Latreille, 1810
GYRINUS Geoffroy, 1762
GYRINUS Geoffroy, 1762
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.
The present species is generally distinctive enough to be recognized without dissection but several others are closely similar, especially G. natator (Linnaeus, 1758) which was last recorded in Britain during the 1920s but still occurs in Ireland, and G. distinctus Aubé, 1838 which remains local in Britain and Ireland.
5.0-7.0mm. A broadly-rounded species with the entire upper surface shiny black and vaguely metallic, the dorsal microsculpture and punctation being very faint. Lateral pronotal margins impressed from the base to about the middle, in natator they are impressed to the anterior angles. Scutellum flat or weakly convex but lacking a median basal ridge. Elytra with all striae comprised of well-impressed rows of punctures and very finely punctured interstices (X40), each with only a few rather random rows, in distinctus they are equally fine but much more dense, and 2 transverse series of punctures before the apex that are much stronger than those of the striae. Ventral surface dark with the mesosternum, terminal abdominal sternite, elytral epipleura and prosternal margins contrastingly pale, in natator the ventral surface is more extensively dark. The difference in elytral micro-punctures between the present species and distinctus can be subtle but the genitalia are very distinct; in substriatus the median lobe is gently narrowed to a truncate apex which is much narrower than the width of a paramere across the apex, in distinctus the median lobe is dilated towards the apex and about equal in width to apex of a paramere. There are also very subtle differences between the genitalia of substriatus and natator, especially in the female and reference to good photos or diagrams will be needed to appreciate this.