Aphelocheirus aestivalis

This species was first described in 1794 by Danish zoologist Johan Christian Fabricius, who studied specimens of it and believed it belongs in a genus known as Naucoris, giving it the scientific name Naucoris aestivalis. Fabricius did not list a specific locality where the specimens were found, only stating that they originate from the region formerly known as Gallia.^[5] However, it is most likely that the specimens originate from France.^[6] British entomologist John O. Westwood would later study more specimens of this insect collected across England and determine that the species belongs in a separate genus. He therefore established the genus Aphelocheirus in 1833, and renamed this species as Aphelocheirus aestivalis, making it the type species of this genus. The generic name combines the Ancient Greek words apheles (ἀφελής, meaning "simple") and cheir (χείρ, meaning "hand"), in reference to the simple structure of the insect's forelimbs.^[7] In 1965, it was reported that the holotype specimen of A. aestivalis studied by Fabricius had gone missing. Therefore, a female specimen collected in France and kept in the Hope Entomological Collections of the Oxford University Museum of Natural History was designated as the lectotype of the species.^[6][8]

Aphelocheirus is the only genus in the family Aphelocheiridae, whose closest known relatives according to phylogenetic analysis are the members of the family Potamocoridae.^[9] The genus is very diverse, with almost 60 species known from Afro-Eurasia alone, including A. aestivalis.^[10] Although A. aestivalis was historically thought to be the only species in the genus to occur in Europe, two additional species named A. murcius and A. occidentalis were reported from the Iberian Peninsula in 1989.^[11] The former of the two Iberian species is so similar to A. aestivalis that some experts have doubted whether they are truly distinct from each other. However, a study published in 2011 finds the two to be closely related but separate species based on molecular analysis and differences in male genital shape, and that a third, unnamed species (originally thought to be a population of A. murcius) is also present on the Peninsula. The cladogram below shows the results of the molecular analysis in this study:^[12]

Measuring 8.5–10 mm (0.33–0.39 in) in body length, Aphelocheirus aestivalis is medium-sized for an aquatic bug.^[12] Females tend to grow slightly larger than males. The body of this insect is flattened, and in the vast majority of individuals is rounded, with small, reduced wings. This form is known as the micropterous morph and is incapable of flight. However, the rarer macropterous morph has a more elongated, ellipsoid body, and is capable of flight due to its large wings.^[6][13] This species is mostly brown or greyish brown, though the head, legs and the edges of the body are yellow, and the eyes are a blackish colour. Entirely black individuals have also been recorded, but this colour is caused by a coating on top of the exoskeleton rather than pigments within it, and is most often seen in older insects. In some individuals, there are dark patches of colour on the head, or yellow markings or bands on the thorax and abdomen.^[6][12]

The head of this animal is slightly longer than it is wide, and bears flattened, narrow eyes. The part of the head made up of the frons and the clypeus (two of the hardened parts of the face) has a rounded cone-like shape pointing towards the front of the insect. Both the scutellum and pronotum (two parts of the thorax) are short and wide, with the latter bearing rounded lobes on the sides of its hind part. The abdomen has a serrated outline, with the serrations being formed by tergites projecting from the sides of each abdominal segment. The sternites on the underside of the abdomen each bear a slight protrusion, together forming a small keel running down the middle of the abdomen's underside.^[6]

This species is widespread, ranging across much of Europe and into northeastern Africa and southwestern Asia. It is found from the British Isles in the west across to the Caucasus and Ural mountains in the east. In Africa, A. aestivalis is known from Egypt, and its range in Asia includes Turkey and Georgia. In Europe, its range stretches from as northerly as southern Scandinavia to as far south as the northern areas of the Mediterranean basin.^[6] Although older sources may claim that it can be found in the Iberian Peninsula, these records likely actually represent the similar-looking Aphelocheirus occidentalis, and a 2011 study has confirmed that A. aestivalis does not occur in this region.^[11][12] While the flightless micropterous morph is found throughout the range of the species, the flighted macropterous morph has only been found parts of central and southern Europe.^[6][14]

Within its range, this insect mainly lives in stony rivers with fast-flowing, oxygen-rich water or in stagnant water sources such as reservoirs and lakes. It is a benthic species, living near the bottoms of water bodies with depths of 0.4–9.0 m (1 ft 4 in – 29 ft 6 in), where it occurs beneath plants, rocks or sediment.^[6] Though primarily a freshwater animal, it can also inhabit brackish water, as indicated by reports of specimens from the estuary of the River Seine in 1898, and from the brackish parts of the Gulf of Finland and Randers Fjord in 1943.^[15][16]