Apion Herbst, 1797
This Palaearctic genus includes about 16 species, of which 9 occur in Europe and 5 extend north to the UK and Scandinavia. All members of the genus are quite distinctive, being entirely red but for the eyes and claws, and as such will not be confused with any other members of the family. They feed on various Polygonaceae; in Northern Europe on species of Rumex L. but elsewhere also on Polygonum L., Calligonum L. and Atraphaxis L. Larvae mine the stems, petioles and roots, causing galls, and more than one species may be present on a single plant. Adults occur year-round and during the summer may generate very large populations. With the exception of A. rubiginosum Grill, 1893, which is of local occurrence in southern and central England and Wales, our species are widespread and either generally or locally common.
Species vary from 2.2-4.5mm, A. frumentarium (Linnaeus, 1758) being our largest member of the family, and are elongate with weakly or strongly rounded elytra. Head quadrate to elongate, pronotum quadrate to transverse, both distinctly punctured, often strongly and densely so. The base of the head is often impunctate and transversely striate, eyes black and relatively large, varying from flat to very strongly convex and protruding, rostrum relatively short, cylindrical and thickened at the antennae insertions, and punctured, at least towards the base. Antennae long and slender, inserted behind the middle of the rostrum, with a distinct and fusiform 3-segmented club. Scutellum visible and usually elongate. Elytra with distinct and strongly punctured striae, interstices at least as wide as the striae, usually broader, finely punctured and pubescent, generally in 2-4 fairly distinct rows, and usually flat or only weakly convex on the disc.
Our UK species may be identified from the following key (adapted from Arved Lompe):
Temples entirely punctured.
Temples broadly smooth and transversely strigose towards the base.
Head more elongate; the temples about 1.5X longer than the eyes and the eyes and temples combined longer than the width across the eyes. Pronotum curved laterally to a strong subapical constriction and with a median longitudinal impression almost reaching the middle. 3.3-4.5 mm.
-A. frumentarium (Linnaeus, 1758)
Head less elongate; the temples at most only slightly longer than the eyes and the eyes and temples combined about the same as the width across the eyes. Pronotum less curved laterally, sometimes almost straight, hardly constricted before the apex, and with a shorter median longitudinal impression. 2.4-3.6 mm.
-A. cruentatum Walton, 1844
Rostrum straight or only weakly curved in lateral aspect. Head slightly elongate, pronotum quadrate or nearly so and elytra flattened dorsally and widest about the middle. 2.5-3.2 mm.
-A. rubiginosum Grill, 1893
Rostrum distinctly curved in lateral aspect. On average smaller species, 2.2-3.1 mm.
Head slightly elongate, pronotum at most only very slightly elongate, elytra broader and distinctly widest behind the middle. Body pale red with sparse and short pubescence. 2.2-3.1 mm.
-A. haematodes Kirby, 1808
Head and pronotum slightly transverse, elytra narrower and longer, more than 3X longer than the pronotum, and only weakly broadened behind the middle, sometimes almost parallel-sided. Body usually deeper red and with longer pubescence. 2.2-2.7 mm.
-A. rubens Walton, 1844