Description

Adult Lymexylids are small to medium sized beetles, 5-40mm, slender and very elongate, parallel-sided and at least to some extent dorsally flattened, most are drab coloured and finely punctured and pubescent. Most species are soft-bodied and superficially Cantharid or Clerid-like in appearance e.g. Elateroides Schaeffer, 1766, but some tropical forms are heavily-sclerotized and resemble Melandryids e.g. Melittomma Murray, 1867. The head is broad, short and visible from above, only rarely strongly deflexed, and with large or very large and conspicuous eyes which form the antero-lateral angle; in Melittomma they are very convex and occupy the entire side of the head, the vertex is smooth and variously, although usually finely, punctured and microsculptured, the only exception being the genus Elateroides where there is a small epicranial pit at the centre, and most species have long and curved or tattering temples; in Lymexylon Fabricius, 1775 they are constricted, forming a distinct neck. The antennae are 11-segmented and short, sometimes very short, filiform to serrate or, rarely, flabellate; when filiform the basal segment is generally elongate, the intermediate segments quadrate and the terminal segment moderately long and pointed, they are never expanded apically or clubbed. In some species they are sexually dimorphic. The mandibles are short and robust, rounded laterally and sharply pointed, the maxillary palps are 4-segmented and generally sexually dimorphic; simple in most females but elaborately developed in most males e.g. Elateroides where segment 3 is expanded into a large plumose structure, the palporgan. In some species they are much reduced or even missing in both sexes. The pronotum varies from transverse to elongate and is always near-parallel in form, the anterior angles tend to be only weakly-defined and the posterior angles near-perpendicular, the basal margin is often bisinuate and there are distinct lateral borders but never lateral teeth or spines. The surface is generally smooth, without any larger structure, although it may be weakly rugose or have small flattened tubercles or small raised areas, and in Atractocerus there is a deep longitudinal impression towards the base. The pro- and meso-coxae are cylindrical and projecting, the meta-coxae cylindrical and often produced posteriorly at the base or excavate to receive the femur, in some groups e.g. Lymexylon they extend laterally to the elytral margin while in others e.g. Elateroides they are shorter. The scutellum is always visible and sometimes characteristically sculptured. The elytra are generally elongate  and diverge from about the centre, the basal part usually having a locking mechanism along the suture, in many species they are individually tapered and separately rounded or pointed apically, the epipleura are usually poorly developed; broadest at the base and narrowing strongly, and in some species they are absent beyond the centre. The surface is smooth and lacking distinct striae, any punctation being very fine, but in most there are 3 or 4 longitudinal carinae which are often only poorly-defined and faint, but in Australymexylon Wheeler, 1986 they are well-developed and distinct. In some groups e.g. Atractocerus the elytra are much reduced, being about as long as the pronotum and much shorter than the first abdominal segment-Linnaeus named the type species Necydalis brevicornis in 1766! The abdomen is always to some extent exposed beyond the elytra and all segments are articulated and movable, the adults need to be seen when active to appreciate this. The hind wings are well-developed with a fairly complete venation, the radial cell being reduced or absent in some groups. The legs are long, slender and finely punctured and pubescent, the femora only weakly thickened, the tibiae long, narrow and almost parallel-sided; the pro-tibiae often with a comb of setae towards the apex but without an apical spur, the meso- and meta-tibiae have a short spur on the inner apical angle. Tarsi 5-segmented, without bilobed segments (c.f. Cantharids etc.), the basal segment usually the longest, 2-5 variously elongate. The claws small and curved, sharp and variously toothed at the base.

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The larvae are distinctive with the head retractable into a hood-like prothorax, and the prothorax is distinctly larger than the meso- or metathorax. Antennae tiny, ocelli absent in later instars. Abdominal segments usually with lateral folds; segment 9 large and variously modified into a long, posteriorly oriented cutting structure or, in Melittomma, it is truncate and castellated along the posterior edge, like a saw-blade, segment 10 is smaller and situated beneath 8 or 9.