Elateroides dermestoides (Linnaeus, 1761)
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POLYPHAGA Emery, 1886
LYMEXYLOIDEA Fleming, 1821
HYLECOETINAE Germar, 1818
ELATEROIDES Schaeffer, 1769
This widespread Palaearctic species occurs locally throughout Wales and the English midlands and there are scattered records from North Somerset, Surrey and Sussex, and it appears to be locally common in South Herts. Adults occur for a brief season between early April and June but they are generally difficult to record, the natural habitat is wooded borders and parkland where they may be swept from foliage in hedgerows or from individual trees, and the females visit flowers although they do not feed, resembling cantharids and easily missed, otherwise they are crepuscular or nocturnal and need to be looked for carefully on damaged trunks or among fallen timber and logs. The species is saproxylic with fungivore larvae developing on in a wide range of hosts including maple, alder, oak, beech and especially birch, and they have also been recorded from a range of conifers including pine, larch, Douglas fir and spruce. Adults live for only a few days and are very energetic; when swept they move very rapidly, constantly running and taking flight within the net, and when observed on wood at night they run over the surface quickly looking for mates or oviposition sites. Females oviposit either on the surface or among bark crevices or in bore-holes, cut ends of logs and slightly fermented timber more than a year old are preferred and egg-laying occurs above 8oC., they are always laid in batches, anywhere from four to about ninety, and each female will lay up to 150 eggs. Larvae hatch within a week or two and consume part of the chorion, freshly emerged larvae display a characteristic twisting behaviour which ensures contact with other larvae and eggs thus the transfer of fungal spores onto their cuticle. After a while they disperse and bore into the wood head-first, inoculating the tunnel as they go, the modified ninth segment is used both in the boring process as well as to keep the tunnel clear of frass so that the spores begin to grow. Tunnels are soon lined with growing host fungus, Ascoidea hylecoeti, as well as various adventive species including Isaria, Aspergillus and Verticillium and the larva feeds and develops within, enlarging the gallery as it grows; the tunnel becomes long, curved and unbranched, penetrating into the xylem in some hosts such as oak and pine but remaining sub-cortical in others such as beech, and fungal enzymes probably render the wood softer for the boring activities. These borings may penetrate to 20cm and are widened by the larva to accommodate its growth;
the distensible head and thorax are used to facilitate movement within the tunnel and it can turn around to eject frass although this is usually performed with the modified ninth segment. The tunnel is sealed in the winter with a plug of wood pulp which is removed in the spring when tunnel widening re-commences. Prior to pupation a wide chamber is constructed close to the ingress hole and sealed internally with wood dust. The pupal stage lasts between one and four weeks, and pupae are readily sexed as the modified male palps are clearly visible on the surface. Females emerge from the wood holding spores in modified abdominal pouches and these will be deposited in a slimy secretion along with the eggs. A commensal relationship exists with the Tyroglyphid mite Histiogaster hylecoeti; third instar nymphs are transported beneath the elytra of Elateroides and leave during oviposition, entering the larval tunnels and probably feeding upon the fungi. The palporgan of the male is thought to an adaptation for locating females.
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This is a large and distinctive species which will soon become familiar in the field; superficially similar to several cantharids but with shorter antennae and long tarsi without bilobed segments. As stated above they are very energetic and need to be tubed quickly for examination. The male is immediately recognized by the greatly expanded maxillary palps which are constantly moving even when the beetle is at rest. Females are generally larger. The typical colouration is entirely testaceous with the elytral apex darkened in the female, and testaceous with the head, pronotum, scutellum and elytral apices darkened in the male. Entirely black males occasionally occur and have been named morio (Fabricius, 1787), and a female form with the head black, called cossis Lewis, 1896, is known from China and Japan.
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6-18mm Head narrowed anterior and posterior to convex eyes; vertex finely and closely punctured and pubescent, with a sharp tubercle by the epicranial pit. Antennae inserted in front of the eyes; serrate in the male, much less so in the female. Female maxillary palps normal with the terminal segment truncate, male with segments two and three greatly enlarged into brush-like appendages. Pronotum transverse and broadest in front of the base, lateral margins bordered and narrowed anteriorly, surface finely punctured and pubescent, the surface sculptured, in the male with an obscurely defined tubercle either side of the middle. Scutellum densely punctured and pubescent, and with an unpunctured and shiny keel at the base. Elytra with prominent shoulders and gently sinuate lateral margins, diverging and rounded at the apex, and each with several weak longitudinal costae. Legs long and slender, the male femora to some extent darkened. Tarsi 5-segmented, all segments elongate. Claws smooth and gently curved, with a well-developed and sharp tooth at the base in both sexes.