Ctesias serra (Fabricius, 1792)
Cobweb Beetle
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POLYPHAGA Emery, 1886
BOSTRICHOIDEA Latreille, 1802
MEGATOMINAE Leach, 1815
CTESIAS Stephens, 1830
The common name refers to the habit of the specialized larvae of being able to obtain food from spider webs under bark, they are not often seen as they remain hidden during the day but are active at night and may be observed on the surface of dry denuded wood on old dead or dying broadleaf trees, typically oak and beech but also from a wide range of species. Adults occur from May to July or August and are nocturnal as well as diurnal, at night they may encountered among bark or on wood, generally near to loose or peeling bark infested with spider webs, and by day may be swept from vegetation around host trees or found among debris in hollows, under protruding bark or in crevices. The typical habitat is old established woodland and parkland where a population may exist on a single tree for many years, but they may also occur in neglected corners of lofts, barns and sheds but they are only associated with wild habitats and, unlike many members of the family, do not enter houses or stores and are not pests of stored products. Larvae occur year-round and may be observed on wood at night even during mild spells in the winter, they feed on the dry remains of insects left by spiders in their webs and are protected from attack by their hosts by their dense covering of long setae which readily become detached and are thought to be irritating to spiders, they also have a behavioural defence mechanism; when a spider is sensed approaching they can vibrate their abdomens rapidly and thus produce confusing signals which disorient the spider and so conceals their position. The species is thought to be univoltine throughout its range, eggs are laid under bark in late spring and early summer and larvae develop over a long period, passing through five instars and overwintering before pupating during the following April and May. Recording this species will generally require searching suitable habitats at night as the larvae are much more readily observed than the adults but they also turn up in extraction samples taken from bark debris etc. through the year. Larvae are distinctive enough to provide reliable records of the species and once observed are instantly recognizable; entirely pale brown with shiny transverse strips of integument showing between the dense pubescence, body broad and rather flattened
with dense short pubescence and several layers of long setae. The lateral margins of all segments have tufts of setae but the distinctive feature is the four dense tufts of erect hastisetae (setae divided at the apex and armed with barbs) on the apical abdominal segments which can be thrust forward over the head when the larva is alarmed, they lack urogomphi and have robust and have quite long legs that allow them to move quickly when necessary. Excellent pictures of various stages can be found HERE. This is a very local and generally uncommon species through much of its range which includes most of Europe from the Mediterranean, including northwest Africa, to the UK and southern Fennoscandia, Caucasus and east through Russia to Siberia, in the UK it occurs locally throughout England and Wales although is sporadic and generally rare in the north of England and absent from Scotland. Due to its secretive and mostly nocturnal habits it is likely to be under-recorded.
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​3.5-5.0mm. Broadly elongate-oval and continuous in outline, entirely shiny dark brown to black, upper surface with random fine punctures, which are a little larger and denser on the elytra, and short dark pubescence. Head transverse with large and very convex eyes and short temples, vertex more or less flat with a single central ocellus which appears as a pale spot. Antennae 11-segmented; 2 basal segments enlarged, 3-8 transverse and 9-11 form an elongate club, each segment expanded internally and segments 9 and 11 elongate-triangular. Pronotum transverse; widest at acute posterior angles and narrowed to a rounded anterior margin, basal margin very strongly bi-sinuate. Prosternum with sharply-bordered lateral antennal grooves. Elytra with convex shoulders, weakly dilated from the basal third and continuously rounded apically. Fully winged. Legs long and slender; femora usually hidden from above, tibiae narrow and parallel-sided and tarsi 5-segmented without dilated or lobed segments.