British insects: the families of Coleoptera

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L. Watson and M. J. Dallwitz

Megasternum Mulsant

Adults. Beetles terrestrial; walking in water or free-swimming by conventional ambulatory motion of the legs, not diving strongly; moving in the water by alternate, walking leg movements. Beetles 1.5–2 mm long; body length/maximum body width 1.3–2.55. Beetles somewhat waisted. The head not covered by the thorax.

Eyes two; not strongly protuberant; without bristles. Mandibles with a well developed mola; with well developed prosthecae. The maxillae with distinct galea and lacinia apically to the palp. The maxillary palps relatively conspicuous. The maxillary palps conspicuously elongated, sometimes longer than the antennae. The maxillary palps conspicuously elongated, sometimes longer than the antennae. The apical segment of the maxillary palps cylindrical to fusiform. The apical segment of the labial palps not expanded apically. Antennae very short; not strongly asymmetric. Antennal scape not swollen. Antennae clubbed. Antennal clubs 3 segmented (the club hairy); without a cupule. Antennal insertions visible from above, or hidden from above.

Prothorax without notopleural sutures. The pronotum not narrowing towards the rear. Mid-and hind-legs not as in Gyrinidae (q.v.). The front tibia very conspicuously asymmetrically expanded and emarginate near the attachment of the tarsus. Hind coxae not extended laterally to meet the elytra; not shaped posteriorly to receive the femur. The hind coxae not produced behind into flat plates partly concealing the hind femora. Hind coxae without the steep transverse declivity characteristic of Dryopoidea. The inner parts (‘processes’) of the hind coxae not incorporated with the metasternum in a flat, median longitudinal keel. Tarsal segmentation formula 5, 5, 5. None of the tarsi with ‘hidden’ segments. The front tarsi with as many segments as the mid-tarsi; 5-segmented. Mid-leg tarsi 5-segmented; pentamerous. Hind tarsi equipped with ‘swimming hairs’. Swimming hairs confined to the tarsi.

Elytra striate or with regular rows of punctures.

Adults not predacious.

Larvae. The larvae elongate and more or less parallel-sided; vestiture restricted to fine hairs or setae; only very lightly pigmented or sclerotized; only very lightly pigmented. The antennae 3 segmented. Frontoclypeal suture between frons and clypeus indistinct or absent. The labrum and head capsule completely fused, with no suture apparent. Apices of the mandibles with a single lobe or tooth. The maxillary palps 4 segmented. The labial palps 4 segmented. The mesothoracic legs with 1 movable claw. Visible abdominal segments 10. Tergum 9 of the abdomen entirely dorsal.

The larvae non-aquatic. Abdominal tergum 8 without amature. The larvae predacious.

Worldwide and British representation. 1 species in Britain (M. obscurum). Recorded from common, Scotland, Northern England, South-west England and Wales, South-eastern England, and Ireland.

Classification. Suborder Polyphaga; Infraorder Staphyliniformia; Superfamily Hydrophiloidea; Hydrophilidae; Subfamily Sphaeridiinae.

Miscellaneous. • Megasternum obscurum (from Joy, 1932).


To view the illustrations with detailed captions, go to the interactive key. This also offers full and partial descriptions, diagnostic descriptions, differences and similarities between taxa, lists of taxa exhibiting or lacking specified attributes, and distributions of character states within any set of taxa.

Cite this publication as: ‘Watson, L., and Dallwitz, M.J. 2003 onwards. British insects: water beetles. Version: 18th September 2012. http://delta-intkey.com’.

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