Tanysphyrus Germar, 1817 

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POLYPHAGA Emery, 1886

CURCULIONOIDEA Latreille, 1802​

CURCULIONIDAE Latreille, 1802​​

BRACHYCERINAE Billberg, 1820

TANYSPHYRINI Gistel, 1848

T. ater Blatchley, 1928

T. lemnae (Paykull, 1792)

Tanysphyrus lemnae (Paykull, 1792)

Duckweed Weevil

This Holarctic species occurs throughout Europe north to the UK and Scandinavia, through Asia and Japan, and across North America from the Great Lakes to Florida. Here it is locally common across central and southern England and Wales becoming scattered and generally scarce further north. In Europe it is generally associated with species of Lemna L. (duckweeds), more especially L. minor L, and to a lesser extent Calla palustris L (bog arum), where it develops among the masses of free-floating thalli on or just below the water surface. Adults occur year-round, they are active from March until late in the year, November or December, among the host, and through the winter among marginal litter and tussocks etc. although we have found them several hundred metres from water and also under bark on floating logs at this time. In North America they also occur on species of Spirodela Scheid, 1839 (Araceae, formerly Lemnaceae), and in Florida it is specifically associated with Pistia stratiotes L. (water cabbage, a pantropical member of the Araceae). Adults are readily sampled by working through masses of Lemna netted from the water surface where, despite not being able to swim, they spend much of their time, or by sweeping vegetation around still-water margins where the host is present as they often climb sedges etc. They mate and oviposit through late spring and summer; pale grey eggs are inserted singly into the tissue of the thalli which is then plugged with secretions and frass. Larvae emerge within a few days, eat their way out of the tissue and feed on the surface of the thallus which they more-or-less consume within 12 hours, they then burrow, crawl or swim a short distance to the next and so on over the next 2 or 3 weeks. Fully grown larvae are about 3mm long and translucent brown or grey with a pale brown head, they walk across the mats of host plant or swim to the water edge where they pupate among roots or in the soil. The cycle from egg to adult, depending upon conditions, takes 16-20 days, and peak numbers occur from late May to early July.

1.2-1.9mm this species is characterized by the tarsal segments, the third is widely bilobed and the terminal segment protrudes very little so that they appear truncate. Body black or dark brown when mature with various patches of broad pale scales to the pronotum, and narrower pale scales to the elytra; generally below the base, behind the middle and before the apex. Head transverse with coarsely faceted eyes that follow the lateral margin. Rostrum about as long as the pronotum and expanded towards the apex; shiny and finely punctured with scrobes narrowly visible from above. Antennae inserted in front of the middle, nearer the apex in the male; scape curved near the base and abruptly expanded before the apex, funiculus 7-segmented, club broad and abruptly set-off. Pronotum broadest at the middle, coarsely and densely punctured and usually with lighter scales arranged a longitudinal line either side of the middle, laterally without or with only scattered scales. Elytra much wider than the prothorax, with prominent shoulders and subparallel in the basal third; broadest behind the middle and evenly curved to the apex. Striae deeply impressed and punctured, each interstice with a row of recumbent setae. Femora dark, legs otherwise pale. Pro-tibiae with a sharp internal tooth before the apex.

Tanysphyrus ater Blatchley, 1928

A further species, the widespread Holarctic T. ater Blatchley, 1928, has recently been recorded from a single site in eastern England. On the continent and North America it is associated with the aquatic liverwort Ricciocarpus natans L. (Purple-fringed Riccia).

Superficially similar to T. lemnae in size and habitus but readily separated by the lack of a subapical tooth on the inner margin of the pro-tibiae and the densely and extensively scaled lateral margins to the pronotum; in T. lemnae these scales form at most a narrow longitudinal band near the top of the margin and generally visible from above. 

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