|Stilbus testaceus (Panzer, 1796)|
|Quoted in Joy simply as local; southern England to Yorkshire, and, with the
exception of the west country, this is also portrayed on the NBN map (April 2009) where there are modern records
scattered below a line from the Humber to the Severn estuary, beyond this there are a few records from Wales.
Thompson, who quotes the species as common in
dry grass and hay, gives additional records from Scilly, the Isle of Wight, Hereford and from Kirkudbright in
Scotland. The beetle is common throughout the Watford area; adults, usually along with other species eg
Meligethes and Miarus, may be seen in numbers on flowers, especially Dandelion (Taraxacum),
in hot weather from April onwards. In the absence of sunshine they descend into the base of the capitulum and
stay hidden but may readily be found by shaking the flowers into a bag. Locally they are likely to occur in any
situation but notably large numbers occur where grass is allowed to mature eg around the ponds at Radlett road
or along the pathway margins bordering the 'wild' areas in Cassiobury park. We have also recorded them commonly
at MV during May and June in town centre gardens.
Soon recognised in the field with a X10 hand lens; by observing them on hands and knees, among the many other small beetles that abound in suitable flowers, the convex form and bicoloured pattern become obvious.
1.8-2.3mm. (Vogt, 1967) Broad oval and very convex, undersurface flat, upper surface dark brown with apical fifth or quarter pale yellowish brown, often the head and pronotum darker, in some of our specimens the elytra are darker. Head very finely punctured (X50), transverse and evenly rounded in front. Eyes continuous with angled part of lateral margins, front and hind margins of eyes weakly sinuate. Antennae entirely testaceus, inserted under front margin of head, two basal segments large, 3-8 elongate but becoming less so towards apex, 9-11 forming a compact and elongate club. Maxillary palps testaceus, last segment curved on inner side, out side straight. Pronotum very finely punctured and indistinctly microsculptured (barely visible at X60), broadest at sharply perpendicular hind angles, narrowed and evenly curved to weakly produced front angles. Lateral and basal margins bordered. Scutellum relatively large, equilateral, microsculptured as elytra. Elytra with a single sutural striae which is present, at least residually as a series of fine punctures, from basal margin to apex, becoming stronger and deeper apically, these converge at the apical angle. Lateral margins bordered. Surface with several longitudinal rows of very fine punctures (X50) and obscure and shallow longitudinal impressions. Microsculpture consists of transverse wavy lines which become visible at X70, in detail at X100, in our experience easiest viewed on top surface about two thirds from base by slowly moving the focal plane above and below the surface of the elytra, the detail begins to become obvious. Legs entirely testaceus.
Description from 4 Watford specimens at X50-120
Identifying Stilbus testaceus
Examination of the elytral microsculpture is essential to separate our three British species of Stilbus. This needs to be done at high magnification, X100 or more, and in strong and diffuse light. In S.oblongus (Erichson) this is reticulate, comprising irregular but overall isodiametric pentagons, hexagons or quadrilaterals which are rarely fused to form elongate cells within the pattern. In S.testaceus (Panz.) the microsculpture comprises wavy transverse lines which occasionally merge but are not cross-linked and do not form cells. In S.atomarius (L.) the microsculpture is a hybrid form; reticulate but interspersed with continuous transverse lines and open transverse cells. These are figured as line drawings in Thompson.
Line drawings of the aedeagi of all three species are featured by Thompson as is the prosternal process which is a good character for separating oblongus from testaceus and atomarius; in oblongus this has 4 or 5 small (difficult to appreciate) setae along the (truncate) apex whereas in teastaceus and atomarius there are betwen 4 and 8.
Vogt's (1967) treatment includes only these three species and follows Thompson's key, including also reproductions of his line drawings.
Joy's key relies on shape (including convexity), colour and size; testaceus is quoted as 1.8-2.4mm while oblongus and atomarius are 1.5-1.8mm. Differences in shape and convexity are very subtle and possibly meaningless without experience, the size range of each species given by both Thompson and Vogt is virtually the same. This Joy key should, therefore, not be relied on.
Vogt, H. 1967 In Freude, H et al. Die Kafer Das Mitteleuropas 7 158-166