Sphaeroderma Stephens, 1831
This is an Old World genus; it includes up to 250 species but in some cases they may be confused with other genera and those recorded from the Nearctic and Neotropical regions, with the obvious exception of a single introduced Palaearctic species, are almost certainly wrongly attributed. The genus occurs throughout the Palaearctic, Oriental and African regions and is represented in Australia by 3 species, the greatest diversity is in the eastern Palaearctic and Oriental regions e.g. only 2 species are known from Europe and much of the Western Palaearctic while 16 are listed for Nepal and Bhutan and 11 for Japan, by contrast more than 50 occur in Sub-Saharan Africa about 10 in Madagascar. Our two European species are both widespread and extend north into the UK where they are widespread and generally common. Most members of the genus are associated with Ranunculaceae and Asteraceae and both our UK species occur on various thistles and knapweeds, they both occur throughout the year and will soon be found when sweeping herbaceous vegetation in most situations.
They are small, 2.3-4.2 mm, convex and strongly-rounded, almost hemispherical; our species are pale brown, which is typical of much of the genus although some from warmer climates are maculate. Among our UK fauna they are easily distinguished, with a little experience, by sight; the colour and shape being unique although, pessimistically, S. testaceum, which is slightly elongate might be mistaken for several Neocrepidodera Heikertinger, 1911 which commonly occur on the same hosts, but these are more elongate and have strong impressions across the base of the pronotum. In the present genus the head is strongly produced in front of convex and prominent eyes and the frontal grooves are strongly impressed, but in life the head is usually mostly concealed by the pronotum. Antennae long and filiform, inserted laterally in front of the eyes and separated by about the length of the first segment. The pronotum is transverse, broadest across the base and smoothly curved to a rounded (from above), apical margin, lateral margins not visible from above, basal margin straight or curved
Sphaeroderma rubidum 1
Sphaeroderma testaceum 1
Sphaeroderma rubidum 2
Sphaeroderma testaceum 2
Sphaeroderma testaceum 3
but produced back to the centre, the surface is very finely punctured and lacks a transverse impression across the base or latero-basal fovea. The elytra and strongly-rounded from rounded or sloping shoulders to a continuous apical margin which completely covers the abdomen, the surface is smoothly convex and finely punctured, without striae but the fine punctures may be partially seriate across the disc. Hind femora greatly enlarged, front and middle femora normally developed. Tibiae smooth externally and with a single apical spur. Tarsi pseudotetramerous with the third segment widely bilobed and the fourth segment diminutive. Claws smooth and weakly toothed at the base. Our species may be distinguished as follows:
Body strongly rounded, often almost circular, pronotum very strongly narrowed from the base and very finely punctured. 2.3-3.5 mm.
Body less strongly rounded, slightly but distinctly elongate, the pronotum less strongly narrowed from the base and more strongly punctured, especially towards the basal margin. 2.5-4.2 mm.
Sphaeroderma rubidum (Graëlls, 1858)
This is a generally common species throughout Europe although sporadic in some northern areas, extendling to the UK and southern provinces of Fennoscandia, east to western Russia, Caucasus and Ukraine and widespread across North Africa from Morocco to Lebanon. Here it is common throughout England and Wales including the islands, and much more local and scarce in Ireland and southern Scotland. Host plants include a range of Asteraceae, especially various thistles (species of Cirsium Mill, Carduus L. and Onopordum Viall) but also saw-wort (Serratula tinctoria L.), safflowers (Carthamus tinctorium L.), burdocks (Arctium spp.) globe artichoke (Cynara cardunculus L.) and cornflowers (Centaurea spp.), adults feed on foliage causing numerous small holes but they also visit flowers and may occasionally be found on foliage of broadleaf trees and shrubs. Typical habitats include anywhere the hosts are common, roadsides, heaths and moors, grassland and arable land, parks and gardens etc., and they are often common on disturbed ground. Adults occur year-round but very few individuals overwinter and only small numbers occur in early spring, they become common from late May or June when the new-generation appears, they remain so until September and persist into the autumn. Mating occurs from late May or June and continues over several months and oviposition occurs from July until October. Eggs are laid singly on the underside of leaves, usually near to a large vein and often towards the middle of the leaf, and larvae emerge within two weeks, upon leaving the egg they immediately bore into the tissue beside the vein and begin to mine, at first the mine is full depth across the leaf and often meanders or forms loops, these early mines contain little frass and have irregular margins but as the larva grows the mines become much wider and more regular, last (third) instar larvae are very broad and completely fill the mine. Larval development takes about a month; fully grown larvae occur from late August into and the majority of these will leave the leaf and enter the soil to form a cell a few centimetres below the surface, these will overwinter and pupate from April to July. The pupal stage lasts between four and six weeks and so new-generation adults appear over a long season, they begin feeding soon after eclosion but most will aestivate through the summer and there is only a single generation each year. Sampling is usually by sweeping suitable vegetation from spring until the autumn and adults will only rarely occur in winter tussock or moss samples.
2.3-3.5mm. Very convex and only slightly elongate, sometimes appearing almost circular in outline, and entirely pale brown when mature. Head transverse with large convex eyes, usually substantially hidden under the pronotum, vertex flattened and finely punctured, antennae inserted anteriorly beside the inner margin of the eyes, the separation slightly less than the basal segment. Antennae 11-segmented with all segments elongate. Pronotum broadest at acute posterior angles and strongly narrowed to a rounded (from above) anterior margin, surface very finely punctured or impunctate in places and lacking any basal grooves or fovea. Elytra quadrate to slightly elongate with sloping shoulders, strongly rounded lateral margins and continuously curved apical margin, surface randomly and finely punctured, without impressions or fovea. Legs long and robust, the hind femora greatly expanded compared with the middle femora, tibiae slightly curved and gradually broadened to obliquely-truncate apical margins. All tibiae slightly expanded at the outer apical angle and bearing a small spur. Tarsi pseudotetramerous with the third segment strongly bilobed, basal segment of all tarsi dilated in the male.
Sphaeroderma testaceum (Fabricius, 1775)
This widespread and generally common western Palaearctic species occurs throughout Europe, except for the extreme west, it extends north into the UK and southern Fennoscandia and east into Russia, Ukraine and Azerbaijan, it is also present on many Mediterranean islands, it extends south to Tunisia and Israel and, following accidental introductions has become established western Canada. Here it is widespread and common throughout England and Wales, widespread but very local and mostly coastal in Ireland, and sporadic and rare in Scotland. Host plants include a range of Asteraceae, mostly various thistles (species of Carduus L, Carlina L, Cirsium Mill, Onopordum Vaill. Ex L. and Cynara L.) but also occasionally knapweed (Centaurea nigra L.) and it can be a nuisance, if not a pest, of commercially-grown artichokes. Typical habitats include grassland, arable land, verges, wooded borders and hedgerow margins etc., wherever the host has become established, the adults fly well and may quickly arrive at new sites or in gardens and they may be especially common on disturbed land. Adults occur throughout the year, at least some overwinter in tussocks or litter etc. and become active early in the spring, they are generally abundant from June until September and may be found into November. Reproduction occurs in early summer and oviposition continues from July until October or November, several eggs are laid beneath both fresh and mature leaves, generally away from large veins and usually several to a leaf, and larvae emerge within two weeks. Fresh larvae immediately bore into the leaf and begin to mine, they produce a long curved mine, often following the edge of the leaf, containing very little frass and with the margins unevenly eaten out so that in places it has a blunt saw-tooth appearance, often several mines will run parallel near the edge and sometimes they form irregular loops. Larvae pass through three instars and are present in various stages of development during the autumn but all will go on to overwinter and it is likely that a winter diapause is necessary before pupation can occur, younger larvae will continue to develop in the spring and pupation occurs from March until July, the first new-generation adults appear from May and it is probably some of those that eclose late that will go on to overwinter. Sampling is straightforward, host material may be swept and adults occur in flight-interception traps etc., but a certain degree of dexterity will be needed as they hop readily when alarmed and seem to vanish, sweeping in cool weather or at night will overcome this. Both adults and larvae may be detected by their feeding signs; larval galleries will soon become familiar in thistle leaves, and adults produce numerous small holes in the foliage as they feed.
2.5-4.2mm. Elongate-oval and very convex, slightly discontinuous in outline with the base of the pronotum narrower than the base of the elytra, entirely orange with only the eyes darker. Head transverse and very finely punctured, usually substantially concealed from above, eyes large and convex, antennae 11-segmented and inserted in front of the eyes, separated by about the length of the basal segment, all segments narrow and elongate. Pronotum transverse, broadest at the base and narrowed to a rounded anterior margin, surface coarsely punctured, especially towards the base, otherwise smoothly convex, without basal fovea or furrows. Elytra broadest in front of the middle, with sloping shoulders and a continuously-rounded apical margin, surface randomly and moderately coarsely punctured. Legs long and robust with femora visible in normal setting, hind femora much larger than the middle femora. Tibiae weakly broadened to an obliquely-truncate apex, slightly sinuate and produced apically to a blunt external tooth, each with a small spur and a series of short, stiff setae about the external apical angle. Tarsi pseudotetramerous, the third segment widely bilobed, male with the basal pro-tarsomere dilated.