Notes on Zygiella stroemi

SRS NEWSLETTER NUMBER 18 February, 1994

Writing in the BAS Newsletter (No: 50) Clive Hambler expressed the belief that Zygiella stroemi (Thorell) is more widespread than present records would suggest.

Support for this view came whilst I was recording spiders at Mid Crook LN.R (SP 773464) during June ,1993. The reserve consists of traditionally managed meadowland on the floodplain of the River Tove but does include a single large oak. I took the opportunity to examine the deeply fissured bark of this tree and was rewarded by a male Z. stroemi and, shortly afterwards, by a female.

The webs, situated at head height, were small and fragile-looking but otherwise not unlike those of other Zygiella spp. Their occupants were coaxed from their retreats with the help of a tuning fork. This finding represents a new county record for Northants (kindly confirmed by Peter Merrett).

Although the Mill Crook site appears to be the most northerly record for this species in England it has long been known from the Black Wood of Rannoch (Perthshire) and one other Scottish site. There seems no obvious reason why diligent searching should not reveal the presence of Z. stroemi in the intervening region.
 
Added by John Partridge at 15:56 on Tue 20th Dec 2011.

Zygiella stroemi (Thorell) on Oak by Clive Hambler

The Newsletter No. 50 November 1987

Zygiella stroemi is considered to be a rare spider in Britain, although sometimes common locally (Jones, 1981). The species has generally been found on the trunks of pines in woods but has also been found in abundance on a hut (For some habitat records from mainland Europe, see Duffey (1953)). By 1985, there were about ten known sites for the species in Britain.

I found Z. stroemi in five new sites during 1986. The records are: Little Wittenham Nature Reserve, Berkshire (SU 573928, 27 May); Bucklebury Common, Berkshire (SU 575697, June); Brasenose Wood N.R. (SP 560050, 24 May)—new to Oxfordshire; Blenheim Palace Park, Oxfordshire (SP 437167, July and August); and Savernake Forest N.R (SU 2167, 23 August)—new to Wiltshire.

I believe this spider is likely to be much more widespread than currently thought, as I have found the species on the trunks of oak (Quercus robur) at all these sites. At Blenheim I also found a few on horse chestnut, sycamore and even on a very old beech tree, with a large population on nearby conifers (Atlantic blue cedars— there were over a hundred webs on very large trees). The habitat in these sites is interesting. Large populations were found at Blenheim and Brasenose. The former site is part of an S.S.S.I. with ancient woodland, very old oaks and extensive parkland, and the latter is also an ancient woodland S.S.S.I. which has a long history of coppice-management and has areas of recent coppicewith- standards in several different age classes. In both sites there are many trees with tall, exposed trunks: these are scattered in the parkland at Blenheim, and include some of the oak 'standards' and trees along the rides in Brasenose. The few specimens found at Little Wittenham were in woodland meadow areas, and at Savemake they were found along a road in the wood—again, in these cases, relatively large areas of the tree trunks were exposed to direct sunlight It is likely that the species has some preference for trunks which provide a certain (dry?) microclimate, and/or which receive a good supply of food. Indeed, J. Newing and I have found a statistically significant bias in the number of webs towards the southern half of tree trunks.

The texture of the tree trunk is very important to Z. stroemi. The species has been noted to use deeplyfissured pine trunks (Jones, 1981), and the same applies to its choice of oaks and other broadleaves: as Jones found, the diameter of the trunk is less important than the depth of the crevices in the bark, although larger trees have generally more highly textured bark I found the species on oaks from about 50 to 150cm diameter at breast height The retreat is made in a crevice, which is generally at least a centimetre deep, and the web is usually spun between two ridges in the bark, across the crevice. Occasionally, the web is spun across to a small twig, or may be spun amongst clusters of twigs by small spiders. Several trees supported at least ten webs each. Oak appears JikeJy to meet the species requirements more frequently than some other deciduous trees, but I expect the species will be found on ash. The height at which I have seen webs, almost certainly of this species, varies from about a half to four metres above the ground; it is likely that they could be found higher and, indeed, it is possible that the species is normally found high in trees, but is able to move closer to the ground on trees where the trunk is not surrounded by undergrowth.

Given the apparently rather specific requirements of the species, it seems most likely to be found in areas where there are many trees with creviced bark, and with trunks of various sizes and degrees of exposure: such conditions may be met most frequently in large woods, parklands, or in coppice-with-standards woods. It is interesting that all five new sites include, or are very near to, ancient woodland, which may have provided historical continuity of suitable trees.

The original habitat of this species over much of the country may well have been on trees like oaks, from which it opportunistically colonised planted coniferous trees with highly textured bark (and certain huts!). It is notable that neither in Little Wittenham, nor in Brasenose, are there any large conifers, nor were there any suitable conifers near the site in which I found the species in Savemake. Z. atrica (C. L. Koch) and Z. x-notata (Clerck) build very similar webs and retreats; I have found webs of adult females of all three species on oaks, sometimes with two species on the same trunk. However, adults and eggs of the two commoner species occur mostly in the autumn whilst I have found adult female Z. stroemi from May to October, with many sizes of immatures at all times in this period. Several females of Z. stroemi with eggs and newlyhatched young were found in mid-July, but an adult female taken in late May laid eggs which unfortunately I let dry out.

I think it may be possible to distinguish immatures of Z. stroemi from their congeners by the dark marks on the carapace. In Z. stroemi, I have always found a pair of dark lines, sometimes fusing at the base; in subadult males this pair of lines may be less distinct In contrast, immatures of the other two species appear very similar to each other and have a more rectangular dark mark. This distinction holds for over thirty immarures of Z stroemi that I have examined in the field with a hand-lens. It may prove most useful with small immatures, as in older specimens the abdominal pattern of Z. stroemi (with white rather than silver blotching) is fairly distinctive. The carapace pattern in Z. stroemi develops while the spiderlings are still in the mother's retreat, although it is not present in newly-hatched specimens.

It would seem worthwhile to seek this species on oaks and other deciduous trees generally. I find pressing my chin against the tree-trunk and peering upwards helpful: not behaviour for the self-conscious arachnologist! A hand-held mist-sprayer is useful for highlighting the webs, and a strong sharp tool may be needed to chip off bark. I am very grateful to Linda Francis (in the original) for making the drawings, to the Northmoor Trust for supporting the work, and to Peter Merrett for checking the new county records.

References
Duffey, E. (1953) On a lycosid spider new to Britain and two rare spiders taken near Oxford. Ann. Mag. not. Hist. (12) 6:149-157.
Jones, D. (1981) Zygiella stroemi (Thorell). News/. Br. arachnol. Soc 30: 9-10.
 
Added by John Partridge at 15:42 on Tue 20th Dec 2011.

Zygiella stroemi (Thorell) by Dick Jones

From The Newsletter No. 30 MARCH 1981

Colonies of this hitherto highly localised species have recently been found at numerous sites in the counties of West Sussex, Hampshire (including the New Forest) and the well-known commons of Surrey, Thursley, Uitley and Chobham; the latter record kindly provided by Mrs F.M.Murphy.

The spider appears to require very deep fissures for its retreat and the Scots Pine (Pinus silvestris) can naturally provide these. On one occasion webs and spiders were found on Corsican pine as well as on neighbouring Scots Pine. The age of the tree and consequent trunk diameter are of no consequence, apparently, provided that the texture of the bark is suitable. The web is placed typically across large gaps in the bark, although several webs have been seen suspended below branch stumps; on one occasion a web spanned a short distance between a pine and a hawthorn branch.

The horizontal diameter of the hub varies between 60 to 100 mm, depending on the size of the spider and the space available to it. As with Z. atrica (which frequently occurs on pine bark.) the web usually has a distinct free zone around the hub which is pulled into a pear shape by the taut signal thread. The 'missing sector' typical of the genus is usually clean and unobscured by additional spirals. The spirals are circa 2mm apart below. The retreat is normally invisible in deep fissures, but a grass stem introduced from above will usually force the resident to drop with surprising swiftness to the ground. To avoid unnecessary disturbance only a small proportion of the webs at each site was thus investigated, but it was evident that not all the webs were tenanted. As described in British Spiders, the web is very delicate, as well as small in size and consequently is difficult to see. I have found that the best chance of spotting the webs is by an oblique view, looking at the sides of the tree rather than the bark directly in front of ones' eyes. The webs seem to occur at a variety of heights from a few centimetres above ground level, to as high as one can reach. Beyond this height the Zygiella webs I have been able to see could have belonged to either of the aforementioned species.

The sexes are very similar in appearance and size. The epigyne of the female is so distinct that she can be easily identified in the field, with the aid of a lens. In the south, adult females have been found from March to September, but only in June were two males discovered running over the bark of pines at Thursley Common.
 
Added by John Partridge at 15:35 on Tue 20th Dec 2011. Return to Summary for Zygiella stroemi