CANAL SPIDERS GET NEW ‘WEB SITE’:
NOTES ON THE SPIDER LARINIOIDES SCLOPETARIUS (Clerck 1757) AT SYKEHOUSE AQUEDUCT.
INTRODUCTION
Situated in a rural landscape of lowland grazing marshes, SSSI hay meadows, hedgerows and green lanes, the Sykehouse aqueduct is located at the northern end of the New Junction Canal at Went Ings (SE/6418; 6518).
In 1998, some 91 years after its opening in 1907, the aqueduct was deemed by British Waterways engineers to be in need of restoration. In advance of the major exercise of sand-blasting, re-priming and re-painting the aqueduct’s ironwork, Doncaster Museum was asked for information on bat sites which might be affected by the proposed maintenance works.
While examining the structure for bat signs during July and August 1998, the presence of numerous large orb-type spider webs, together with a conspicuous bespeckling of innumerable spider droppings on the protruding metal lip around the base of the aqueduct structure, drew attention to a remarkable population of the uncommon riparian spider Larinioides sclopetarius (Clerck).
VIADUCT DIMENSIONS
The approximately 160m long aqueduct has three spans consisting of a southern section of 16.2m, crossing the southern soak drain of the river Went, a central 49.3m section spanning the river Went itself, and a northern section of 15.8m crossing the Went’s northern soak drain.
The iron casing of the canal is constructed of sections some .83m wide and 3.3m deep, each attached to a succession of protruding steel ribs. At intervals of approximately 2.5m the casing is braced by a 1m wide cradling structure. The overhanging parapet of the tow path, the protruding iron lip at the base of the aqueduct casing some 3.3m below and the iron ribs set .83m apart and the paired steel girders of the cradling structures formed the outer framework and anchoring points for the series of huge orb web snares.
SPECIES DESCRIPTION
Females of L. sclopetarius reach up to 14mm in length and have a tendency to flatness of body, reminiscent of the still flatter Nuctenea umbraticus (Clerck) (Bristow 1971). Jones (1983) and Roberts (1995) refer to its distinctly velvety appearance, and that the dark markings of the head, carapace and abdomen are conspicuously highlighted by clear white, characteristically angular, outlining.
DISTRIBUTION AND STATUS
In Britain, Jones (1983) and Roberts (1995) describes it as very locally distributed throughout England and Wales and generally rather uncommon. In Watsonian Yorkshire, Smith (1982) describes it as uncommon, being recorded from only eight 10km squares (SE/14; 42; 50; 61; 63; 64; 65; 71). These are mainly in lowland regions, six being within the Humber Head Levels natural area, three of which are in the Doncaster region (Smith 1982 & 1985).
HABITAT
Although Smith (1982) alludes to its presence on waterside bushes, this behaviour seems to be restricted to casual occurrences by wandering adult males. Bristow (1971) and Roberts (1995) refer to it being largely restricted to buildings, bridges and fences near water. This is certainly the case along Doncaster’s New Junction Canal and South Yorkshire Navigation where colonies are invariably found below bank level on concrete or metal structures overhanging water.
To test this assertion, an examination of the habitat selection of orb web spiders along the New Junction Canal corridor undertaken in September 1998 showed that L. sclopetarius never occurred above bank or towpath level and curiously, no specimens were located on the associated masonry buttresses, spiders and webs were only found on the sections of the steel and ironwork structure over water. On riparian vegetation its place was generally taken by Tetragnatha extensa (L.), on tall herbage by L. cornutus (Clerck) and Meta segmentata (Clerck) and on waterside brambles and other shrubs by Araneus diadematus (Clerck).
It is tempting to suggest that the prevailing lowland distribution and the sub bank level preference of L. sclopetarious colonies may indicate that this species had a habitat niche on the overhanging undercut banks of meandering rivers of alluvial plains in lowland Britain. Such situations are relatively rare in modern highly engineered ‘v’ shaped river profiles so the solid metal and concrete structures with overhangs and long drops to water beneath, associated with navigable rivers and canals, may form an ideal if sparse artificial habitat substitute.
COLONY SIZE, ORIENTATION AND SEX RATIO
On 2nd August, by lying on the towpath parapet and looking down at the aqueduct structure with 8×40 magnification binoculars, it was possible to undertake a section by section count of occupied webs. A total of 233 adult and large sub adults were counted which with the 25 specimens removed on 30th July produced a total of 258, making it easily the largest population counted in Yorkshire. Only 2 (.7%) specimens were males, this low proportion possibly being accounted for by mortality this late in the breeding season.
Although the population was distributed along both the eastern and western facing aspects of the three aqueduct spans, the numbers of spiders counted within each .83m section of the canal casing showed most (74.8%) were on the eastern (down wind and down stream) side and that the largest concentration (39.9%) were located in the south eastern section of the central span (see figure 1). The reason for this phenomenon can only be conjectured. Perhaps the eastern side was more sheltered from the prevailing south-westerly airflow and may have been more conducive to flying insects. The spiders occupying webs on the western aspect, through receiving sunlight for longer into the afternoon and evening, may be more vulnerable to diurnal bird predation.
TRANSLOCATION
On the evening of 30th July 1998 a butterfly net attached to a telescopic fishing rod, giving a reach of some 5m was used to collect specimens from the aqueduct’s nearest (south eastern) sections of the aqueduct’s central span. With one sweep, 24 adult females and 1 male were secured and transferred to suitable sections of the metal footbridge over the canal and to metal bank piling at the southern approaches to the aqueduct. This was a timely exercise since by September the aqueduct metalwork had been sand-blasted, primed and repainted and no spiders remained.
WIND DISPERSAL
Although the translocation was beneficial in enabling some adult (possibly fertilised and gravid) females to survive, the exercise at this time of year was perhaps a token gesture. In warm, fine conditions on 2nd August, hundreds of spiderlings which had emerged from cocoons secured beneath the parapet overhang, were watched climbing to the sunlit and windswept parapet, producing strands of silk and ‘ballooning’ into the brisk southerly breeze. Although no suitable habitat was visible down wind of the aqueduct, a proportion of these aeronauts could possibly have been dispersed to sites at Southfield Reservoir and along the Aire/Calder Navigation some 500m to the north.
GREGARIOUS BEHAVIOUR
The rather bedraggled, often huge (up to 2.7m2), orb webs frequently linked up with adjacent webs and although the spiders were well separated, they readily moved from web to web when disturbed without obviously exhibiting aggressive behaviour to their neighbours.
The section by section count mentioned above revealed that although the .83m x 3.3m aqueduct section, were frequently occupied by only one spider, figure 2 shows that up to 14 adult and sub-adult specimens could be accommodated apparently without exhibiting aggressive behaviour to each other. Significantly, while being transported in the confines of the butterfly net, the 25 spiders did not seem to either avoid or attack each other, a phenomenon rare in web-spinning spiders which I have only previously encountered with the colonial Steatoda bipunctata (L).
ACKNOWLEDGEMENTS
I would like to thank local angler Mr. Tony Brian for the use of the extendible fishing rod, and Mr. Bill Richardson, British Waterways security guard, for assistance during the nocturnal element of the survey.
REFERENCES
Bristowe, W. S. (1971). The world of spiders. Collins, London.
Jones, D. (1983). Country life guide to spiders of Britain and Northern Europe. Hamlyn, London.
Roberts, M. J. (1995). Collins field guide, spiders of Britain and Northern Europe. HarperCollins, London.
Smith, C. J. (1982). An atlas of Yorkshire spiders. Privately published, York.
Smith, C. J. (1985). Additions to an atlas of Yorkshire spiders. Privately circulated.
C. A. HOWES