Notes on Erigone welchi

From The Newsletter No. 21 May 1978

Erigone welchi has always been considered to be a rare species since it was first recorded by Dr. A. Randell Jackson from Co. Donegal in Ireland during September 1908 and named after Mr. E. Welch of Belfast who first collected the specimens described by Jackson (1911). Apart from a female in the Jackson Collection labelled "Carlisle 1911" and those referred to above, there were no other published records until Holm recorded the species from an inundated Carex meadow in the Virihaure area of Swedish Lapland about 90 miles north of the Arctic Circle (Holm, 1951). Since then there have been records in Britain from Hampshire, Perthshire, Argyllshire (Tiree), Radnorshire (now Powys), Caernarvonshire (now Gwynedd) and Co. Clare in Ireland. I believe the species has also been taken in Finland but I do not have the reference for this. Apart from the Welsh records (1975) all of the previous ones, including those from Sweden, have, as far as I am aware, been for no more than one, two or three specimens on each occasion. However, E. welchi is not really so rare as it appears to be if one knows its habitat requirements. Most of the Erigone species need small open spaces within which the webs can be constructed. E. atra and E. dentipalpis spin webs across small depressions in the soil or in grazed grassland. E. longipalpis is at the base of open grasses and rushes or similar micro-habitats in tidal estuaries. E. capra in wet cattle trodden pasture or wet mossy places. E. tirolensis and E. psychrophila in depressions under stones at high altitudes, and E. arctica and E. vagans are generally found in similar stoney habitats on coastal beaches and sewage beds respectively. But all these are generalisations; there is little doubt that these little spiders will colonise other micro-habitats which are somewhat similar. I have collected E. welchi at Migneint Bog in Gwynedd and at Ffos Llwyd in Powys. Both sites are very similar in character, the former being near to the source of the River Conway and the latter near to the source of the River Elan and both are blanket bogs with eroded peat hags with sphagna in the depressions, at an altitude of 1400 feet (430 m.) above sea level, The spider is most frequently found in September in wet sphagnum. I do not mean where the moss grows in luxuriant cushions or in those loose attenuated sphagna such as Sphagnum recurvum and similar species. One has to look carefully at those very wet areas in a quaking bog where the sphagnum is completely saturated, of a dark green colour, and growing only just above the level of the water.

Searching such an area, from reasonably firm ground, one can then see numerous tiny Erigone webs spun across depressions in the moss. The webs are rarely more than 45 mm in width and made visible by minute droplets of water adhering to the threads giving the webs a silvery appearance, similar to those of E. atra on a dewy grass lawn. Collecting the spiders is less easy on account of the treacherous bog surface. Scooping out a handful of the moss containing a web only results in a soggy mess on the collecting sheet when the web is lost and destroyed and only rarely does the tiny occupant reveal itself. The best method is to spread a large polythene sheet over part of the bog and adopt either a kneeling or completely prostrate posture to look for the spiders, One is sure to get wet knees in one position and wet elbows in the other because the water tends to run in over the edge of the sheet.

But armed with a stiff grass stem one can usually separate the strands of the web until the occupant becomes visible when capture by a pooter is possible. The web is a three dimensional structure and not a simple sheet web and the upper structure has to be removed to reveal the spider suspended under the sheet. Each sex can be taken in such webs, but the spider is very difficult to see against the dark green sphagnum. Several webs can be examined without success.

The species is certainly smaller than the size of 2.5 mm given in British Spiders Vol. II. Those of each sex which I have taken are barely 2 mm in length with the male carapace measuring 1 mm and that of the female 0.8 mm. Holm submits that Swedish specimens are smaller than those of Jackson and states a carapace measurement of 1.25 mm for Swedish specimens against 1.45 by Jackson; however, he has misquoted Jackson's measurement of 1.20 mm. E. welchi is probably our smallest British Erigone species. The long ventral tibial apophyses on the male palps each curve laterally inwards, a character which Holm has also remarked upon.

While adults of many of the Erigone species can be found at almost any time of the year I tend to believe that all these species are seasonal in reaching peak populations of adults and this period is usually early in October when aerial dispersal of the mass population takes place, and, before this happens the males have found the females in their webs at ground level and mating has been completed. But one cannot be dogmatic about this especially in the knowledge that small aerial dispersals do take place to a lesser degree at other times of the year. It may be that at least some of the Erigone species are reaching maturity in small numbers at almost all times and indulge in aerial dispersal after mating. More research is needed. When intensive collections are being made of ground-living spiders such as Erigone species the web structures are inevitably destroyed in the pursuit of specimens, We seem to know remarkably little about the web structure, prey and life histories of nearly all the micro-linyphiids. It is certain that many of these tiny species which occupy very sheltered habitats such as those provided by deep leaf litter in woods and are not exposed to the rigours of winter can be found as adults all the year round and in this respect are similar to spiders which inhabit caves. In these habitats the range of seasonal temperatures is less extreme than, for example, on a sphagnum bog which can be frozen solid in the winter months. In comparison with other linyphiid species there is some evidence to suggest that, because the Erigone species spin webs on open places, their life history is a seasonal one. On the 22nd January 1970 out of 27 different adult spider species collected in holly, beech, oak and sweet chestnut leaf litter in the Forest of Dean only one female specimen of Erigone atra was taken. On 29th January 1975 in freezing conditions at 1,500 ft (460 m), 13 different species of linyphiids, all adults, were collected in a sphagnum, Juncus and Polytrichum habitat on Migneint bog and there were no Erigone species. The following day, 30th January 1975, at Llyn-tyn-y-Mynydd sedge marsh 35 different spider species were collected when the genus Erigone was represented by only one specimen; a female of Erigone atra. On two of these occasions there were large numbers of both sexes of certain species: At the Forest of Dean 63 specimens of Microneta viaria; 27 specimens of Monocephalus fuscipes; 24 specimens of Centromerus dilutus where, in open spaces, there was a good deal of snow on the ground. At Llyn-tyn-y-Mynydd 29 specimens of Aphileta misera were taken and 8 specimens of Drepanotylus uncatus at a time when the open water was frozen over.

A good deal might be discovered about the seasonal life cycle of the linyphiid spiders if one was to study specimens kept for observation in a simulated natural habitat. One might start with the relatively large species such as Allomengea scopigera, Stemonyphantes lineatus and Walckenaera acuminata. Everyone is familiar with the elongated turreted head of W. acuminata in the male sex, but without perhaps realising that this head, normally seen in an upright position, is usually inverted and points downwards from the sheet web, This is the normal position of the spider when it makes its web, feeds, moults, mates and moves, suspended from silk lines.

References;
Randell Jackson, A. (1911) On a spider new to science recently found in Ireland. Irish Nat. 20 (20).
Holm, A. (1951) The mountain fauna of the Virihaure area in Swedish Lapland, Araneae. Lunds Universitets Arsskrift N.F., 2, 46
Locket, G.H. and Millidge. A.F. (1953) British Spiders. Vol. II. Ray Soc. London.
 
Added by John Partridge at 17:19 on Sun 8th Jan 2012.

THE HABITAT OF ERIGONE WELCHI JACKSON By J. R. Parker

From The Newsletter No. 21 May 1978

Erigone welchi has always been considered to be a rare species since it was first recorded by Dr. A. Randell Jackson from Co. Donegal in Ireland during September 1908 and named after Mr. E. Welch of Belfast who first collected the specimens described by Jackson (1911). Apart from a female in the Jackson Collection labelled "Carlisle 1911" and those referred to above, there were no other published records until Holm recorded the species from an inundated Carex meadow in the Virihaure area of Swedish Lapland about 90 miles north of the Arctic Circle (Holm, 1951). Since then there have been records in Britain from Hampshire, Perthshire, Argyllshire (Tiree), Radnorshire (now Powys), Caernarvonshire (now Gwynedd) and Co. Clare in Ireland. I believe the species has also been taken in Finland but I do not have the reference for this. Apart from the Welsh records (1975) all of the previous ones, including those from Sweden, have, as far as I am aware, been for no more than one, two or three specimens on each occasion. However, E. welchi is not really so rare as it appears to be if one knows its habitat requirements. Most of the Erigone species need small open spaces within which the webs can be constructed. E. atra and E. dentipalpis spin webs across small depressions in the soil or in grazed grassland. E. longipalpis is at the base of open grasses and rushes or similar micro-habitats in tidal estuaries. E. capra in wet cattle trodden pasture or wet mossy places. E. tirolensis and E. psychrophila in depressions under stones at high altitudes, and E. arctica and E. vagans are generally found in similar stoney habitats on coastal beaches and sewage beds respectively. But all these are generalisations; there is little doubt that these little spiders will colonise other micro-habitats which are somewhat similar. I have collected E. welchi at Migneint Bog in Gwynedd and at Ffos Llwyd in Powys. Both sites are very similar in character, the former being near to the source of the River Conway and the latter near to the source of the River Elan and both are blanket bogs with eroded peat hags with sphagna in the depressions, at an altitude of 1400 feet (430 m.) above sea level, The spider is most frequently found in September in wet sphagnum. I do not mean where the moss grows in luxuriant cushions or in those loose attenuated sphagna such as Sphagnum recurvum and similar species. One has to look carefully at those very wet areas in a quaking bog where the sphagnum is completely saturated, of a dark green colour, and growing only just above the level of the water.

Searching such an area, from reasonably firm ground, one can then see numerous tiny Erigone webs spun across depressions in the moss. The webs are rarely more than 45 mm in width and made visible by minute droplets of water adhering to the threads giving the webs a silvery appearance, similar to those of E. atra on a dewy grass lawn. Collecting the spiders is less easy on account of the treacherous bog surface. Scooping out a handful of the moss containing a web only results in a soggy mess on the collecting sheet when the web is lost and destroyed and only rarely does the tiny occupant reveal itself. The best method is to spread a large polythene sheet over part of the bog and adopt either a kneeling or completely prostrate posture to look for the spiders, One is sure to get wet knees in one position and wet elbows in the other because the water tends to run in over the edge of the sheet.

But armed with a stiff grass stem one can usually separate the strands of the web until the occupant becomes visible when capture by a pooter is possible. The web is a three dimensional structure and not a simple sheet web and the upper structure has to be removed to reveal the spider suspended under the sheet. Each sex can be taken in such webs, but the spider is very difficult to see against the dark green sphagnum. Several webs can be examined without success.

The species is certainly smaller than the size of 2.5 mm given in British Spiders Vol. II. Those of each sex which I have taken are barely 2 mm in length with the male carapace measuring 1 mm and that of the female 0.8 mm. Holm submits that Swedish specimens are smaller than those of Jackson and states a carapace measurement of 1.25 mm for Swedish specimens against 1.45 by Jackson; however, he has misquoted Jackson's measurement of 1.20 mm. E. welchi is probably our smallest British Erigone species. The long ventra1 tibial apophyses on the male palps each curve laterally inwards, a character which Holm has also remarked upon.

While adults of many of the Erigone species can be found at almost any time of the year I tend to believe that all these species are seasonal in reaching peak populations of adults and this period is usually early in October when aerial dispersal of the mass population takes place, and, before this happens the males have found the females in their webs at ground level and mating has been completed. But one cannot be dogmatic about this especially in the knowledge that small aerial dispersals do take place to a lesser degree at other times of the year. It may be that at least some of the Erigone species are reaching maturity in small numbers at almost all times and indulge in aerial dispersal after mating. More research is needed. When intensive collections are being made of ground-living spiders such as Erigone species the web structures are inevitably destroyed in the pursuit of specimens, We seem to know remarkably little about the web structure, prey and life histories of nearly all the micro-linyphiids. It is certain that many of these tiny species which occupy very sheltered habitats such as those provided by deep leaf litter in woods and are not exposed to the rigours of winter can be found as adults all the year round and in this respect are similar to spiders which inhabit caves. In these habitats the range of seasonal temperatures is less extreme than, for example, on a sphagnum bog which can be frozen solid in the winter months. In comparison with other linyphiid species there is some evidence to suggest that, because the Erigone species spin webs on open places, their life history is a seasonal one. On the 22nd January 1970 out of 27 different adult spider species collected in holly, beech, oak and sweet chestnut leaf litter in the Forest of Dean only one female specimen of Erigone atra was taken. On 29th January 1975 in freezing conditions at 1,500 ft (460 m), 13 different species of linyphiids, all adults, were collected in a sphagnum, Juncus and Polytrichum habitat on Migneint bog and there were no Erigone species. The following day, 30th January 1975, at Llyn-tyn-y-Mynydd sedge marsh 35 different spider species were collected when the genus Erigone was represented by only one specimen; a female of Erigone atra. On two of these occasions there were large numbers of both sexes of certain species: At the Forest of Dean 63 specimens of Microneta viaria; 27 specimens of Monocephalus fuscipes; 24 specimens of Centromerus dilutus where, in open spaces, there was a good deal of snow on the ground. At Llyn-tyn-y-Mynydd 29 specimens of Aphileta misera were taken and 8 specimens of Drepanotylus uncatus at a time when the open water was frozen over.

A good deal might be discovered about the seasonal life cycle of the linyphiid spiders if one was to study specimens kept for observation in a simulated natural habitat. One might start with the relatively large species such as Allomengea scopigera, Stemonyphantes lineatus and Walckenaera acuminata. Everyone is familiar with the elongated turreted head of W. acuminata in the male sex, but without perhaps realising that this head, normally seen in an upright position, is usually inverted and points downwards from the sheet web, This is the normal position of the spider when it makes its web, feeds, moults, mates and moves, suspended from silk lines.

References;
Randell Jackson, A. (1911) On a spider new to science recently found in Ireland. Irish Nat. 20 (20).
Holm, A. (1951) The mountain fauna of the Virihaure area in Swedish Lapland, Araneae. Lunds Universitets Arsskrift N.F., 2, 46
Locket, G.H. and Millidge. A.F. (1953) British Spiders. Vol. II. Ray Soc. London.
 
Added by John Partridge at 17:18 on Sun 8th Jan 2012. Return to Summary for Erigone welchi