Previously published in the SANSA News (South African National Survey of Arachnida), No. 21, May-Sept 2014, as Webs at Vermont Pan, Hermanus.
Hermanus people enjoy nature. During April 2014 a lady walking around the Vermont Salt Pan was intrigued by the extensive webs covering the plants bordering the water. On closer inspection it transpired that these webs are constructed by thousands of spiders. The mass of webs indicated of the size of the food source supporting such a vast number of spiders, as well as praying mantises, damselflies, etc.
The Vermont Pan is a local depression between the Onrus Mountains to the north and rocky outcrops and sand dunes to the south. The Pan was originally fed from a small waterfall, run-off and seepage from the mountains and a wetland to the west. With development some of the natural run-off was rerouted and replaced with surface run-off from built-up areas and leaking sewage systems. These caused the Pan to become highly rich in nutrients during certain periods of the year. Due to the Pan enrichment it is frequented by a variety of birds, especially the lesser flamingoes[1,2]. The main aquatic plant is Cladophora spp.
Analyses indicate that the Pan salt content can reach 50x seawater levels just before drying up. Since 1928 effects of housing development and roads and the planting of blue gum trees has had a devastating impact on the Pan being either dry or full for extended periods. When full the water level then reaches into the plants, mainly Cyperus spp., bordering the Pan. This is where the spiders build horizontal orb-webs in or close to the water . By late afternoon the webs are very thick and extensive (Fig. 1). But in the early morning webs are indiscernible as they are removed daily and digested. The intact web has an open hub with up to 30 viscid spirals. The spider hangs over the hub with the front legs stretched forwards. During periods of inactivity the spider will sit on plant stems with the body and legs pressed to the substrate. If the spiders fall in the water they are able to rapidly walk on the surface[4,5].
On conferring with Dr. Ansie Dippenaar-Schoeman (Plant Protection Research Institute, Agricultural Research Council) the reddish-brown spiders were identified as long-jawed water spiders of the family Tetragnathidae, possibly the genus Tetragnatha sp. (Figs. 2+3). They have elongated well developed chelicerae (“jaws”) with strong spurs and teeth (Fig. 3, jaws (cheliserae + fang), 8 eyes in two rows and an elongated abdomen. The spiders photographed are males (Fig. 3). In adult males some segments of the palp are modified into secondary copulatory organs (Fig. 3, copulatory organ)4.
These spiders feed mainly on mosquitoes and gnats (“muggies”) and other spiders4. Mosquitoes and muggies are currently hatching in abundance. During the late afternoon the exoskeletons of these larvae form layers at the water edge and amongst the vegetation. Sometimes the muggies can be observed as clouds late afternoon and early evenings – to the dismay of local residents. With all the horizontal webs just above the water, the spiders must enjoy a feast of trapped morsels.
Dr Vic Hamilton-Attwell
- Harding, W.R. 2000. Vermont Pan Assessment. For Overstrand Municipality. 18pp
- Euro-Africa. 2003. Pollution study of the Vermont Pan.
- Discussions and newspaper clippings collected from a number of local residents.
- Dippenaar-Schoeman, A.S. & Joque, R. 1997. African spiders. An identifcation Manual. ARC – Plant Protection Research Institute, Biosystematics Division, National Collection of Arachnida. 392pp
- Dippenaar-Schoeman, A.S. & Van den Berg, A.M. 2010. Spiders of the Kalahari. ARC – Plant Protection Research Institute, Biosystematics Division, National Collection of Arachnida. 114pp
- Silberbauer, M.J. & King, J.M. 1991. Geographical trends in the water chemistry of wetlands In the south-western Cape Province, South Africa. Afr. .J. Aquat. Sci. 17 (112) p82-88. 1991.