Cave Life of Wales

Life in the cave - Spiders and millipedes



The familiar cave spider is the large Meta menardi. The spider needs space to spin the characteristic orb web which is small considering the size of the spider. It is a large, dark brown, troglophilic cave spider that prefers to inhabit dim areas and areas where the light is reflected. It also appears to be slightly deeper in a cave that has two entrances close by, and in winter. Some arthropod troglodytes move towards the threshold of the cave at wetter times of the year – in winter this may change the dynamics of the food web. It also prefers vertical shafts, presumably more prey might fall in. The webs are orientated away from light – at 90 degrees to the light. The web is found in the shadows e.g. in phreatic pockets and scallops – by spinning them here they may trap fungus gnats seeking shade, or maybe the web is harder for prey to spot and avoid. The web is not spun across the passage unless there is nothing else to anchor web to, it is often parallel to the wall. The spider primarily uses its web as a supportive platform rather than using it to actively trap prey (as with most orb-spinning spiders) as the webs are not stickey.   Some of the outlying threads of the web do act as "tripwires" however to alert the spider to the presence of prey on the walls and ceiling. They feed on woodlice, millipedes, slugs, earthworm, flies (Culex pipiens in particular), maybe the smaller Metellina merianae (but there is no evidence of this). It can gorge itself and may store food by trussing it in the web.

Meta menardii with a woodlouse, Porth yr Ogof.

The female lays a clump of eggs that and spins a silk cocoon around them for protection. You may see these hanging from the ceiling.

Metellina (=Meta) merianae is found in the threshold, often at the entrance itself where it spins a larger, finer orb web across the passage to catch creatures flying in and out of the cave. The spider is a similar shape to Meta menardii, but smaller, brown and grey in colour with spots on the legs.


 Metellina merianae in Porth yr Ogof.


In the cracks by the cave entrance you might see Nesticus cellulanus. It is a smaller, paler spider that builds a fine, criss-cross platform web that is attached to the walls by longer threads that have a sticky 'gum' drop near the base. The Nestiscus web traps crawling and flying insects and in this way it avoids competing for the same food as M. merianae.

You may also see sheet webs by the entrance containing silk tunnels. These are made by Tegenaria sp. but the spider is usually well hidden in a crevice behind the web.

There a number of 'money spiders' (e.g. Porrhomma sp.) found in caves. P.convexum is common – but there are few records in Wales. In the dark zone of Ogof y Ci you may find Britain’s only troglobitic spider, Porrhomma rosenhaueri. It is a straw coloured blind spider – but it is only 2mm long and probably hiding in a crack so you will have to look carefully to find one. Other 'money spiders' include Lessertia dentichelis and Lepthyphantes pallidus. Porrhomma sp. webs are often in obscure cracks and crevices in the walls of the cave, and the spiders even harder to spot.

Porrhomma rosenhaueri on flowstone.

All spiders are predatory but they themselves may not have many predators in the cave. Bats do not appear to be interested except possibly Natterer's bat (Myostis natterei) in caves during cold weather or when hibernating. Young spiders (spiderlings) have to shed their skeletons five to ten times (moult or ecdysis) to grow, and often the discarded remains are mistaken for dead spiders. The newly emerged spider is slightly paler for a while.

PSEUDOSCORPIONS are small and abundant arthropods but are difficult to spot as they are only 2-4mm in length. They are aggressive hunters and use their enlarged pedipalps to catch prey. Once it has sucked up the contents of its prey it can survive for weeks even months without another meal.

Pseudoscorpion Roncus lubricus, Otter Hole


MITES are small relatives of spiders. There are a number of mites found living in Welsh caves and some are probably troglobites. Rhagidia spelaea is relatively common (found in Ogof Ffynnon Ddu and Ogof Clogwyn). Some mites are truly microscopic and go unnoticed. They live in a variety of habitats - some free living, some parasitic on plants and animals, others preying on e.g. Collembola. Eugamasus magnus and E. loricatus have been found in Porth yr Ogof and Eglwys Faen. There should be some water mites to add to the list of mites found in Welsh caves. In Ogof Ffynnon Ddu Rhagidia sp. that prey on Collembola are widespread but infrequent (like many cave creatures).

Three Calyptostoma velutinus mites on the cranefly Limonia nebeculosa, Porth yr Ogof


Parasitic ticks and mites in caves are associated with bats.

TICKS The most common tick on bats is Ixodes vespertilionis, the female lives on the blood of the bat and is closely associated with caves.


The Pauropoda ‘small feet’ are tiny (1mm), blind creatures similar in shape to centipedes with 9-11 pairs of legs that are generally found in soil feeding on dead plant matter. They look a bit like springtails but if you blow on them they run quickly backwards. They are thought to be scavengers. Although there are no records from Welsh caves there are cave records from elsewhere in the country and they might be found in accumulations of detritus in the threshold zone.
The Symphyla are slightly larger (still under 1cm) blind, white centipede-like organisms that also live on dead plant matter in soilJuveniles have six pairs of legs, but over a lifetime of several years they add an additional pair at each moult, so that the adult instar has twelve pairs of legs. Scutigerella causeyae has been recorded from Ogof y Ci.  Symphylella isabellae has been found in Guzzle Hole, elsewhere in the country it has been found far into the dark zone.

Millipedes (Diplopoda – ‘twin feet’ have two legs per segment). Millipedes date back to over 425 millon years ago, making them the oldest terrestrial animals. Those we see today are remarkably similar to their ancestors. They are adapted for pushing their way through soil and leaf litter as they feed on decaying plant matter including wood. Millipedes are docile creatures and often curl up if they feel threatened, but also have passive defence mechanisms including caustic and noxious chemicals. Some release cyanide, which smells a bit like bitter almonds. Their eggs hatch into a 6 legged first stadium which can be confused for an insect. It moults a number of times, gaining segments and legs each time until it becomes the adult. Some species die after mating and therefore only live for one year, whilst other species may live for five years. The short life cycle of some species means that at certain times of year it is difficult to find adults. Cave forms often have a thinner shell and more sensory bristles than those on the surface.  Millipedes are one of the larger creatures in the cave ecosystem and include a number of cavernicolous species, many of which are blind.

The Order Polydesmida (the flat backed millipedes) get their name from the broad extensions to the body armour giving them a flattened appearance. They include Nanogona, Polydesmus, Brachydesmus, and Oxidus and have distinctive flattened bodies with about 20 segments, grow to 20-40 mm long and vary in colour from whitish to brown. Some of these species produce hydrogen cyanide.
The Eyed Flat-Backed millipede Nanogona polydesmoides (=Polymicrodon polydesmoides) is a common, caverniculous species and widely distributed in Britain. It is paler than Polydesmus angustus, light brown in colour with distinct rounded lobes at the edges of its plates, and has ocelli (eyes). Although common in Britain it is much scarcer on the continent.
The Common Flat Backed millipede Polydesmus angustus is widely distributed in Britain and sometimes found in caves. The bodies of the blind adults are 14 to 25 millimeters in length flat, brown, with 20 segments. The plate segments covering the back are ridged along their lengths (they are more angular than N. polydesmoides). The antennae and legs are longer than in most other millipedes. As a defence mechanism they can secrete almond smelling fluids (cyanide) to repel predators.
Brachydesmus superus is small (8-10mm), blind, pale (light brown/off white) with 19 segments. It is considered caverniculous. On the surface most records of adults are from March to June with few records of adults during August and September. This suggests it has an annual life cycle and dies after breeding in summer, although a few early maturing adults may overwinter.

Brachydesmus superus on rotting wood, Porth yr Ogof


Another common caverniculous species is the blind Spotted Snake Millipede Blaniulus guttulatus. The red spots down the sides are stink glands (ozadenes). This millipede may be more common in caves during winter when the colder temperatures encourage the millipede to go deeper into the soil. A similar spotted species, but with eyes, is likely to be Cylindroiulus sp.

Spotted snake millipede Blaniulus guttulatus, Ogof y Ci


Centipedes are normally regarded as soil organisms and there have been few recordings from caves. They are active predators with poison claws feeding on nematodes, mites and other smaller insects.  The most common species that might be encountered is Lithobius microps (=L. dubosqui), a small (to 10mm), robust, red-brown, rather short-bodied species with 15 pairs of legs. Immature ones can be hard to identify as features are added as they grow such as antennae segments, teeth etc. It might be regarded as a possible cavernicole although unlike some other centipedes and many of the millipedes found underground this species has ocelli (simple eyes). When disturbed it may curl up like a millipede. After each moult centipedes may have a violet tinge for a few hours.







Copyright © 2007