Strawberry spider

The brightly-coloured strawberry spider (Araneus alsine) is rare and highly localised in Scotland, living in small, damp clearings in deciduous woodland.

Global distribution

The strawberry spider occurs in much of Europe, from Britain and Scandinavia through France, Germany and Poland to Italy, Romania and Bulgaria. It also occurs in Turkey but is apparently absent from Ireland.

Distribution in Scotland

The strawberry spider has a very localised distribution in Scotland, having been found only at Killiecrankie in Perthshire and at Corrimony, near Glen Affric, prior to 2002. Subsequently, it was recorded near Fort Augustus and Spean Bridge, before being found on Dundreggan during a spider survey there in 2009. Since then, it has been found in other parts of Glen Moriston, in Glen Garry and at Loch Arkaig, with the Glen Moriston records comprising the bulk of the currently-known population in Scotland.

Elsewhere in the UK, the strawberry spider has been recorded from a number of sites in the southeast of England, and there are a few records from Lincolnshire and south Wales. This gives it an unusual overall distribution in Britain, the reasons for which are not immediately apparent.

The strawberry spider has not been assessed for its conservation status at an international level, so it does not feature on the IUCN Red List of Threatened Species. However, it is included in the national Red Lists for several countries, including Belgium (where it is listed as Vulnerable), Denmark (where it is classified as Endangered) and Norway (where it is rated as Near Threatened). In Britain, the strawberry spider is also listed as Near Threatened and Nationally Scarce.

The strawberry spider is a relatively large spider in the Araneidae family. This is the family of spiders that make orb webs – the spiral, wheel-shaped webs that are commonly found in gardens, verges and woodlands – and it gives rise to the species' alternative common name, the orange wheelweaving spider.

The strawberry spider is very distinctively coloured, being bright orange to deep red, with a pattern of pale spots on its abdomen, which provides the resemblance to a strawberry that is indicated by its name. As with most spiders, the female, at 7-13 mm. from the top of the head to the tip of the abdomen, is larger than the male, which reaches a size of 5-6 mm and is slightly darker in colour. The female has a larger, more spherical abdomen, and this becomes particularly enlarged when she is pregnant.

The strawberry spider's body consists of two main parts – the cephalothorax (which includes the head and the attachment point for the legs) and the abdomen. There are four pairs of legs, which are covered in hairs and spines. Like most spiders, the strawberry spider has eight eyes, and these are arranged in two rows, with four in a central, square formation and a pair set to each side, one above the other. The eyes are 'simple', in that they do not have the multiple facets that are found in the compound eyes of insects.

A pair of appendages that are attached to the front of the cephalothorax are called palps. In the male, these are much larger, looking like miniature boxing gloves, and are used during mating to transfer sperm into the female's genital opening. The spider's mouth is on the front underside of the cephalothorax, and is flanked by the chelicerae, or fangs, which are used to bite prey and inject venom.

At the rear end of the abdomen the spider has three pairs of spinners, which extrude the silk that is produced by glands in the abdomen itself. The strawberry spider spins a small web, which is situated in a horizontal plane, a few cm. above the ground. The web is circular in overall shape, but is made from a continuous spiral thread of silk, and is almost invisible to the unaided human eye. A stronger section of thread, known as the signal thread, is spun from the hub or centre of the web to the spider's retreat, where the spider waits, with one leg on the thread, to detect any movement that indicates that prey has become ensnared. 

The female strawberry spider makes a retreat for herself, in which she spends much time, waiting either for prey to be captured in her web, or for a male to appear, for courtship. The retreat is made from a leaf, usually from a birch tree (Betula spp.), which she sews together with her silk into a conical shape, with the pointed end facing upwards. This is attached to some low vegetation, which in Scotland is almost always the leaves of purple moor grass (Molinia caerulea), at a height of about 15-20 cm. above the ground.

Occasionally leaves from another tree such as oak (Quercus petraea) may be used instead, and instances have also been found where the leaves of bog myrtle ( Myrica gale ) have been used, attached in amongst the living foliage of the plant itself. The retreat provides both a good vantage point to see her web, which is spun beneath the retreat, and also excellent protection from predators, as the spider is invisible from above.

 

Like all spiders, it would appear that the strawberry spider feeds on a wide variety of small flying insects, mainly two-winged flies in the order Diptera, that get caught in its web. The spider injects captured insects with venom and then digestive juices, after which it sucks out the liquefied body of its prey.

Mating takes places in June or July, and begins with the male transferring the sperm produced by the testes in his abdomen to a tiny web that is specially constructed for this purpose. He gathers the sperm from the web into his palps and then approaches the female, performing a swaying motion under a female's leaf retreat, at a distance of 10-15 cm. If the courtship is successful, the male inserts his palp into the female's genital opening, or epigyne. Once the sperm has been transferred, a 'cap' from part of the palp breaks off, sealing the female's opening, so that no other males can mate with her. After fertilisation, the female lays her eggs and spins a special spherical sac to cover them, for protection from predators and parasites.

Egg-laying continues until August, and a large number of spiderlings will hatch from each sac. As with other members of the Araneidae family, the spiderlings remain together en masse for a short time before dispersing. The young spiders go through a series of moults, as they outgrow their hard external cuticle. They overwinter as sub-adults and reach maturity in the early summer of the following year. Adult females are generally seen from June to October, while mature males are usually observed in June and July.

The strawberry spider is quite specific in terms of its habitat requirements. In Scotland it occurs in damp clearings in deciduous woodland, where the vegetation consists of bog myrtle, purple moor grass and meadow flowering plants. The trees surrounding the clearings provide the leaves that are used by the females for their retreats, and those are sited in the open, up to 25 metres from any trees. Very small clearings appear not to provide suitable habitat, and the same holds true for large open areas that are too far from trees.

Predators often have a regulatory function on the populations of the species they feed on, but this is probably insignificant in the case of the strawberry spider, because of the random nature of the prey caught in its web. Spiders themselves are taken by other predators, which can include insects, spiders, birds, mammals and reptiles, and some of these are likely to prey on the strawberry spider.

Spiders are also susceptible to attack by parasitic wasps, which lay their eggs in the spiders' egg sacs. The wasp larvae eat the eggs or the developing spiderlings, before pupating to form a new generation of adult wasps. Nematode worms also affect spiders, growing as parasites in their abdomens, and a number of tiny parasitic red mites feed on some spiders, attaching themselves to various parts of the spiders' bodies. Whether these affect the strawberry spider is unknown.

As a colourful denizen of clearings in wet woodlands, the strawberry spider is one of the more charismatic invertebrates in the Caledonian Forest but there is still much to discover about its ecology.

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