Friday, 23 October 2015

Eresidae: velvet spiders

Credit: Viridiflavus, licensed under CC BY-SA 3.0 via Wikimedia Commons.

The Eresidae is a small family of about 100 species, mainly distributed in the Old World. They are interesting as several species have independently evolved social behaviour: closely related individuals live in groups through their lives, cooperating in building communal nests and capture webs, prey capture and brood care. Thus, they are able to subdue larger prey, which they also share. There is just one British species, the Ladybird Spider, Eresus sandaliatus, which has managed to survive against the odds. They live in sandy, sunny slopes in lowland heather, building their tubes in the ground. Its common name only applies to the adult male (top shot), which, in contrast to the uniform velvety black of the females, it is a striking spider, with black legs ringed with white and a bright scarlet abdomen with six paired black spots. Females rarely if ever leave their burrows, while males wander in the spring in search of females. They are cribellate spiders, that is, they produce woolly silk that they brush with comb-like bristles on their rear legs and trip threads cover and surround each burrow.

Suicidal maternal care
Ladybird Spiders have a slow development and a high degree of maternal care. Males take three years to reach maturity, females take four. Males mature in the autumn, but overwinter as adults. In the spring, males emerge from their burrow and wander in search for mature females. After mating, the female produces her only clutch of eggs and guards them in her burrow. When the spiderlings hatch, she feeds them by regurgitating a nutritious liquid created as her internal organs liquefy. After a few days, she stops feeding the young and lets the spiderlings climb onto her body and feed from her, her disolved organs prepared to maximise the nutritional value to their young. Before dispersing, the spiderlings share the maternal burrow, adding to the silk threads.

Ladybird spiders are highly endangered in the UK and listed on Schedule 5 of the Wildlife and Countryside Act. Between 1816 and 1906 a total of six males and one female had been collected in the Bournemouth and Poole area, but none after that. The species was feared to be extinct, as the lowland heathland where they had been found had been lost to land development. In 1958 W.S. Bristowe stated in his book The World of Spiders:
I do not despair of Eresus being found in Britain again, and [...] I hope clues may help some lucky person to experience the thrill of feasting his eyes on the brilliant beauty of the wandering male...If anybody finds Eresus before I do, I shall feel no resentment.
Bristowe narrates a tantalising account of a shopkeeper describing a spider just like a male Eresus, his family had observed in Cornwall in 1932. He did visit the exact site, but he was not to be the lucky re-discoverer. He goes on giving many tips on the best way to look for the species, informing the reader on its natural history, which he was familiar with from his experience with the species in mainland Europe.
 Curiously, Bristowe died in the early days of 1979, the same year that the species was rediscovered in the UK in a small remnant of heath in Dorset, and he was probably never aware of the rediscovery. By 1992 the population was estimated to comprise fewer than 40 individuals and a captive breeding plan was established, first trialling the methods with the less endangered Danish spiders od the same species. As concurrent habitat management of the existing population was very successful and the population of the original site increased substantially, native spiders have been reintroduced to suitable heathland areas within the historical distribution range. The reintroduction program, which is ongoing, has been a success and there are now eight established populations in various patches of suitable habitat (you can donate here).

1 comment:

Midmarsh John said...

That is a real beauty.