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.

Conservation
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).

Segestriidae: Tube dwelling spiders

This is a small spider family of which only three species of the genus Segestria are found in Britain, only one of them widely distributed. They are elongated spiders, with only six eyes and the first three pairs of legs facing forward. They have a characteristic dark abdominal pattern, reminiscent of an adder's. They live in silk tubes in cracks in logs, walls, under stones and cliff faces. The tube opening is surrounded by at least six radial threads extending quite a way away, which act as signal threads to the spider sitting inside the funnel and are quite characteristic of the genus. The spider is active at night, when she comes out to her retreat's entrance, poised with her forward facing six legs touching the radial lines, awaiting passing insects.
Green fangs
Segestria florentina (top photo taken in Sheerness, Kent, courtesy of Robert Jaques) is a large spider that can reach 22 mm of body length. It is almost black with iridescent green chelicers. It lives near sea ports and it is thought to have been introduced from at least the 19th century with produce from the Mediterranean, from where it is native.

Maternal care
The female makes her egg sac inside her retreat and remains with her spiderlings until they disperse. Matriphagy, where the mother dies and the spiderlings eat her, has also been reported.

Black Lace-Weaver wanderer

 In springtime male Black Lace weaver spiders roam in search of females. This one was dangerously walking on the pavement this morning, cold and wet from the night's rain. I took some quick shots and took him home for a white background session. The male palps look like they are holding white balls, these are the characteristically white and very visible palpar organs in this species.
 Black-lace weaver females (Amaurobius ferox), as their smaller relatives Amaurobius similis, have remarkable maternal behaviours, including guarding the spiderlings, feeding them using unfertilised eggs and then finally, encouraging them to eat their own bodies (matriphagy). For a fantastic video documenting this spider's maternal behaviour see this Arkive entry.
 More info in the British Arachnological Society page.

Scary spider story

For a belated Halloween celebration how about some cannibalism on spiders? I have posted before on maternal behaviour in spiders, but, after coming across this species a few times in the last few days (a male above) I had to write on its bizarre, and utterly horrifying behaviour. Amaurobius are common spiders, with three British species which often live on holes in house and garden walls, fence posts or tree trunks. Their holes' entrance is surrounded by a characteristic radial and random web of white silk, with adheres to prey like velcro. These spiders are mainly nocturnal and can trap relatively large prey such as bees and droneflies, presumably as the sit nearby the entrance hole.

Amaurobius hole in a wall...

...and its owner with a trapped a dronefly

The unusual behaviour relates to reproduction. Females lay their egg clutch during June and July inside their holes and sit over it for three weeks, when she opens the egg sac and the spiderlings hatch. The interaction between the spiderlings and their mother stimulates her and inhibits the maturation of the eggs of a second clutch. The female is not agressive and instead solicits the young to migrate to her ventral side and lays immature eggs, the so called trophic eggs, for the spiderlings to eat. 

(from Kim and Roland 2000)

Spiderlings deprived of this extra food do not survive as well, and gain weight much more slowly. A few days later, the female dies and the spiderlings promptly cannibalize her, gaining further weight and increasing their chances of survival when they disperse. The trophic eggs are fertilized and apparently viable, so the female is actually increasing the survival of her first clutch by sacrificing a possible second. Does she produce more or less offspring overall as a result? Kim and Roland, in experiments in which they compared spiderlings with access to trophic eggs with deprived spiderlings, estimated that the mother produced an average of 104 viable spiderlings of the first clutch at day 6 when fed with trophic eggs, compared with a total number of 102 when they removed females from the first clutch and allowed to lay again. This similar number of total spiderlings is, however, misleading as six out of the 10 females removed from their offspring either had abortive clutches or ate them themselves, suggesting that the viability of the second clutch as a replacement clutch is decreasing and the eggs of the second clutch could be specialising in their function as trophic eggs. Trophic egg feeding might have evolved as a way of reducing intra-clutch cannibalism at a time in the spiderling's life when they cannot produce the sticky silk required to catch their own prey. Next time you see Amaurobius holes in your wall, spare a thought for these hair-raising behaviours happening so near you at night.

More information

Kim KW, & Roland C (2000). Trophic egg laying in the spider, Amaurobius ferox: mother-offspring interactions and functional value. Behavioural processes, 50 (1), 31-42 PMID: 10925034