A quick update from here in the spider lab! Last week we had visitors from the Bookend Trust in Australia who are putting together a documentary called Sixteen Legs. The project (actually it is a suite of projects) is a fantastic collaboration between scientists, artists, writers, school kids and performers. The documentary will showcase cave fauna, and more specifically, the bizarre biology and behaviour of the Tasmanian Cave Spider (Hickmania troglodytes) .
The Tasmanian Cave Spider are huge spiders with leg spans up to 18 cm! They belong to an ancient, relict group (the Hypochiloidea) which dates back to Gondwanaland.
Tasmanian cave spiders are troglophiles, meaning they are adapted to live mostly in caves or in neighbouring dark places like hollow logs, although unsuspecting humans do occasionally find them in their bathrooms. Animals that live permanently in caves are called troglobites, and cannot survive away from their cave habitat.
Troglofauna often have special adaptations to living in dark habitats such as a loss of skin pigmentation and eyesight. To cope with living underground they may have increased sensitivity to smell, hearing and touch. Monsoscutid harvestmen (Opiliones) found in the cave systems of New Zealand have a unique adaptation to allow them to find food in such dark places. Unlike most harvestmen, these species have relatively well developed eye sight that allows them to orientate to the shining light of glow worms; their favourite prey.
Niall Doran and colleagues have been studying the biology of the Tasmanian Cave Spider for 20 years and have revealed some incredible aspects of their natural history that makes the basis of the Sixteen Legs film. The courtship behaviour of this species is a particularly drawn-out but fascinating process, sometimes lasting more than 5 hours. Males locate females, probably using smell given their dark location, and then proceed to pluck the web with his first two pairs of legs to attract the female’s attention.
Once the female appears convinced by the male’s courtship performance and chilled out sufficiently for the male to approach further, mating begins. This is a rather complicated arrangement, involving the male attempting to lock the female’s front legs together with his own legs. Although smaller in body size, males have elongated forelegs that play an integral part of their mating rituals. Once the female is locked in place, males use their very long second pair of legs to secure the female further, using a little crook on the legs to embrace her around her chelicerae (jaws). This locks the female’s jaws apart so she cannot attack the male while he is in such a vulnerable position.
The stage is now set for successful sperm transfer, which the male does by inserting one of his palps (a modified leg-like structure that holds sperm) into the female’s genital opening on the underside of her abdomen.
This beautiful dance played out between the couple is not the only interesting aspect of their natural history. After mating, female’s create a large silken egg-sac that hangs from the cave wall. The egg-sac is double-layered meaning that the eggs themselves are housed in a separate compartment from the outer sac wall, which keeps them protected from fluctuations in humidity and from possible fungal and bacterial infection. This turns out to be really important because the eggs have an incredibly long gestation time; it takes nine months for them to develop, just like a human baby!
As well as telling the story of the Tasmanian Cave Spider and their cave-dwelling friends, Sixteen Legs will include tales from people and their perspectives of spiders. From arachnophobes to the scientists that dedicate their lives to studying spiders, Sixteen Legs will cover it all. On their way back from filming a few scientists and celebrities in the UK, the team stopped in at NUS to chat to me and Chia-Chen Chang, another PhD student in the Li lab. We talked about my current spidery research as well as my previous work on harvestmen in the caves and forests of Waitomo, as well as how I actually ended up being interested in arachnids.
For more detailed information about the behaviour of the Tasmanian Cave Spider, which is where I got all the stories from for this blog, check out the paper in Journal of Zoology: here.