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Tasmanian cave spider (Hickmania troglodytes). Image Credit: Joe Shemesh © SIXTEEN LEGS/Bookend Trust

The underground world of spiders

  • BY Joanna Khan |
  • May 09, 2017

Spiders can be found even in the most inhospitable of places thanks to their amazing capacity to adapt.

IT MAY NOT COME as a surprise that spiders like to lurk in the dark corners of your garden shed, but they can be found in even gloomier places. Cementing their place in the nightmares of humans worldwide, spiders are also rulers of the underworld. Caves, that is.

Last month, scientists from the San Diego Natural History Museum caught people’s attention when they announced the discovery of a new species (and genus) of cave-dwelling spider in Mexico. The huge, furry Sierra Cacachilas wandering spider (Califorctenus cacachilas) was found in a small mountain range in Baja California Sur, Mexico, by a team of very surprised scientists.

Dr Maria Luisa Jimenez, a spider expert involved in describing the new species, said at the time that "in all my experience over the years collecting spiders on the peninsula, I had never seen a spider this large".

But don’t worry Australia, you aren’t missing out – we have our very own cave spiders right here. They have been underground for hundreds of millions of years, yet scientists are only just touching the tip of the iceberg when it comes to understanding them. Scientists abseiled and squeezed their way through underground systems in Tasmania to discover the giant, spindly-legged, Tasmanian cave spider, and are now working to understand its peculiar habits.

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A huge diversity of spider species live in cave environments – with many more to be discovered. (Image: Joe Shemesh © SIXTEEN LEGS/Bookend Trust)

Spiders, and other arthropods, inhabit subterranean spaces all over the world. Some spend their entire lifecycles underground, and are so highly adapted to caves that they cannot survive on the surface. Other species transition easily from below to above ground, generally living closer to cave entrances. A third group of species fit somewhere in between, the cave remains their stronghold, but they can also be found in other environments.

“World experts of different disciplines recognise the value of caves as ideal evolutionary laboratories,” say Dr Stefano Mammola and Dr Marco Isaia of the University of Torino, Italy, who co-authored a review paper on cave spiders in the journal Proceedings of the Royal Society B last month.

“Their [cave spiders] study gives insights into the adaptation of life to extreme conditions, which may unravel deep evolutionary processes that can be transferred to other ecosystems,” adds Stefano, lead author of the paper.

For example, a cave spider from Mexico will be completely different to a spider in the Pilbara region of Western Australia – where 12 new species were recently discovered in close proximity, each unique to its own underground void.

According to Stefan and Marco, cave spiders are extraordinary organisms that exhibit huge diversity of lifestyles, behaviours and body types (morphology). The smallest cave spider known to scientists (Anapistula ataecina) was discovered in Portugal and measures less than 0.5mm, while the largest one (Heteropoda maxima), discovered in Laos by a German arachnologist has a whopping leg span of about 30cm. These spiders “provide two examples of the extraordinary morphological diversity of subterranean spiders,” says Marco.

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Tasmanian cave spider with her unique egg sack. (Image: Joe Shemesh © SIXTEEN LEGS/Bookend Trust)

Welcome to the underground world of cave spiders

Hundreds of millions of years ago, spiders would have loitered in a diverse range of nooks and crannies, not dissimilar to the present. However, in the last 380 million years the Earth has faced multiple mass extinctions and extreme climatic changes. Yet spiders persisted, seeking refuge in caves during periods of uncertainty on the surface. While spiders certainly wouldn’t have been alone in their underground retreats, they were able to remain underground and thrive, even when conditions above ground had become hospitable once more. Although, in the tropical realm it is more likely that spiders actively made their way into caves, colonising what were previously unexploited habitats.

The spiders in your backyard probably only live for a couple of years, or even as little as just one season. In order to fit into the varied urban environment, these common spiders have sacrificed the longer life span that more primitive spiders possess, instead retaining a variety of body forms. However, cave-dwelling spiders have bridged that gap and don’t have to navigate such a trade-off. Not only are they long-lived, they are also highly adapted to their dark, energy-poor subterranean world.

“Generally in caves, the environment is relatively food-poor compared to the surface, so one way of dealing with that is to have a very low metabolic rate, which allows them to reduce their food intake and survive through periods that would otherwise result in starvation,” says Dr Niall Doran, director of the Bookend Trust and a research associate at the University of Tasmania. “Cave spiders don’t need to eat very much, in fact everything about their lifecycle is slow,” he says.

“Most spiders, when they produce an egg sac, they just chuck them in like marbles thrown inside a pillowcase,” explains Niall. The Tasmanian cave spider (Hickmania troglodytes) can’t afford to have such a blasé attitude toward reproduction. Life underground is a delicate balance mediated by narrow ranges of temperature, humidity, and light. In order for offspring to survive, the female spider must ensure her eggs can withstand the environmental conditions, and that the young will emerge at exactly the right time.

A female produces a large, pear-shaped egg sac, which hangs from the roof of the cave like a milky stalactite. Within the sac, the eggs are cushioned in a spindle-like ball, protected from the sides of the encasing sac and insulating them from fluctuations in temperature, humidity, and air flow. “There seems to be a point each year when droplets form on the outside of the egg sac, driven by the humidity balance, and when the droplets appear consistently that’s the cue for the young to emerge,” says Niall. “The female builds the egg sac with a weakened point at the top, which water droplets gradually bleed through.”

For the finale of this incredible reproductive display: the young emerge after a staggering nine months inside the egg sac, the amount of time in which some spiders complete their entire lifecycle from egg to death. The fascinating lifecycle of the Tasmanian cave spider is the subject of a new documentary called Sixteen Legs produced by Niall. An unusual reproductive cycle is just one way the Tasmanian cave spider has harnessed the underground domain; it also has a small body and long thin legs – about as big as a dinner plate – to be able to feel and move in the dark.

Scientists believe that millions of spiders and insects have yet to be discovered and described, and with so many subterranean spaces yet to have had human eyes laid on them, one can only imagine the creatures waiting to be found in the darkest of corners.

“So far more than 1000 subterranean spiders have been described, belonging to nearly 50 families out of 113 currently known,” say arachnologists Stefano and Marco.

“However, the recent descriptions of an array of cave spiders, especially in the Tropics, shows how there are still lots of surprises in store.” 

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