Trapping trapdoor spiders

Bleeding diving petrel
Taking a blood sample from a diving petrel.

Midnight on Codfish Island. The full moon is riding to its zenith. There’s a high wind blowing and the stars are shining bright. I am in amongst the large rear-dunes, leaning against an almost vertical sandy wall. My face is pressed up against a small opening, a little round tunnel a few centimetres across. And I am calling into the void. “Kuaka, Kooooo-ack-aaaaaaa”. I repeat this process for a few minutes, feeling a little self-conscious. And then a stirring in the darkness. A muted “kuaka” call comes from within. A small beak pushes out of the gloom. “Kuaka”. The bird, a South Georgian Diving Petrel, looks inquisitively around and takes another step forward. I reach my fingers in carefully, not wanting to disturb this fragile burrow built in sand, and clasp the bill of the bird. I slowly drag the bird out of the burrow. The kuaka sits in my hands, named by the Maori for the call it gives. I spend the next 20 minutes preening it for feather lice and taking a blood sample. Kuaka spend most of their time at sea. When on land they are in their dune burrows. A difficult species to work on! Enticing them out of their burrows is the only safe way to get this bird in the hand.

As biologists we often work on species that are difficult to handle. Currently I have students working on a range of animals that present their own challenges. There are fur seals (on land for short periods, as well as big and bitey), white-tailed deer (cryptic in dense native bush), bellbirds (difficult to get close to in scrubby habitat), Himalayan tahr (prone to hanging out on shear slopes in mountainous areas) and trapdoor spiders (live in burrows). All of these species require some thought as to how to find and interact with them. Trapdoor spiders should be the easiest. Vikki Smith, my PhD student, is looking at a New Zealand group of trapdoor spiders called Cantuaria. These species are found all around New Zealand. There are even populations of one of the species living in hedgerows here at Lincoln. Trapdoor spiders build burrows (which may or may not have a trapdoor). They are ambush predators and rarely leave the burrow. If a researcher comes along looking for a Cantuaria individual they flee down their burrow from which they can only be dug out (difficult as the burrows can extend for a considerable distance as well as the high risk of collapsing the burrow and burying the spider that you are after).

Tenebrio molitor beetle tethered.

Vikki wanted to develop a method for safely extracting spiders from their burrows. What would bring spiders to the door of their traps? Vikki reasoned that they would be interested if a large prey item was walking near their burrows and that when they attacked the prey then she could block the spider’s escape back down its burrow. In a paper about to be published in Arachnology (with supervisors Cor Vink, Rob Cruickshank and Adrian Paterson) Vikki details her method. Vikki took Tenebrio molitor beetles and passed a loop of cotton around the join between the thorax and abdomen and behind the first set of legs (or two loops for larger beetles) to form a tether. The beetle would then be placed near the trapdoor entrance and would begin walking about. The vibrations from the beetle would be detected by the spider who would lunge from its home to grapple the beetle. Vikki would then quickly place a small trowel in behind the spider to stop it escaping down the burrow while jerking the beetle away to safety.

Cantuaria dendyi (a local trapdoor spider).

Vikki trialled this method around southern New Zealand (including Codfish Island) collecting over a hundred specimens throughout the year. She was successful at most sites. Other arthropods to be tempted out of burrows were different trapdoor spider genera, wolf spiders and tiger beetle larvae. Vikki found that the spiders had to be collected at night (when they are most active) and that the beetles had to be warm to move (a problem on cool winter evenings). This cunning method means that we have a way of trapping trapdoor spiders and allows us to make great progress in understanding their biology. All in all this seems to be an excellent new way to access a difficult species. Perhaps there are others? I wonder if Vikki ever tried whispering “kuaka” down a burrow?

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