The stars of Tasmanian caves – what lies behind the silk curtain

Humidity‐dependent mechanical and adhesive properties of Arachnocampa tasmaniensis capture threads

Piorkowski, T.A. Blackledge, C.‐P. Liao, N.E. Doran, C.‐L. Wu, S.J. Blamires, and I.‐M. Tso, 2018, Journal of Zoology, vol. 305, pp. 256-266

Navigation is impossible. The rush of the stream is the only sound I can hear and dizzyingly seems to come from all sides. Water is everywhere and just seems to fall out of the air. The ground is either made jagged and sharp by rocks or soft and squishy with mud. The darkness is so thick I can’t see my own hands no matter how close I hold them to my face or how wide I open my pathetic human eyes. The only thing I can see is overhead; a faint shimmer of hundreds of dotted lights (Figure 1). If I were a winged insect I might fly towards these lights thinking they were stars in the night sky shining through the forest canopy. In an effort to escape this horrible disorientation I would find myself trapped in a sticky curtain of silk, as I am not in a forest, I am deep within a cave. What was supposed to be salvation has become my worst nightmare.

Figure 1
Figure 1 – Hundreds of Tasmanian glowworms illuminating a cave. Copyright: SIXTEEN LEGS/Bookend Trust, photo by Joe Shemesh

Life in caves is harsh; given the persistent darkness, extreme humidity (often >95% relative humidity [RH]) and subsequent low productivity and nutrient availability, few organisms are capable of full-time residence. Animals that can survive here often display specialized characteristics, such as loss of pigmentation, elongated limbs and reduced ability to retain water, that would make inhabiting other environments difficult to impossible. For cave animals, finding food is quite difficult and it usually comes from the outside world in the form of detritus or wandering animals, such as a fly that has lost its way in the night. Sometimes, unique strategies are required to catch a tasty morsel.

Figure 2
Figure 2 – The silk curtain of a Tasmanian glowworm. Copyright: SIXTEEN LEGS/Bookend Trust, photo by Joe Shemesh

The Tasmanian glowworm (Arachnocampa tasmaniensis) produces a blue, bioluminescent glow from their abdomens used for luring prey. They build silken traps, or curtains, by anchoring a horizontal support line to the cave ceiling from which they hang sticky, beaded capture threads (Figure 2). Glowworms produce their capture threads from specialized glands in their mouthparts (Figure 3), which are used for arresting and adhering mobile prey that are hauled up for consumption (Figure 4). The sticky glue that coats the internal silk fiber is 99% water and quite susceptible to drying out, as is the animal itself. This opens the question of whether glowworms need the high humidity of the wet cave habitat to survive and thrive, as in, avoiding life-threatening desiccating and successfully foraging for food.

Figure 3
Figure 3 – Glowworm producing sticky capture threads. Copyright: SIXTEEN LEGS/Bookend Trust, photo by Joe Shemesh
Figure 4
Figure 4 – Glowworm eating a captured insect. Copyright: SIXTEEN LEGS/Bookend Trust, photo by Joe Shemesh

Our work demonstrated that glowworm capture threads were rubber-like and sticky when tested at cave-like humidity (>90% RH), but brittle and virtually non-adhesive at a more temperate humidity (30% RH). Stretchiness of the threads increased by over tenfold and stiffness reduced by three orders of magnitude, or by 1,000 times. Work of adhesion, which is force transferred over a distance, i.e., the wings of a struggling fly flapping against the silk curtain it is stuck in, increased by five orders of magnitude when in high humidity! This dramatic difference in mechanical and adhesive performance highlights that high humidity is, in fact, a requisite for capture thread functionality.

Figure 5
Figure 5 – Glowworm threads hanging from the cave ceiling. Copyright: SIXTEEN LEGS/Bookend Trust, photo by Joe Shemesh 

It is difficult separating myself from the majestic, glowing overhead light show in Mystery Creek cave of southern Tasmania. As I exit the cave, I find emotional detachment from this place almost as difficult as navigating through its mud, boulders and darkness. I guess conditions that are harsh for some are favorable to others. I turn on my eighty-lumen headlight to marvel at the thousands of wet, beaded silk strands gently dangling overhead one last time (Figure 5). As a visitor to glowworm country I am thankful I am not a fly, as I might not be leaving.

Dakota Piorkowski

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