Snare Traps and Night-vision Goggles: Net-casting Spider on a Hunt

Not all webs are made equally

For most people the idea of a spider web is either a cobweb at home or the more popular radial structure that resembles a wheel with spokes. There are over 48,000 species of spiders found throughout the world. With this astounding diversity comes an equally impressive mix of how spiders use their silk. Orb-weaver spiders build web structures that fit our typical description of one — think of the web on Spiderman’s suit. While spiders in the family Theridiidae build ‘scaffolding webs’ that are tastefully complex 3D structures, the Agelenidae family decorate the ground with ‘funnel webs’ that make it a minefield for other invertebrates.

Despite the many structural variations, most spiders that use their silk to hunt have evolved to build large webs and wait for prey to entangle themselves in them. One among a few interesting exceptions to this trend is the net-casting spider. This spider uses its silk to build a net-like trap that it casts on unsuspecting insects that end up below it or even fly too close it.

Eight-legged scientist

Preparations for the hunt begin at dusk as the net-casting spider begins weaving its net. Spiders primarily produce silk from a specialised organ called the spinneret; some spiders like the net-casting spider produce an additional form of silk from a separate organ called the cribellum. Using its spinneret, the spider creates an anchor line (dragline) from which it is suspended and radial lines to hold its horizontal position. The production of cribellate silk then begins as the spider carefully uses the calamistrum (comb-like like structures) on its rear-most legs to ‘comb’ out cribellate silk. This silk is then carefully weaved into its signature capture net. To the naked eye, the net looks like it is made of fuzzy lines. Take a closer look (with a microscope) and you will notice that each fuzzy line consists of a core line with silk coiled around it. This affords the capture net immense flexibility and improves its efficacy in ensnaring the spider’s next meal.

Both hunter and scientist, the net-casting spider goes on to fine-tune its preparation. With multiple radial lines tethered towards the substrate (a leaf, the ground, or any surface below where the spider thinks its next meal will be found), the spider holds onto these lines with its middle pairs of limbs. With its head pointed down towards the substrate, and last pair of limbs pointed in the opposite direction, it proceeds to slowly lower itself by allowing its ‘dragline’ to carefully slip through its fourth pair of legs. This sequence resembles the famous scene from the first ‘Mission Impossible’ movie where the protagonist lowers himself on a dragline towards the floor, but stops inches away. The spider does just this and stops close to the substrate, tapping it with its front limbs to get a final estimate of distance.

Once the calibration of distances is set, the spider ascends along the dragline back into position. Through the process of determining distance during its test, the spider’s dragline will now allow it to free fall towards prey without crashing into the substrate. Aside from the careful preparation and testing, some net-casting spiders have been seen to leave target markers on the substrate using faecal matter. In the pitch darkness of the forest, these markers act like laser pointers on a gun for this precision hunter.

Night-vision goggles

For a hunter that relies on the dark forest night to surprise its prey, target markers wouldn’t mean very much if the spider had no way of seeing them in low to no light. That’s where one of the most powerful set of night-vision goggles in the natural world comes into play. The net-casting spider is equipped with night vision so advanced that even the faintest photons of starlight reaching the forest floor are enough for it to see its prey. It possesses one of the most powerful set of eyes in the animal kingdom.

One way of gaining some perspective on the power of their eyes is aperture. Often written as an ‘f-number’, it is quite simply the ratio between the focal length to the diameter of the pupil. Lower the f-number, greater the amount of light entering the lens. This is why photographers would refer to a camera lens with an aperture of f/2.8 as a ‘faster’ or more light-sensitive than one with an aperture of f/5.6. While the human eye is capable of f/2.1 in really low light conditions, specialised predators like owls have apertures around f/1.13 in low light and cat eyes are known to go even lower. To hunt with remarkable precision in near darkness, the net-casting spider has an aperture of f/0.58. Combined with very large light receptors inside their eye, the net-casting spider absorbs 2,000 times more light than the human eye for the same field of view.

And that’s a wrap!

With meticulous preparation in place and advanced night-vision in check, the hunt begins. The net-casting spider takes its position, suspended above the substrate holding the net with its first pair of limbs. When an unsuspecting insect walks across the substrate within view of the spider, it rapidly lets the dragline slip and fall towards its prey. The rapid descent is combined with expanding the highly flexible net moments before reaching its prey. The prey is then incapacitated with venom and wrapped up into a wholesome meal.

At times the net-casting spider may find itself set up for a strike when potential prey chooses to fly near it rather than walk beneath it. The spider has adapted a very special technique to find themselves an ‘in-flight meal’. In a moment that combines the flexibility of yoga with the speed of a karate chop, the spider performs a ‘backward strike’. From its suspended position, the spider stretches backwards and expands its net to snatch its next meal straight out of the air. This type of attack happens thanks to the spider’s added sensitivity to subtle vibrations in the air. Net-casting spiders were seen performing ‘backward strikes’ in response to vibrating tuning forks that simulated the vibrations that may be caused by the flight of small insects.

A hard day’s night

As dawn approaches, the net-casting spider prepares to rest through the day. However, sleeping during daylight, with eyes as powerful as theirs might be comparable to trying to sleep while looking at the sun through a pair of binoculars. While some animals like the cat have adopted reducing their pupils to a thin slit to deal with daylight, the sensitivity levels of the net-caster require a more drastic solution. As the spider assumes its cryptic daytime position to rest each morning, it begins to rapidly destroy the ‘rhabdomere membrane’ in its eye that is responsible for its incredible lowlight vision. What’s more astounding is that it re-synthesises this layer each night in just about an hour.

Beauty of ogres

One of the common names given to this remarkable spider is the ogre-faced spider. A reference to a rather unpleasant looking mythical creature, which makes a comparison to the spider’s frontal appearance. When we observe nature, our worldviews often seep through, colouring our observations with shades of stereotypes. While analogies, metaphors, and anthropocentric comparisons can be useful in meaningfully communicating our experiences with each other, they can also be limiting, misleading, and not too polite to the natural wonder in question. In my own time spent around these gentle forest dwellers, the only traits immediately evident to me were patience, discipline, and an uncompromising attention to detail.

Our world is a tapestry of life. Like birds, plants, and net-casting spiders, we are threads in this delicately balanced system. Each thread is just as important as the other, each thread interdependent on the other, and each thread capable of inspiring the other; even the ogre-faced ones.