Leg Hairs Prevent Spider From Sticking to Its Own Web

Biological Strategy

Eleanor Banwell

Image: Kristian Golding / Flickr / Some rights reserved

Physically Assemble Structure

Living systems use physical materials to create structures to serve as protection, insulation, and other purposes. These structures can be internal (within or attached to the system itself), such as cell membranes, shells, and fur. They can also be external (detached), such as nests, burrows, cocoons, or webs. Because physical materials are limited and the energy required to gather and create new structures is costly, living systems must use both conservatively. Therefore, they optimize the structures’ size, weight, and density. For example, weaver birds use two types of vegetation to create their nests: strong, a few stiff fibers and numerous thin fibers. Combined, they make a strong, yet flexible, nest. An example of an internal structure is a bird’s bone. The bone is comprised of a mineral matrix assembled to create strong cross-supports and a tubular outer surface filled with air to minimize weight.

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Capture, Absorb, or Filter Organisms

Many living systems must secure organisms for food. But just as one living system must capture its prey to survive, its prey must escape to survive. This results in capture and avoidance strategies that include trickery, speed, poisons, constructed traps, and more. For example, a carnivorous plant called the pitcher plant has leaves formed into a tube that collect water. Long, slippery hairs within the tube face downward. When insects enter the tube seeking nectar, they lose their footing and slide inside, unable to climb out and escape being eaten and digested by the plant.

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Move in/on Solids

To obtain needed resources or escape predators, some living systems must move on solid substances, some must move within them, and others must do both. Solids vary in their form; they can be soft or porous like leaves, sand, skin, and snow, or hard like rock, ice, or tree bark. Movement can involve a whole living system, such as an ostrich running across the ground or an earthworm burrowing through the soil. It can also involve just part of a living system, such as a mosquito poking its mouthparts into skin. Solids vary in smoothness, stickiness, moisture content, density, etc, each of which presents different challenges. As a result, living systems have adaptations to meet one, and sometimes multiple, challenges. For example, some insects must be able to hold onto both rough and slippery leaf surfaces due to the diversity in their environment.

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Arachnids

Class Arachnida (“spider”): Spiders, mites, ticks, scorpions

Arachnids get a bad reputation, but they play an important role in the ecosystem by controlling insect populations. Almost half of the species in the class are spiders, and most live in terrestrial environments. Unlike insects, most arachnids have two body sections: the cephalothorax, which is the head and thorax fused together, and the abdomen. Mites and ticks, on the other hand, have only one body section that is relatively flat. This body shape comes in handy for the species that are parasitic, because it makes it harder for their hosts to bite or scratch them off.

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Leg hairs of orb-weaver spiders allow it to build its web without getting stuck.

Some spiders capture their prey with the help of a sticky web. Strands of web silk are coated in droplets of glue that stop insects that fly into the web from escaping. Once the web has been made, spiders walk slowly and carefully across it ensuring they only touch structural strands, which are not coated with glue. While they are building their webs, however, they have to touch the sticky strands to attach them to the scaffold. The glue is non-specific and strong, so how are spiders able to construct their webs without getting stuck?

Orb weaver spiders’ legs are coated with hair-like structures called “setae”, which all point down towards the end of the leg. When a spider pushes down on a sticky strand, it doesn’t push down directly onto the strand, but past it, so the web strand slides along the surface of the leg. As it does this, the web line becomes snagged on the hairs, enabling the spider to direct the glue-coated silk down and force it to adhere to the structural strands. The hairs are branched, which stops strands from sliding all the way to the base of the hair, where they might contact the spider’s leg and become stuck.

When the spider removes its leg, it pulls it up past the newly attached strand. Droplets of glue slide down the setae to the tips where, because the tips are so narrow and sharply pointed, they easily disengage.

Many small adhesive forces combined create a strong adhesive force. Because the spiders’ leg hairs are randomly distributed, the sticky droplets slide off the ends one at a time, which prevents the forces combining to become too strong. In this way, the spider can manipulate sticky silk strands and position them while building its web, and then safely and easily pull its leg away afterwards.

Image: Leah Lowrey, Michael Smith / Public Domain - No restrictions

Magnified views of trichobothria reveal the density with which they can cover the spider's legs.

Image: Janice Haney Carr, Leah Lowrey, Michael Smith / Public Domain - No restrictions

Image: Ianaré Sévi / CC BY SA - Creative Commons Attribution + ShareAlike

Image: Mark Rosen / CC BY SA - Creative Commons Attribution + ShareAlike

References

“Orb-weaving spiders construct webs with adhesive silk but are not trapped by it….Video analyses of behavior and experimental observations of isolated legs pulling away from contact with sticky lines showed that the spider uses three anti-adhesion traits: dense arrays of branched setae on the legs that reduce the area of contact with adhesive material; careful engagement and withdrawal movements of its legs that minimize contact with the adhesive and that avoid pulling against the line itself; and a chemical coating or surface layer that reduces adhesion.” (Briceño and Eberhard 2012:337)

Journal article

Spiders avoid sticking to their webs: clever leg movements, branched drip-tip setae, and anti-adhesive surfaces

Naturwissenschaften, 99(2012): 337–341 | 02/03/2012 | Briceno RD, Eberhard WG

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