Spiders’ Leg Hairs Help Find Flying Food

beige and brown spider on light pink flower

Biological Strategy

Spiders

Mary Hoff

Image: Volker Framenau / Flickr / CC BY NC - Creative Commons Attribution + Noncommercial

Sense Touch and Mechanical Forces in a Living System

Perceiving touch enables living systems to detect other living systems around them and environmental conditions, such as air movement, water currents, and temperature. This ability can help them sense danger as well as opportunity, as when a Venus flytrap’s hairs sense the presence of an insect to eat. Sometimes, a living system senses touch or mechanical forces at a coarse scale; other times, at a sensitive scale that detects very subtle differences. For example, a human elbow is not nearly as sensitive to textures as human fingertips. Fingertips have dermal ridges and many nerve endings that increase sensitivity, enabling them to explore the environment in detailed ways. Elbows don’t need to sense at that level of detail.

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Sense Sound and Other Vibrations From the Environment

For living systems, sensing sound and other vibrations is important for communicating and detecting conditions within their environment. Living systems must locate a signal’s source so that they can move toward it (such as when it is food or a potential mate) or away from it (such as when it is a predator). To prompt an appropriate response, living systems must sense these signals, recognize their amplitude or volume (which is sometimes very low), and determine their direction. Living systems must be attuned to signals relevant to them and able to distinguish these from irrelevant sounds to avoid expending unnecessary energy. For example, owls’ ears are asymmetrically placed. This enables them to detect sounds more accurately, which helps them locate small prey at night and avoid wasting energy chasing down irrelevant sounds.

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Sense Motion

Perceiving motion is important for a living system to sense where it is in relation to a moving environment, which is critical in locating resources or wayfinding. This applies whether the environment itself is in motion (such as water movement coming from a nearby fish) or the living system is moving within a stationary environment (such as a bird flying through the air). Because motion dampens over distance and the cost of missing those motion signals is high, living systems must be quite sensitive to these signals. For example, fast-flying big brown bats have microscopic, stiff, domed hairs on their wing membranes that act as a sensor array to monitor flight speed and airflow conditions.

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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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Tiny leg hairs help spiders sense subtle air movement by conducting it directly to nerve cells.

Introduction

All animals eat—but first, they have to find their food. For predators, that means being able to detect the presence, location, and movement of their often-elusive prey. Wolves and their kin use their keen sense of smell. Eagles and other raptors rely on exceptional eyesight. For bats, it’s all in the ears.

For hunting spiders, the secret to success is touch––information gathered by hundreds of hairs protruding from their lanky legs.

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 Carr / Centers for Disease Control and Prevention / Public Domain - No restrictions

At high magnification, the cup shaped base into which trichobothria are set becomes visible.

Image: Luc Viatour / wikimedia commons / GFDL - Gnu Free Document License

The legs of a jumping spider sport specialized hairs that can detect the presence of nearby prey.

Image: Clemens Schaber / Interface /

Long, thin leg hairs known as trichobothria help a spider sense motion in the air around them. Courtesy of Clemens Schaber

The Strategy

Each of a spider’s legs has at least 20 (and potentially more than 100) sensory structures called trichobothria. Each consists of a thin hair of varying length and thickness embedded in a membrane at the bottom of a cup-shaped base. Below the membrane, the hair connects to three or four nerve cells.

Because of variations in size and location, each trichobothrium provides unique information about the location and movement of the prey in relation to the spider.

When an insect flies nearby, it sets the air around it in motion. The movement of the air—even the vibrations from sound—sets the hairs moving, too. Since each hair is suspended in a flexible membrane (instead of being held tight at the base), even the slightest movement is conducted right to the nerves.

Because of variations in size and location, each trichobothrium provides unique information about the location and movement of the prey in relation to the spider. In a split second, the nervous system is able to combine the information from the many trichobothria into a clear picture of where and when the spider needs to strike in order to capture its meal.

Learn how to recognize trichobothria from other spider hairs with the above video from Wild South Australia.

The Potential

The hypersensitivity to touch and vibrations modeled by trichobothria’s system of a long thin receptor suspended in a flexible membrane could be emulated at varying scales using different materials to detect different kinds of signals. At a large scale, they could help analyze underwater currents or detect the presence or activity of sea life. At the micro scale they could provide alternative options for sound detection in a variety of products.

In addition, a spider’s ability to use information gathered from and synthesized by a network of many sensors to detect the location and motion of an object could be used to improve our ability to do the same. Manned or unmanned aircraft could adapt the strategy for greater sensitivity to environmental clues to avoid collisions with birds, and ground vehicles could use it to improve maneuvers such as parallel parking or backing up. The sensory capacity might be built into wearable devices or clothing to make it possible to detect motion beyond the range of vision.

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Biological Strategy

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Sticky Hairs on Leg Aid in Prey Capture

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Last Updated May 24, 2022

References

“When the trichobothrium is deflected, the lower edge of the helmet presses against the end of the dendrite that inserts on the side in the direction of deflection (Fig. 7). Different deflection directions stimulate different sensory cells. This has the remarkable effect that a single trichobothrium “looks” in different directions with its three or four sensory cells, so that it can signal the direction of a stimulus to the spider (if the afferents are suitably connected to the central nervous circuitry).” (Barth 2002:93)

Book

Trichobothria - the Measurement of Air Movement

A Spider's World | 2002 | Friedrich G. Barth

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Reference

“Therefore, it seems that the ‘design’ of filiform hairs evolved to maximize mechanical sensitivity to higher frequencies (or equivalently the sensitivity to fast signals, such as transient and fast fluctuations), rather than to optimize the response at the frequency where the angular velocity is maximal. From an evolutionary point of view, this suggests the existence of strong pressures selecting for high sensitivity at higher frequencies.” (Bathellier et al. 2011:1141)

A … fly flying towards a stationary alert predatory spider produce signals in which the progressively increasing amount of higher frequency airflows signify imminent encounter [3,14]. The filiform hairs of prey and predator arthropods may have attained the mechanical response properties close to the physical limit over the course of evolution to precisely assess such close encounters.” (Bathellier et al. 2011:1142)

Journal article

Air motion sensing hairs of arthropods detect high frequencies at near-maximal mechanical efficiency

Journal of the Royal Society Interface | December 14, 2011 | Brice Bathellier, Thomas Steinmann, Friedrich G. Barth, Jérôme Casas

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