Researchers have discovered that jumping spiders, and quite likely other spiders too, can hear sounds over much greater distances than previously thought.
Spiders don’t have ears or eardrums and instead rely on tiny hairs on their legs to sense air perturbations and sound. New research shows these hair vibrations get converted into neural signals we all know as hearing, and experiments show that jumping spiders are able to hear and react to sounds from sources much farther than originally thought.
The Research
The paper by Shamble et al of Cornell University (www.cell.com/...) presents the behavioral and neurophysiological evidence that jumping spiders perceive and respond to airborne acoustic stimuli, even when the distance between the spider and the sound source is relatively large (∼3 m) at sound levels comparable to human conversation.
Researchers at Cornell University implanted tiny electrodes in a region of jumping spiders’ brains that would show whether sound was being processed. They then subjected the spiders (Phidippus audax) to various sounds and measured brain activity. The experiments were set up to make sure the spiders were responding to sound traveling through the air and not vibrations in the test equipment. One of the sounds was the familiar frequencies made by a wasp when it flaps its wings (80 Hz tone), which the spiders responded to by freezing — a typical fear response. Neurophysiological recordings from auditory-sensitive neural units in the brains showed responses to this low-frequency tone. Other frequencies (80 - 380 Hz), at human speaking volume levels were also met by neural responses. The spiders responded to sounds at distances of up to 10 feet.
High-speed cameras demonstrated that these sounds were picked up by sensory hairs that shook back and forth. After identifying an acoustically sensitive neural unit, the researchers used a linearly actuated micro-shaker to mechanically drive a single long sensory hair on the patella of the foreleg. Both airborne stimuli and direct mechanical stimulation of the hair generated increased neural activity across multiple frequencies (64–256 Hz).
Accidental Research
The researchers actually discovered this Spider sense by accident. It started with a fortuitous chair squeak.
"We were doing these studies on how the brains of jumping spiders perceive vision," Shamble explained, using technology that allowed them to monitor neuron firings in a spider's brain and convert them into human audible sounds. Shamble's colleague and co-author Gil Menda was working in the lab when he backed up his chair, which let out a loud squeak.
"The neurons started firing and we thought, oh, that's kind of weird," Shamble says. To test this, Shamble clapped — first, right next to the spider. The neurons fired. He continued to back up, expecting the firing to stop, because they thought spiders could "hear" only at very close range.
So, they decided that this required further investigation and created the experiments that led to this discovery.
Behavioral Relevance of Sound
Researchers surmise that sound detection can benefit these spiders in multiple ways, such as prey and threat detection. The observed frequency sensitivity of P. audax seems to make it well suited to detecting acoustic signals generated by potentially dangerous flying insects, such as predatory wasps and small-headed flies. Dominant acoustic frequencies of these flying insects are ∼100 Hz. The ability to detect these predators may be especially important for jumping spiders since unlike sedentary web-dwelling spider species, they are diurnally active, moving through the environment in search of prey and potential mates—a lifestyle that likely increases exposure to similarly diurnal predators such as wasps. Furthermore, unlike their web-dwelling cousins, jumping spiders lack the built-in defensive advantages that accompany life on a web. Thus, the ability to detect the presence of threats—even before they become visually apparent—could provide these animals with an important fitness advantage.
Spider Anatomy
Spider bodies include two tagmata (sections or segments), eight jointed legs, no wings or antennae, the presence of chelicerae and pedipalps, multiple simple eyes, and an exoskeleton, which is periodically shed.
The internal anatomy is shown below. I had no idea !
Respiratory System
In primitive spiders, there are two pairs of book lungs, however in more modern spiders one of these pairs has become modified into a pair of tubular tracheae.
Book lungs consist of an atrium or space into which numerous layers of membrane bound tissue extend. Blood (Hemolymph) flows through these plates and gases are exchanged. The gases pass in and out by a diffusion process, which is relatively inefficient.
In more active spiders, tracheae replace part of the book lung or the whole book lung. The tracheae are tubes that do not branch but run from the opening on the outside directly into the tissues and organs.
In jumping spiders, both the book lungs and the tracheal system are well-developed. Resting and low-activity oxygen consumption rates can be met by the lungs or the tracheae alone, while high oxygen demand activities require both.
Spiders do not actually breathe; there is no active pumping of air into and out of those small bodies. Insect respiration takes place by passive diffusion into and out of the book lungs and tracheal tubes.
Spider Nervous System
Smithsonian researchers report that the brains of tiny spiders are so large that they fill their body cavities and overflow into their legs. As the spiders get smaller, their brains get proportionally bigger, filling up more and more of their body cavities. The central nervous systems of the smallest spiders fill up almost 80 percent of their total body cavity, including about 25 percent of their legs.
The human brain has around 100 billion neurons; a mouse has about 70 million. The Portia spider is estimated to have about 600,000 neurons, putting it midway between the quarter million of a housefly and the one million of a honey bee.
The sensory information detected by spiders can be categorized as -
- touch and vibration, including sound
- proprioceptor input (position/posture of body appendages)
- visual and thermal signals
- taste, pheromone detection and probably some internal chemical signals.
Jumping Spider Vision
All jumping spiders have four pairs of eyes with one pair being their particularly large anterior median eyes, providing 360 degree vision. The eyes can zoom in and out and they can turn up and down and left and right. The spider can also turn its carapace (breast) more than 45 degrees to look around.
Unlike eyes of other insects, spider eyes are simple eyes, not compound; there is one lens in each eye. Jumping spiders don’t have an iris like we do, and their lens is solid, not flexible, which makes focusing difficult. They can tilt their heads to help with focusing. Their eyes also include an elongated tube with a second lens at the end, which is flexible. By adjusting the angle and shape of the inner lens, the spiders can focus and zoom in on at the target, in full color. In principle, they can see an even broader spectrum of colors than we can, including UV.
The field of view of the primary eyes is quite narrow and the retinas have only about a thousand receptors compared to the 200 million or so of the human eye. But spiders can swivel these tiny eyes across the scene in systematic fashion, panning, zooming, scanning, processing visual information and building up a more accurate picture of what lies ahead.
Overall, jumping spiders’ visual acuity exceeds by a factor of ten that of dragonflies, which have by far the best vision among insects; in fact the human eye is only about five times sharper than a jumping spider's.
Spider Locomotion
Spiders are unusual in using the body fluid pressure or hydrostatic pressure from their blood (or hemolymph) to move. Spiders use hydrostatic pressure to extend their legs, but muscles to flex the legs. Spiders are able to control their heart rate to control hydrostatic pressure. The faster the heartbeat, the more force is produced due to higher hydrostatic pressure. With this simple mechanism, spiders have adapted to develop various means of locomotion. Many spiders also utilize silk and natural wind in a special locomotion called ballooning. blogs.cornell.edu/...
When jumping they can generate up to 8 times their resting pressure. This means they can run very fast and jump really high. The fastest spider (Giant House Spider – Agelenidae: Tegenaria duellicia) can run 1.73 ft/s. This is very fast considering that the size of the spider is only 0.59 inches. They can move 34 times their body length every second! The Jumping Spiders can jump up to 25 times their body length using their 3rd and 4th leg pairs.
For jumping spider lovers, here is a video of jumping spiders in slo-mo.
Spider Hairs
Spider have tiny hairs covering their legs and other appendages. The hairs play a role in the senses of touch and smell. They are capable of detecting air movement and sound, typically to detect prey, predators and other threats.
Their ability to detect sound over short distances has been known for a while.
Research by physicist Brice Bathellier of the Institute Of Molecular Pathology in Vienna, who co-authored a study of trichobothria hairs in the Journal of the Royal Society Interface, suggests that each hair acts like a single, independent ear — not a network of ear parts. Each hair is its own ear that responds to a narrow range of low frequencies, filters out background and high-frequency noise and zeroes in on biologically relevant information, such as an unwary cricket’s hopping or a spider’s sneaking.
The new research by Shamble and his colleagues show that these same hair are capable of detecting sound from distances of 3 meters.
After identifying an acoustically sensitive neural unit, the researchers used a linearly actuated micro-shaker (Physik Instrumente) to mechanically drive a single long sensory hair on the patella of the foreleg at a range of amplitudes (maximum displacement 10.8 μm). Both airborne stimuli and direct mechanical stimulation of the hair generated increased neural activity across multiple frequencies (64–256 Hz).
Spider Neuron Monitoring
Spiders had long been thought inaccessible to neural recordings because their bodies are filled with pressurized fluids. As they are pumped from one chamber into another, the fluids help spiders move their limbs to walk, run, or jump. But that internal hydraulics system poses a technical challenge for neurobiologists.
Glass electrodes, typically used to record from invertebrate neurons, are too fragile to poke through the spider’s exoskeleton. Researchers had tried to surgically expose the brain first, opening windows through which to insert the electrodes. The spiders didn’t fare well.
“Once you poke a hole in it, the blood squirts out and the animal dies within a few seconds or a minute,” says Cornell University neurobiologist Ronald Hoy, whose team pioneered the new technique.
Hoy’s group used a much stronger tungsten electrode typically used in vertebrates, such as mice and monkeys. Inserted carefully, these probes could push directly through the spider’s body into the brain without the need for extra cuts.
To prepare the spiders for neural recording, the researchers first put the naturally skittish animals in the refrigerator for a few minutes to slow them down. Then, with a pair of forceps, they loaded the spiders—which range from a few millimeters to just over 10 millimeters in length—into a custom 3D-printed plastic harness positioned in front of a video screen. Strategically placed dabs of wax secured the animals in place.
An extracellular tungsten microelectrode (4 MΩ, MicroProbes), advanced using a motorized hydraulic microdrive, was guided through a small hole in the cuticle and into the spider’s brain. The wound was small enough (about 150 micrometers around) that only a tiny bead of fluid escaped, which sealed into a clot around the electrode and stabilized the preparation.
Spider “Intelligence”
How intelligent are spiders?
One could argue that any organism that walks, runs, jumps, catches prey, avoid predators, has senses of vision, hearing, touch, taste and smell, responds to various stimuli in the environment and goes through elaborate mating rituals is by definition intelligent. But spiders go even beyond that.
Unlike web-spinning spiders that wait for food to land in their webs, jumping spiders use their specialized visual system to actively stalk and pounce on their prey. The hairy arachnids often scuttle along circuitous routes to avoid detection, tackling their target from above. Female jumping spiders also size up the gyrations and gesticulations of males performing courtship dances.
In the 1980s and 1990s, Robert Jackson of New Zealand’s University of Canterbury demonstrated that Portia fimbriata, a member of the spider-snacking subfamily Spartaeinae, methodically plans winding detours to sneak up on prey spiders. Portia can even find hidden prey, suggesting that the predator can visualize its prey's location and a path to get there.
Some jumping spiders of the genus Portia hunt other spiders in ways that seem intelligent, outflanking their victims or luring them from their webs. Laboratory studies show that Portia's instinctive tactics are only starting points for a trial-and-error approach from which these spiders learn very quickly how to overcome new prey species.
Duane Harland, a researcher affiliated with the Canterbury University spider lab, says there is a disconcerting plasticity in Portia's gene-encoded knowledge of the world. If one population of Portia can recognize an egg-carrying Scytodes but specimens from another region can’t, then this seems something quite new – a level of learning somewhere in-between the brain of an individual and the genome of a species.
Closing Remarks
Scientists still don’t have a clear picture of how the spider’s tiny brain circuits incorporate signals from each of the eight eyes and other senses to produce such a rich repertoire of behavior. But if future studies can provide insights, Hoy sees potential applications: for example, helping engineers design miniaturized robots with multiple cameras or sensors.
It is a humbling thought that there is so much we do not understand yet about life and intelligence here on earth.
References
- Airborne Acoustic Perception by a Jumping Spider — www.cell.com/…
- Despite lacking ears, spiders can hear you talk across the room — www.zmescience.com/…
- Visual Perception in the Brain of a Jumping Spider - www.cell.com/…
- Spider Brains — www.findaspider.org.au/…
- Spider Brains and legs — newsdesk.si.edu/...
- The structure of the central nervous system of jumping spiders of the genus Phidippus (Araneae: Salticidae) - peckhamia.com/…
- Do spiders have a mind? www.dichotomistic.com/…
- Inner Workings: Inside the mind of a jumping spider — www.ncbi.nlm.nih.gov/...
- Air motion sensing hairs of arthropods detect high frequencies at near-maximal mechanical efficiency - rsif.royalsocietypublishing.org/…
- A spider's tactile hairs — www.scholarpedia.org/article/A_spider's_tactile_hairs
- Hoy Lab at Cornell - hoylab.wordpress.com/…
Notes
Main image of Phidippus audax attributed to Thomas Shehan at www.flickr.com/…
P.S. I am not a biologist, but several of you are. Besides, who does not love spiders? So, discuss away — especially about spider senses and intelligence.