Image: Jumping spider with large median eyes – © Thomas Shahan 2012
On the sensory structures of #arachnids, both shared and unique
Get to know more about how these fascinating creatures perceive their environment
Eyes and Photoreceptors
For organisms to survive in their environment, some sort of way is needed to gather information from the environment so that the individual organism can adjust its behaviour accordingly. The use of chemical signals and mechanical force is generally thought to be the oldest forms of sensory inputs used by organisms.
Although some arachnids do posses sensory organs that are photosensitive, vision is usually rudimentary and most arachnids focus on mechano- and chemosensitive hairs (called sensilla) for the reception of stimuli, and have evolved an exceptional ability to locate prey and mates through vibrations. Most of the sensory organs are located on the extremities of arachnids and are thus located in order for the accurate determination of the type and direction of a stimulus
For most arachnids sight is not nearly as important as mechanical or chemical stimuli, and are rather used for orientation either towards (positive phototaxis) or away (negative phototaxis) from light. The basic arrangement usually consists of a pair of median eyes and a number of lateral ocelli (secondary eyes). The lateral ocelli are extremely sensitive to subtle changes in light intensity, while the median eyes have greater visual acuity and spatial discrimination, but lower absolute sensitivity. Although many arachnids posses eyes and or ocelli, many have also lost these sensory organs altogether. Some pseudoscorpions, mites, harvestmen and many cave-dwelling arachnids have evolved to function without any visual sensory system, completely relying on tactile hairs. Apart from these two photoreceptors, some arachnids also have extraocular light sense organs on their bodies, such as the extraocular light sense organs on the tail of scorpions or the abdomen and legs of mites and ticks.
Jumping spiders on the other hand greatly rely on their visual system for orientation, courting, jumping, prey location and prey capture. This can be easily be seen simply by looking at the size of the eyes compared to rest of the spiders body.
Image: Scorpion with small median eyes and small secondary eyes next to mouthparts – © Jan A. Neethling 2018
The most common type of sensilla are tactile hairs. These fine hairs are tapered toward the tip, have no pores in the hair cuticle and respond to mechanical tactile stimuli, similar to our touch sensation. The tapering of the hair shaft is important as this allows the hair shaft to bend and distort without breaking when contacting a surface. The distortion of the hair shaft also results in a non-linear response to stimuli, thus although a very small stimulus would result in an action potential in the neurons, the response of a strong stimuli would result in only a slightly higher action potential, and thus not over stimulate the central nervous system. These sensory hairs are located all over the body, but like most sensory organs are concentrated on the extremities. Tarantulas look hairy since their bodies are usually covered in a myriad of tactile hairs of varying sizes. These help them feel their way around their environment as they move around at night. Scorpions have many long tactile hairs on their pincers which they use to both navigate their environment as well as detect prey.
Image: Honduran Curly Hair Tarantula (Tliltocatl albopilosus) – © Jan A. Neethling 2015
Image: Scorpion pincer with tactile hairs – © Jan A. Neethling 2018
Trichobothria are specialized sensory hairs that are extremely sensitive to air currents. Each hair emerges from, and articulates within an independent socket. These very fine sensory hairs do not bend when deflected by airflow and have a fixed plane of articulation, irrespective of the direction of the air current. In some trichobothria the surface is ornamented with numerous feathery projections. These enlarge the surface area and thus the air resistance of the hair, enhancing its sensitivity. Many are also curved, and this is most probably a sensitivity adaptation, as a curved hair gives rise to a greater surface area, and thus more air resistance than a straight sensory hair. Trichobothria may occur singularly, but more regularly occur as a group of hairs of varying length. This variance in length gives each hair its own frequency at which it is deflected, with the combination of these differing lengths giving the organism a greatly increased spectrum of air frequencies to which it can respond. Trichobothria are used extensively in both systematics and classification as both the number and arrangement remains the same within individuals of a species, but differ between different species.
Image: Pseudoscorpion pincer with trichobothria – © Jan A. Neethling 2019
Image: Trichobothrium anatomy – © Encyclopaedia Britannica 2012
These hairs come in a variety of shapes and sizes, and can be differentiated from pure mechanosensory hairs by the presence of pores in the hair shaft that facilitate the contact of sensory nerves with the outside environment. They are used by arachnids to “taste” their surroundings and particular objects of interest. Their function is to respond to chemical stimuli, such as pheromones, and these hairs are principally distributed on the tips of sensory appendages, such as the pedipalps of many arachnids, the first legs of whip spiders and whip scorpions (Ambypygi and Uropygi), the chelicerae (mouthparts) of spiders and scorpions (Araneae and Scorpiones), and to a lesser extent on the surface of the body. Some chemosensory hairs serve a dual function as mechano-receptors.
Other types of sensilla that are less numerous, but still just as important, are hygro-sensitive sensilla that respond to changes in the relative humidity of the environment, thermo-sensitive sensilla that respond to changes in temperature, and sensilla that can determine pressure. These sensilla are less arranged alone on the appendages and are rather located as components of pit organs, such as those found on the distal tarsi of arachnids.
Image: Tarantula footpad with claws. These footpads contain many chemosensory hairs with which the spider can taste its environment with. – © Jan A. Neethling 2015
Slit Sensilla and Lyriform Organs
These sensory structures are unique to arachnids, and are found in every order except for Palpigradi. They occur as clefts in the cuticle of the exoskeleton and function in the detection of minute compression forces acting on the exoskeleton, causing it to deform slightly, thus in a sense they act as proprioreceptors. These forces are generated during a number of behaviorally significant occasions, such as locomotion and substrate vibrations, and allow the organism to respond accordingly.
The slits are covered by a thin membrane that can expand or contract. As compression forces act on and cause the exoskeleton to deform, the walls of the slit compress closer to, or further away from another and this results in the deformation of the membrane contained within these clefts. Mechanosensory neurons then pick up these deformations. These organs, like many sensory organs in arachnids, occur mostly on the extremities but are not confined to these areas. Slit sense organs are found across the body, like on the sterna plates, pedipalps and tail of scorpions.
Slit sensilla are categorized into three groups depending on the number and proximity of the clefts to one another. The first type, isolated slit sense organs, have a minimal distance of 100µm to the next closest cleft. The second type, group slit sense organs, are characterized by one slit measuring at least 30µm in length that is situated closer than 100µm of another slit sense organ. Lastly the third type, compound slit sense organs or lyriform organs , are characterized by the very close parallel or side by side spacing of two or more slit sense organs. Lyriform organs can contain up to 29 separate slits and are surprisingly consistent in distribution over the body within orders.
Image: Scanning electron photographs of a lyriform organ of a spider, demonstrating the close arrangement of the slit sensilla. – © F.G. Barth 2004
Scorpion Sensory Structures
In scorpions the trichobothria only occur on the pedipalps. In terms of light sensitive receptors, scorpions possess three types; the medial eyes can form rudimentary images, the lateral eyes function in the detection of light intensity and the third type of light receptor is located on the tail and is composed of extraocular sense organs.
Location of prey is done through the orientation to vibrations caused by the prey that travel through the substrate and are detected by the tarsal sense organs. There are two organs located at the ends of the walking legs namely, the basitarsal compound slit sensillure and tarsal sensory hairs, each tasked with detecting a certain type of vibration wave.
Pectines are sensory structures unique to scorpions. They are comb-like structure located posteriorly of the walking legs on the ventral side of the body. The comb is composed of two flattened base appendages that are very mobile and serve as support for a multitude of pegs that in turn act as foundations for rows of contact chemosensory sensilla. The pectines are used for scanning the substrate in the context of feeding, mating, and locating the home burrow. Also important is the presence of a large slit running alongside the pectines. This slit houses chemosensory hairs important for the detection of olfactory stimuli and chemical gradients.
Image: Scorpion ventral view – © Jan A. Neethling 2018
Image: Scorpion Pectines – © Jan A. Neethling 2020
Camel Spider Sensory Structures
Sulifugids, also known as camel spiders or wind scorpions, are ferocious predators of mostly arid region of the world. They possess a massive pair of chelicerae that are used to dismember prey. They possess two medial eyes that are situated on a raised ocular tubercle, and rely extensively on their long tactile hairs for information about their surroundings.
The pedipalps are long and leg-like. They often contain backward facing spines that help in the capture of prey. They also contain suctorial organs that help in the negotiation of vertical surfaces. The first pair of legs are used as tactile organs, and contain various sensory hairs. These legs also possess the sensilla ampullacea, which are pits in the cuticle that house olfactory organs.
Lastly, the solifugids possess a series of special enervated sensory structure called malleoli or racquet organs. These organs are located on the coxa and trochanter of the fourth pair of walking legs. Although a sensory function is commonly accepted, the exact sensory function of these organs has yet to be discovered.
Image: Solifugid malleoli, also known as Racquet organs – © Warren Savary 2006
Tick Sensory Structures
In ticks (Acari) the first pair of legs are equipped with numerous sensory organs and are waved in the air above the body of the arachnid in a manner similar to an insect’s antennae. These arachnids also possess multi-porous sensilla that function in the detection of olfactory stimuli.
In order to locate mates and hosts, these arachnids utilize a range of sensory queues derived from multiple types of sensory hairs located in their Haller’s organ, a sensory structure unique to the Acari. This structure is located at the tip of the first pair of legs and is composed of multiple pits that protect and house several types of chemosensory sensilla, with the whole complex functioning in the detection of olfactory, thermal and humidity stimuli.
Image: S.E.M. picture of a tick indicating the position of Haller’s organ on the tips of the front legs – © Belozerov, Kok & Fourie 2004
Image: Haller’s organ of a tick – © University of New Haven’s Lyme Disease Research Group 2020
Spider Sensory Structures
Spiders possess an extensive array of sensory organs and appendages to help them survive in their environment. The pedipalps are the main sensory appendages, containing a diverse array of mechanosensitive and chemosensitive hairs. Trichobothria are numerous and are found on the pedipalps and the walking legs. Each walking leg contains a tarsal sensory structure called the “tarsal pore organ” which functions as a hygroreceptor. Together with these tarsal sense organs, spiders also possess scolopales on the distal ends of the walking legs. These proprioreceptors function in strain detection that is coupled with the movement of the tarsal claws. Hair plates on the coxa of the walking legs of spiders are used to gage the proximity of the walking legs to one another. These plates are covered with tiny tactile hairs that cause an action potential in their associated neurons as soon as an adjacent legs coxa comes close enough to touch these sensilla.
The slit sense organs, or lyriform organs, of arachnids are the most extensive of these sensory systems found in arachnids. Lyriform organs are only found on the appendages, and are especially numerous on the walking legs, close to articulation points. They are used for the detection of vibrations produced by mates, prey, and predators, and in the measurement of skeletal strains caused by muscular activity and hemolymph pressure.
In terms of olfaction, although it is generally accepted that spiders do possess the ability to receive and interpret olfactory stimuli, there is still debate as to the location and form of a spider’s odour receptors.
Lastly, vision in spiders is most prominent in jumping spiders (Salticidae). These are some of the most vision dependent animals, and they use their remarkably well adapted vision in for many of their normal day to day behaviour, including courting and prey capture.
Image: Honduran Curly Hair Tarantula (Tliltocatl albopilosus) – © Jan A. Neethling 2015
Walking Leg Modification in Whip spiders and Vinegaroons
In Whip spiders (Amblypygi) and Vinegaroons (Uropygi) only three of the four walking legs are used for propulsion. The first pair is extensively modified into long antennae-form sensory appendages that are waved in front of the organism to explore its surroundings. These arachnids are nocturnal animals and as such, mechanical and chemical signals are extremely important as they provide the majority of information about the organism’s surroundings. These mechanical and chemical stimuli are, for the most part, received by the myriad of sensory sensilla and pits of the elongated front legs.
The front legs are covered in a variety of sensory organs. These include bristles that function as contact chemoreceptors, trichobothria, pore hairs, club hairs that serve as hygroreceptors and tarsal claws that are located at the very tip of the legs and contain a pore at their tips, thus the claws serve as contact chemoreceptors.
In terms of vision, these arachnids do have eyes, two median eyes and a number of lateral eyes, but these are used more for the detection of fluctuations in light intensity than for prey detection and capture.
Image: Whip spider on rock- © Jan A. Neethling 2018
Image: Damon medius demonstrating first pair of legs modified into long antennae-like sensory structures – © Naturhistorischen Museum Basel 2016
Walking Leg Modification in Ricinulei
In ricinulids the second pair of walking legs, not the first as in the whip spiders and vinegaroons, are elongated and used as sensory appendages. This is also where many of the sensory hairs are located. Most of the mechanosensory hairs are located on the legs, while most of the chemosensory hairs are located on the pedipalps, no eyes are present.
Ricinulids possess slender, movable pedipalps with tiny chelae used to grasp prey. As they move, these arachnids periodically touch the surface they are moving on with their pedipalps. The pedipalps thus serve as short range contact chemosensory appendages that compliment the long range sensory capabilities of the elongated second pair of walking legs. Some of the sensory hairs found on the pedipalps include sigmoidal sensilla with smooth shafts that serve as contact chemoreceptor and chela sensilla, found on the movable claw. The latter function as chemosensory organs that “taste” prey items as the ricinulids grasps them. Other sensory organs include slit sensilla used to gage torsion forces of the chela and a “deep pore organ”. The latter organ can be described as a deep pit with a number of sensory hairs inside. The pit protects these hairs from mechanical damage and the whole assembly functions in the detection of olfactory, thermal and humidity stimuli.
Images: Hooded Tickspiders – ©Marshal Hedin and Fernandez & Giribet 2008 – 2015
Text: Jan Andries Neethling