Most arachnids are seekers of the dark, hiding within the crevices of tree buttresses or between leaves, roots, rocks, and debris. By day they appear to be virtually nonexistent, but at night emerge by the masses as evidenced by constellations of eyeshine both large and small. For those of us with lights our paths are far less treacherous, but small critters must tread around carefully to avoid an untimely fate.
Wandering spiders of the genus Cupiennius (family Trechaleidae), in particular, are a staple of the nightly forest atmosphere, perching arboreally to ambush arthropods and even small vertebrates like lizards and frogs. Of the three common species, the spotted banana spider (C. getazi) is most variable in color pattern— ranging from brown to bright orange— but it can be reliably identified by characteristic dots on the underside of all legs. The two remaining species are more subtle in their diagnostic features. The red-thighed wandering spider (C. coccineus) attains the largest body size and as per the common name, females are adorned with bright orange on the femora of the first two pairs of legs. Males are generally less warm in color and have mottled black markings on the legs. The tiger wandering spider (C. salei) is often confused with C. coccineus, but they can be differentiated by the lack of orange patches in females and the males having equally-spaced stripes on the legs.
Vibrational stimuli that propagate through the substrate are one of the fundamental sensory elements that Cupiennius spiders rely on to initiate behaviors. Since the early 1980s, C. salei & C. getazi have been extensively studied as models for vibrational sensation and communication, encompassing anatomy, electrophysiology, integration of sensory inputs, signal production and content, sexual selection, and species interactions. Mechanistically, vibration-sensitive hairs (called sensilla) reside on two indentations and one joint of the tips of the legs. Each area differs in both sensitivity and the range of frequencies that excite their associated neurons, and further work on the lyriform organs (whose name and appearance is likened to the strings of a lyre) suggests they act as a filter that selectively responds to vibrational information used in courtship/competition and predator-prey interactions. Prior to courtship, a male will first encounter a silken line made by a female and in response to the pheromones, he will produce vibrational signals by moving the abdomen up and down. Although the body never contacts the substrate, the physical movement generates low frequency (75-115 Hz) waves that have a regular rhythm. If a female is nearby, she will respond with vibrations, and the two will continue the crosstalk until they are situated together. At this stage, the process becomes highly tactile along with rapid oscillations of the legs and repositioning of the body which signals the degree of receptivity. Females are selective in their preference for male vibrational signals, and differences in signal properties between co-occurring species may be a mechanism that promotes reproductive isolation.
I have always wished that scorpions had more variation in color and patterning, but nevertheless, they are probably my favorite order of arachnids to encounter and interact with— and their strong ultraviolet glow also helps! When exposed to UV light, chemical reactions occur within the scorpion cuticle, which produces a characteristic neon blue-green glow. This process is distinct from the production of pigments that simply absorb and reflect light or from microstructures that produce iridescence via light scattering. Although scorpions take the glory of the glow, other arachnids are similarly capable, including harvestmen, millipedes, and spiders. More broadly, fluorescence has been documented in mantids, phasmids, orthopterans, beetles, a plethora of eyes and ocelli in other arthropods, frogs, geckos, fish, flowers, chameleon bones, flying squirrels, and puffin beaks! Organisms often differ in the wavelengths at which they fluoresce due to differences in the properties of the fluorescent compounds they possess. In many species, fluorescence is only visible for a short “window” of wavelengths, and their glow can easily be missed when using a standard UV flashlight. However, in some taxonomic groups (such as scorpions), fluorescent properties may be highly conserved and therefore reliably detected. As the phenomenon of UV fluorescence becomes increasingly well-known, it will surely continue to pop up all around the tree of life.
The function of UV fluorescence in scorpions has long remained a mystery, though it has been hypothesized to play a role in conspecific recognition, prey attraction, as a warning coloration (aposematism), or simply be a byproduct of physiological development of the cuticle— none of which have received support from empirical or ontogenetic data. Recently, yet another potential function has been proposed, thanks to behavioral experiments that draw from an understanding of the scorpion visual system. As scorpions are exposed to UV light, their strength of fluorescence degrades, and in tandem, they tend to more strongly avoid the UV light source. Moreover, scorpions are more likely to move when subjected to light composed of shorter wavelengths, even when their lateral and medial eyes were blocked. Similar light conditions occur after the onset of nightfall, which coincides with the highest movement rates of scorpions in nature. Because scorpion vision and photoreceptors on the posterior body are attuned to green and UV light, it has been suggested that scorpions use their own faint fluorescence to regulate activity levels. So, the scorpion cuticle transduces UV light into blue-green fluorescence, which helps gather information about the external light environment. This mechanism would be beneficial for scorpions to find refuge from predators during the night, more specifically, locations where their cuticle receives the least amount of ultraviolet light from the night sky.
Write-up on Amblypygi coming soon!
Write-up on Phoneutria also coming soon!