Sensing the world with hairs and pits
All spiders are hairy - like us. Spider hairs are part of the hard outer cuticle (the spider's 'skin'). Unlike us, spiders use their hairs (which are often greatly modified) to perform an amazing variety of tasks - for sensing their world, in courtship displays, for moving around, and in defence and prey capture.
Sight is our most important sense. However, most spiders have very poor eyesight and many are active during the dark night hours. With a few notable exceptions, their sense of the world is obtained through specialised sensory hairs and pit-like sense organs in the cuticle. These hair and pit sensillae are especially associated with the spider's limbs and mouthparts. Connected at their bases to sensory nerves, they provide spiders with the sensory experience and feedback needed to survive and function in their world.
Hinged, touch-sensitive hairs on the legs help the spider to move freely about its terrain and are important in interactions ranging from mating to prey capture. Chemically sensitive, hollow-tipped hairs found at the ends of the limbs and on the mouthparts are the spider's tasting organs - those on the pedipalps are used to sense the chemicals associated with mate recognition.
Another stimulus to which spiders are incredibly sensitive is vibration. Vibrations transmitted through air, substrate surfaces and even water can be sensed by spiders.
Air vibrations and currents are detected by slender, vertical hairs called trichobothria, usually found on the upper surface of the three outermost limb segments. These hairs are easily deflected and their 'ball and socket' basal hinge allows them to respond to air movements coming from any direction. This high sensitivity to air-borne vibrations can alert the spider to the wing beats of an approaching moth or fly, or the presence of a wasp predator.
Substrate vibrations, induced in web, leaf , ground or water surfaces by nearby disturbances (such as prey, mate or predator movement), cause slight movements in a waiting spider's limbs. These movements can be directionally sensed by tiny 'strain gauges' in the cuticle called slit organs (or lyriform organs when several are grouped together, as at the tarsal-metatarsal joint) that react to any slight deformation of the cuticle. The sensitivity is such that spiders can readily distinguish prey movement vibrations from those of a courting male or the background noise of wind.
Slit organs are also somehow involved in the spider's ability to 'memorise' directions, for example, the return route to its burrow after a hunting trip. A number of tiny 'slit organs' placed around the bases of the silk spinning spigots on the spinnerets appear to be involved in the sensory monitoring of the movements of the spigots and the emission of silk during spinning, the control of which is still poorly understood.
Near the end of each limb is a tiny pit, dome or peg-like structure called the tarsal organ. They appear to function primarily as moisture and temperature detectors.
Internally placed joint receptors and external sensory hairs (usually short hairs adjacent to joint membranes or longer hairs that span joints) keep the spider informed of its limb positions (proprioception) during movement. This ensures that the spider moves in a coordinated way.
Protection, aggression and grip
American tarantulas use the barbed irritant hairs (urticating hairs) on the abdomen to protect themselves against predators like lizards and mammals. When threatened, these spiders brush their back legs rapidly across the back of the abdomen. This sends clouds of loosely attached barbed hairs into the eyes, nose or mouth of the predator chasing them. The spider escapes while its pursuer is distracted by the highly irritant hairs.
Some spiders make noises to attract mates or as warning signals. Australian tarantulas are called Whistling Spiders because they 'whistle' when threatened. To make the whistling or whirring sound, they rapidly rub together modified hairs on their mouthparts. This is called stridulation. Both male and female whisting spiders have these organs. The sound may help scare off predators like small carnivorous marsupials when the spiders are caught hunting out of their burrows.
Most spiders have strong, thick modified hairs called spines on the limbs. Spines are often more numerous and larger on the front legs. They assist in capturing and holding prey or in helping males hold females during mating.
Getting a grip
Spiders like tarantulas, huntsman and jumping spiders have densely packed hairs called scopulae on their feet (tarsal and metatarsal leg segments). Scopulae give the legs lots of clinging power and allow the spiders to walk easily on smooth surfaces like tree trunks, leaves, ceilings and window glass. They also help these spiders to hold on to struggling prey.
There are two types of scopulae - brushes and claw tufts. Both may have hundreds of scopulate hairs. Each scopulate hair may have hundreds of little branches or 'end feet'. As a result, there can be more than a million of these tiny 'end feet' in contact with a surface, providing enormous frictional clinging power. This effect is aided by the capillary forces of an extremely thin layer of water on most surfaces.