Trichobothria are mechanoreceptors located on the dorsal surface of tarsus, metatarsus, and tibia of walking legs and pedipalps of spiders. They are thin hairs emerged from a cup shaped sockets sensitive to minute air movement.

Trichobothria are cuticle mechanoreceptors highly sensitive to small air movement. Dahl (1883) first suggested that these structures are auditory mechanoreceptors and named them Hörhaare. Later, he introduced the term trichobothria (Dahl, 1911). Further studies (Palmgren, 1936; Pauly, 1956; Schömann, 1956; Hoffmann, 1965) show that trichobothria respond to airborne stimuli such as gas particle displacement and have to be regarded not as sound detectors, but rather as “touch-in-distance” mechanoreceptors. Electrophysiological studies (Görner, 1965; Christian 1971, 1972; Barth, 2002) support this conclusion and suggest a specific mechanism of sensory input transduction.

Trichobothria are located on the tarsus, metatarsus, and tibia of spider legs. As a rule, they are organized in groups or rows, especially on the dorsal leg surface. The large wandering spider Cupienius has almost 1,000 trichobothria: about 100 on each leg and about 50 on each palp.

Ctenidae: Cupienius coccineus (female), Talamanca Reserve San Gerardo, Rivas, San José, Costa Rica. Photo credit: Tom Murray/Flickr.

In gnaphosid spiders trichobothria are arranged in 1-3 rows: one dorsal row presents in all ground spiders and one or two lateral rows from both sides. As a rule, web spiders have much fewer trichobothria than ground spiders. This difference demonstrates higher importance of these sensory organs for ground wandering hunters than web spiders who more rely on information they receive from web vibrations. The arrangement of trichobothria is species specific and often is used for spiders’ classification and study of their evolutionary relationships. Trichobothrium has two components: 1) long thin hair or trichos and 2) cup shaped cuticle socket or bothrium. The ground plan of trichobothria construction in general is similar among spiders (Görner, 1965; Barth, 2002; Haupt, 2003; Felix, 2010) and other arachnids (Foelix&Schabronath, 1983; Haupt, 1996). It has the bowl-shaped socket in the cuticle with a single long hollow inside filament at the center (see Fig. 1).

Figure 1. Trichobothria of Drassylus aprinus (Banks, 1901).

Typical hair length of this organ vary from 100 μm up to 1400 μm. The hair shaft of trichobothrium has a feathery-looking surface curved distally. The feathery surface of hair increases drag forces and thus creates a high mechanical sensitivity to air movement. The plane of the curvature is inclined from the leg’s main long axis at an angle of 5° to 30°. Depth and diameter of the sockets increase with the length of the hair. However, the angle through which the hair can be deflected between the two edges of the socket remains constant at 25°- 35°, because the socket of a long hair is not only broader but also deeper. Stretched across the bottom of the socket is a cuticular membrane that bears the hair shaft.

Figure 2. Trichobothrium of house spider Tegenaria domestica (Clerck, 1757): b - bothrium; c – cylinder; ha – hair; he – helmet; m – membrane; n – distal nerve process (after Görner, 1965).

The length of the hairs increases distally on every limb section. All hairs have similar shape and are lightly inclined distally towards the tip of the leg (see Fig. 3). The socket is surrounded by distal and proximal plates that may have distal and proximal margins. Shape of the plates varies among spiders and often have more or less high ridges.

Figure 3. Innervation of a trichobothrium of the house spider Tegenaria domestica (Clerck, 1757): longitudinal section. When the hair shaft is deflected (dashed line), the helmet structure (He) pushes against the dendritic terminals (T), triggering the generation of nerve impulses. Rlc – receptor lymph cavity; Sh – sheath cells, Hy – hypodermis; M – articulating membrane (after Christian, 1973).

Proximally, hair rests inside a convex cap or helmet associated with dendrites of sensory neurons. Air movement pressing on the hair of the trichobothrium, bends it. The hair deflection energy is converted in sensory cells into the action potential. The sensitivity of trichobothria is amazing. The long hair shaft is suspended in a very thin (0.5 μm) cuticular membrane. Even a very light air current of around 1 mm/sec makes it quiver. An energy less than a quantum of green light is sufficient to excite the sensory cell. Different deflection directions stimulate different sensory cells, which number vary from 3 to 11. This creates effect of ability to monitor air movement from different directions. Each sensory cell reacts to displacements within a particular sector with a light overlapping among sectors.

The trichobothria receptors have distinct frequency tuning, with best frequencies between 50 and 120 Hz, which perfectly fit to the buzz produced by a flying nearby fly. The lack of spontaneous activity and marked phasic nature of the responses to trichobothria, which react mainly to the speed with which they are deflected, make these sensilla ideally suited to monitor the typically very inconstant stimuli. The irregular noise does not stimulate these sensilla, whereas a sinusoidal stimulus at 100 Hz is responded to in a few seconds.

Trichobothria allow spider not only to sense presence of the fly at distance of as much as 30 cm, but to identify its movement speed and direction to make a successful jump even in the absolute dark or when the spider is blinded. Usually this drama lasts less than 1.5 sec. The spider uses trichobothria not only to detect prey but also as a warning system against predatory wasps (Palpimonidae) and parasitic Neuroptera (Mantispidae).