Trichobothria are elongate setae, present in the arachnids, and various orders of insect

Insect

Insects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae...

s, that function in the detection of airborne vibrations and currents.

Morphology

Unlike the ordinary setae, which are tapered, the trichobothria have the same gauge on all their extent. They fit into the bottom of a broad and deep cup to which connects a membrane with extreme flexibility which adds an extraordinary mobility to them. The least air vibration is able to put them moving and to excite the small group of sensory cells which ensures their innervation.
See figures below for details (from Foelix, 1996):

Distribution

Trichobothria are present in most orders of the Arachnida, except in Solifugae

Solifugae

Solifugae are an order of Arachnida, known as camel spiders, wind scorpions and sun spiders or solifuges, comprising more than 1,000 described species in about 153 genera...

, Ricinulei

Ricinulei

The Order Ricinulei is a group of arachnids known as hooded tickspiders. In older works they are sometimes referred to as Podogona., 60 extant species of ricinuleids have been described worldwide, all in the single family Ricinoididae. They occur today in west-central Africa and the Neotropical...

 and Opiliones

Opiliones

Opiliones are an order of arachnids commonly known as harvestmen. , over 6,400 species of harvestmen have been discovered worldwide, although the real number of extant species may exceed 10,000. The order Opiliones can be divided into four suborders: Cyphophthalmi, Eupnoi, Dyspnoi and Laniatores...

 (Grassé, 1949). Although the distribution of trichobothria on the bodies of arachnids is often used by systematists (especially in Scorpiones and Pseudoscorpiones), few interordinal patterns are apparent (Shultz, 1990).