Trichobothria

A Trichobothrium is a simply constructed Tastsinnesorgan, which is found in many arthropods. The trichobothria are probably several times convergently developed in the course of evolution.

Structure and Function

Since arthropods have a hard and rigid armor of chitin, mechanosensitive ion channels have to be supplied in nerve membranes mechanical external stimuli by Cuticulastrukturen. Corresponding Mechanosensillen are available in different functional contexts, and accordingly varied are the charming filtering and coupling Cuticulastrukturen.

At currents of the medium (usually air movements ) react Fadensensillen ( filiform ) highly sensitive. In arachnids, these sensilla are called trichobothria, because the threads spring from a Cuticulagrube ( Bothrion = pit). The operation of the Fadensensillen can be illustrated by way of example in these trichobothria: On the legs and pedipalps of arachnids these trichobothria are present. The largest so far identified at a number of spider trichobothria is in the Central American nocturnal, still sitting on insects lurking hunting spider Cupiennius about 100 pieces per leg. The fine in diameter from 5 to 15 microns thick filaments protrude only 100 to 1500 microns from the Cuticulaoberfläche. This makes them even by slightest air movements already deflect, they are introduced by a thin membrane stretched in the cuticle. With increasing filament length decreases the excitation threshold, it is at air velocities of less than 1 mm / s The angle of deflection is limited by the edge of the Cuticulagrube a maximum of 25 ° to 35 °.

Perception with trichobothria

Sensilla have according to their mass (density about 1.1 g / cm ³), its frictional resistance to air and the torsional forces in the joint has a certain resonance frequency. Flying insects produce air vibrations of frequencies up to about 600 Hz, and behave as trichobothria as low-pass filter, they react to such low frequencies. The longer the yarn hair is, the better low frequencies are transmitted. This allows the vibration sensitivity adapt to the important frequency range for its life. A film still air forms at each overflowed area. From this so-called boundary layer, the hair must protrude so that the trichobothria can react properly. With decreasing flow rate, the thickness of this boundary layer grows. The hair length is optimally adjusted when it is inversely proportional to the square root of the frequency of stimulation. So how long trichobothria can be almost equally well to deflect in all directions, there is instead in the short direction selectivity that is not caused by the asymmetric edges of Cuticulagruben, but by the properties of the joint membrane. At the insertion of three or four mechanosensitive dendrites trichobothria are innervated, it responds each dendrite to a specific Auslenkungssektor. So the directional sensitivity arises only when Transduktionsprozess by the position of the dendrites in the joint. It is not yet thoroughly explored whether the adequate stimulus of pressure on the dendritic membrane is.

The transmission of force from the air particles on the hair happens viscous, the hair is taken away by the friction of air particles on the hair shaft. Friction carry fine Fiederungen on hair that force transmission. Most Trichobothria the amplitude ratio between the air and particle motion Haarauslenkung 1:2 to 1:1. There are also individual Trichobothria where the Haarauslenkung is greater than the air particle motion. The Haarauslenkung correlated with the air particle velocity and the acceleration, the latter is also known as particle velocity. The sound velocity in the near field is only noticeable because it decreases very rapidly with distance, realistic, values ​​of up to 30 cm. This sensory acquired room is related to the body size of a spider sufficient.

The hunting spider Cupiennius can salei with these hundreds of vibration sensors that total rezipieren by different hair lengths a sufficiently wide range of air vibration levels and frequencies make up their prey. They can even catch flying insects from the air. For example, they may notice a buzzing fly from 30 cm and a beating with the wings fly from 70 cm distance. The main food of spiders are running around on the leaves insects, they produce in their radius of 20 mm air currents, reacting to the trichobothria, then the attacking spider can facilitate the orientation towards prey.