Cleft sense organ

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Fissile sensory organs are the sensors for substrate vibrations in arthropods , e.g. B. in weaving spiders . Split sense organs can be combined to form a lyre-shaped organ . Several, up to 20 parallel, split sensory organs form a group that serves to perceive the stimulus. Most of the time, the sensory organs are located on the joints of the walking legs. They enable z. B. the spiders the recognition of prey or enemies through substrate vibrations.

functionality

The stimulus is transmitted from the tarsi as a voltage change in the cuticle to a sunk 15 to 120 µm long membrane, in the middle of which two mechanosensitive dendrites end in a coupling cylinder . The stimulus is focused on a dendrite area of ​​only 1 µm². The gap sense organs react already in oscillation amplitude of the tarsi from 1 to 10 nm. With the vibration acceleration correlates the neural response from frequencies of 40 Hz, the threshold is approximately 8 mm / s. The relationship between stimulus and response is logarithmic , so that a receptor encodes the naturally occurring accelerations of 10 to 1,000 mm / s², but cannot resolve the differences in amplitude with difficulty.

The up to 20 parallel split sensory organs of a lyre-shaped organ increase the vibration sensitivity and the coding range, because individual split sensory organs can be tuned to different frequencies. Spiders discover running prey, among other sensations such as visual perception, primarily through substrate vibrations. Plant leaves, for example, transmit vibrations in a large frequency range of up to 5,000 Hz with little damping . After 18 cm, the output amplitude drops by half.

Both dendrites of an organ respond to changes in tension in the cuticle only phased, one dendrite with a spike and the other with a burst . It is not yet fully understood whether rapid adaptation is part of the signal transduction process . It prevents permanent tension, for example from the spider's own body weight in the leg cuticle, from exciting the fissile sensory organs. This enables a good ratio between signal and noise and thus a high level of vibration sensitivity.

In a lurking position, the eight tarsi of the spider scan the circular 360 ° surrounding area like feelers on the substrate. With Cupiennius salei (genus Ctenidae) this scanning circle has a diameter of 10 cm, which leads to amplitude differences of up to 17 dB and time differences of up to 10 ms between the diagonally positioned tarsi . Thus the strongest or first vibrating leg of the spider shows the exact direction to the stimulus source. Orb web spiders, for example, can align themselves to the point of vibration with an accuracy of 3 to 4 °. It is not yet known exactly whether these signals also contain distance information; they could be present in amplitude or in spectral stimulus differences between differently positioned tarsi.

literature