Subgenual organ

The subgenual organ is an organ in insects that is involved in the perception of sound. The name (Latin sub-below and genus-knee) refers to the location of the organ just below the knee in the tibia of all legs in most insects. It is associated with sensing ground movement (less than 1 nm of displacement can be enough) and sometimes also of sound. Crickets and katydids have particularly well-developed subgenual organs which are associated with a tympanal organ on the tibia but probably don't share the same function. Lepidoptera and Hymenoptera usually have less well developed subgenual organs, in the former they are suspended on the subgenual nerve, in the latter cone shaped. In Orthoptera and Blattodea it often lies in the neighbourhood of other sensory organs containing scolopidia and is then called the SGO complex. Diptera and Coleoptera lack such an organ completely, Panorpa has only one sensory neuron, while some parasitoid wasps have as much as 400 scolopidia in their organs. In some insects, the organ is split in two parts. In Periplaneta cockroaches, a sensitivity down to 2 nm of displacement has been determined. The subgenual organ is particularly important in parasitoid wasps, where it is a major way of finding suitable target animals in substrates. The sensitivity of the organ varies from species to species, in Orthoptera, Hymenoptera and Lepidoptera it is between one and more kHz, in Hemiptera only a few hundred Hz.

In Teleogryllus commodus, each larval stage forms one scolopidium that contributes to the organogenesis, while in the bushcricket Ephippiger ephippiger all scolopidia (22-24 in total) are already present in the first larval stage. In the latter, the organ increases in size proportionally to the growth of the limb containing it and has the shape of a fan. In the cricket Teleofryllus commodus all scolopidia are formed by the third larval stage and the organ has already its final shape by the time of egg hatching. A bilobar structure in the locust embry forms the precursor of the subgenual organ. Axon growth from the subgenual organ in grasshoppers is contingent on semaphorin I.

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