Tentorium (invertebrates)

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The tentorium is an internal skeletal structure in the head of insects , the rest of the Hexapoda (Entognatha) and the millipedes . It arises from finger-shaped invaginations of the exoskeleton , which are generally called apodemes . These so-called tentorium branches can remain free inside the head or combine to form a common structure. The task of the tentorium is the internal stiffening of the hollow head capsule against mechanical stresses such as B. can arise from the work of the mouthparts. The structure also serves as a starting point and abutment for numerous muscles. Since the tentorium arises from an inward inversion of the head capsule, its attachment point is usually visible on the outside in the form of a small indentation, which is called the tentorium pit. The four branches result in two anterior and two posterior tentorium pits. These are usually within the head seams. Their position is different in different insect orders.

In the winged insects ( Pterygota ) the tentorium has a characteristic x-shaped shape, which is created by the union of the tentorium branches inside the head. Here, either the posterior tentorium branches form a wide connection (called the tentorium bridge), to which the narrower anterior branches connect, or all four branches unite centrally to form a wide plate, the so-called corpus tentorii. The shape and development of the tentorium is characteristic of various insect orders and other kin groups.

In millipedes, the tentorium forms a movable structure that is connected to the mandible base. Muscles attached to the tentorium can cause the mandibles to move like a lever. This structure, known as the "swinging tentorium", is considered to be one of the most important autapomorphies of millipedes. The homology of the structures referred to here as tentorium with the tentorium of the Hexapoda is controversial.

The name of the structure goes back to the shape with a central plate and four arms, which reminded the descriptor of an open tarpaulin of a Roman military tent (see Tentorium (tent) ).

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