Thorax (arthropod anatomy)

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The thorax is the midsection (tagma) of the hexapod body (insects and entognathans). It holds the head, legs, wings and abdomen. It is also called mesosoma or cephalothorax in other arthropods.

It is formed by the prothorax, mesothorax and metathorax and comprises the scutellum; the cervix, a membrane that separates the head from the thorax; and the pleuron, a lateral sclerite of the thorax.

In dragonflies and damselflies the mesothorax and metathorax are fused together to form the synthorax. [1] [2]

In some insect pupae, like the mosquitoes', the head and thorax can be fused in a cephalothorax.

Members of suborder Apocrita (wasps, ants and bees) in the order Hymenoptera have the first segment of the abdomen fused with the thorax, which is called the propodeum. The head is connected to the thorax by the occipital foramen, enabling a wide range of motion for the head. [3]

In most flying insects, the thorax allows for the use of asynchronous muscles.

The thorax in various types of insect
Heteroptera morphology-d.svg
  In Heteroptera
Housefly anatomy-key.svg
  In Diptera
Scheme ant worker anatomy-numbered.svg
  In Formicidae

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The mesothorax is the middle of the three segments of the thorax of hexapods, and bears the second pair of legs. Its principal sclerites are the mesonotum (dorsal), the mesosternum (ventral), and the mesopleuron (lateral) on each side. The mesothorax is the segment that bears the forewings in all winged insects, though sometimes these may be reduced or modified, as in beetles (Coleoptera) or Dermaptera, in which they are sclerotized to form the elytra, and the Strepsiptera, in which they are reduced to form halteres that attach to the mesonotum. All adult insects possess legs on the mesothorax. In some groups of insects, the mesonotum is hypertrophied, such as in Diptera, Hymenoptera, and Lepidoptera), in which the anterior portion of the mesonotum forms most of the dorsal surface of the thorax. In these orders, there is also typically a small sclerite attached to the mesonotum that covers the wing base, called the tegula. In one group of insects, the Hemiptera, the dorsal surface of the thorax is typically formed primarily of the prothorax, but also in part by the enlarged posterior portion of the mesonotum, called the scutellum; in the Coleoptera, the scutellum may or may not be visible, usually as a small triangular plate between the elytral bases, thus similar in position to the Hemipteran scutellum. In Diptera and Hymenoptera the mesothoracic scutellum is also distinct, but much smaller than the mesoscutum.

The metathorax is the posterior of the three segments in the thorax of an insect, and bears the third pair of legs. Its principal sclerites are the metanotum (dorsal), the metasternum (ventral), and the metapleuron (lateral) on each side. The metathorax is the segment that bears the hindwings in most winged insects, though sometimes these may be reduced or modified, as in the flies (Diptera), in which they are reduced to form halteres, or flightless, as in beetles (Coleoptera), in which they may be completely absent even though forewings are still present. All adult insects possess legs on the metathorax. In most groups of insects, the metanotum is reduced relative to the mesonotum. In the suborder Apocrita of the Hymenoptera, the first abdominal segment is fused to the metathorax, and is then called the propodeum.

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Zaitzevia elongata is a species of riffle beetle which is wingless and flightless. It is found on several of the Ryukyu Islands in Japan in the gravel substrate of streams.

References

  1. Theischinger, Günther; Hawking, John (2006). The Complete Field Guide to Dragonflies of Australia. Collingwood, Victoria: CSIRO Publishing. p. 303. ISBN   978-0-643-09073-6.
  2. Tillyard, R. J. (1917). The Biology of Dragonflies (PDF). Cambridge University Press. p. 20. Archived (PDF) from the original on July 3, 2017.
  3. Legner, Elrich Fred. "General Summary of the Hymenoptera". An Introduction to the Study of Entomology.