Ento. 101: External Structure – the Thorax

Click to enlarge

Fig. 1

Re-cap: In previous posts I covered the insect integument and the first tagma – the head, with mouthpartseyes and antenna.The insect body is divided into three tagma: the head, thorax and abdomen (Fig. 1). In this post I will cover the the second tagma, the insect thorax, which is the physical centre for locomotion. Click on any image to enlarge.

Fig. 2

The insect thorax has three segments. The anterior segment, closest to the head, is the prothorax, with the major features being the first pair of legs and the pronotum. The middle segment is the mesothorax with the major features being the second pair of legs and the anterior wings. The third and most posterior segment, abutting the abdomen, is the metathorax, which features the third pair of legs and the posterior wings. Each segment is dilineated by an intersegmental suture (indicated by dotted lines in Fig. 2)

Each segment has four basic regions. The dorsal surface is called the tergum or notum. The two lateral regions are called the pleura (singular: pleuron) and the ventral aspect is called the sternum. In turn, the notum of the prothorax is called the pronotum, the notum for the mesothorax is called the mesonotum and the notum for the metathorax is called the metanotum. Continuing with this logic, we also have the mesopleura and metapleura as well as the mesosternum and metasternum.

Fig. 3

The prothorax on this conehead is shielded by a large pronotum that extends from over the cervix(neck) on the anterior side to cover the mesothorax and most of the metathorax on the posterior side, as well as covering the pleura on the prothorax. (Fig.3)

Fig. 4

Because the meso and metathorax hold the wings, they have a combined name called the pterothorax (pteron=wing). (fig. 4) The forewings (called the tegmina in Orthoptera) arise between the mesonotum and the mesopleura, and the hindwings articulate between the metanotum and metapleura. The legs arise on the meso. and metapleura.

Fig. 5

The mesothorax and metathorax each have a pleural suture (mesopleural and metapleural sutures) that runs from the wing base to the coxa of the leg. The sclerite anterior to the pleural suture is called the episternum (serially, the mesepisternum and metepisternum).The sclerite posterior to the suture is called the epimiron (serially, the mesepimiron and metepimiron). Spiracles, the external organs of the respiratory system, are found on the pterothorax, usually one between the pro. and mesopleoron as well as one between the meso. and metapleuron.

The ventral view or sternum (Fig. 5) follows the same convention, with the prosternum under the prothorax, the mesosternum under the mesothorax and the metasternum under the metathorax.

The notum, pleura and sternum of each segment have a variety of different sclerites and sutures which vary greatly from order to order, and they will not be discussed in detail in this section.

More on the thorax can be seen in the tutorial by Dr John Meyer at NC State University.

Next: The legs and wings


Gordh G. and D.H. Headrick.  A Dictionary of Entomology. Cabi 2001.

Romoser, William S. The Science of Entomology, pp. 32-36. Collier-MacMillan 1973.

Resh, Vincent H. and R. T. Cardé, Eds. Encyclopedia of Insects, pp. 19-21. Elsevier 2003.

Grimaldi, David and M.S. Engel, Evolution of the Insects, pp. 125-128. Cambridge University 2005.


This entry was posted in Anatomy, Conocephalinae, Ensifera, Entomology 101, Insect, Orthoptera, Tettigoniidae and tagged , , , , .

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