The insect thorax is a surprisingly complex piece of anatomy. All at once, the insect thorax serves as housing for muscles, the mechanism of walking and flight, and, in some cases, serves as the stage on which an insect presents itself, as in the case of the actually harmless Death’s Head hawkmoth (Acherontia atropos) seen above with the characteristically skull-shaped pattern on its thorax.

 

What is the Thorax?

The thorax is the second of three major tagma, or segments that make up an insect’s body.

The thorax of an insect is where all appendages for locomotion are, including legs and wings. The thorax consists of three segments, known as the prothorax, the mesothorax, and the metathorax, in that order. Each thoracic segment bears one pair of legs. If the insect is capable of flight, the meso- and metathorax bear the wings; collectively, the wing-bearing segments of insects are called the pteryothoracic segments. The prothorax, which is the very first segment of the thorax, comes after the head, and the cervix, a neck-like membranous region that helps connect muscles from the head to the thorax. The job of the prothorax is usually to serve as a place of attachment for the first pair of legs, although in some insect orders, such as Orthoptera (Grasshoppers), Coleoptera (Beetles), and Blattodea (Cockroaches), the prothorax can also serve as a hard plate, that protects the pteryothoracic segments.

A prothorax that has taken the shape of a hardened plate can be most clearly seen on a grasshopper. The plate covers the cervix, and the anterior part of the mesothorax. The mesothorax comes directly after the prothorax, and bears one pair of legs, and, if the insect is capable of flight, the first pair of wings. The metathorax then comes after the mesothorax, and also bears one pair of legs, and the second pair of wings. The insect thorax has gone through incredible evolution and change in order to accommodate flight. Because not all insects fly, there are differences between the thorax of flying insects, and non-flying insects, as well as differences between the thorax of larval flying insects, and adult flying insects.

Primary Segmentation or Secondary Segmentation?

Primary segmentation occurs in non-sclerotized insects, or larvae and soft bodied insects. Primary segmentation works by physically shortening the segment in order to move the insect along, like how a caterpillar or inch worm moves.

From the diagram on the right, you can see how to muscles within the thoracic are directly attached to the thoracic segment. When those muscles contract, the whole segment contracts, making the inch worm scrunch up.

Secondary segmentation occurs in sclerotized insects, insects that have hard plates over their thoracic segments. The hardened plates most likely evolved as a protective device. Because the hardened plates, or sclerites, cannot move like an inch worm does, a membranous region is created (called the conjuctiva), where the segments telescope into one another. Except for the most primitive lineages, most insects have some degree of secondary segmentation, in the thorax and abdomen. Secondary segmentation is often most evident in the abdomen. Terga overlap each other posteriorly, such that the posterior border of the tergum overlaps with the anterior border of the tergum behind it. Along the anterior edge of each tergum is a costa (a rib or rib-like structure) called the antecosta (and the associated suture is called the antecostal suture, as the name implies, it is a true intersegmental suture). The antecosta, however, is frequently not exactly on the leading edge of the tergum, and the scleritized strip anterior to it is the acrotergite. Immediately anterior to the acrotergite is a membranous region (conjuctiva) marking the region between adjacent terga. From doing comparisons of the terga and sterna structures throughout different insects and arthropods, we see that the antecosta marks the true intersegmental boundary, and the acrotergite belongs to the preceding body terga. The functional dorsal part of the tergum combined with the acrotergite is called the Alinotum.

Pterothorax

As I explained above, the pterothorax is the collective name of the meso- and metathorax, which hold the wings. The overall structure of the entire thoracic region is fundamentally the same as the abdomen, but there are modifications for locomotion, and the degree of modification depends on the flying ability of the insect. A very important modification that the thorax has that the abdomen does not is sclerotization in the pleural region.

The pterothroax itself has many extra strengthening ridges, and the conjuctivae is reduced, making the pterothroax a very rigid structure, because the segments cannot telescope. Here is a basic diagram of all the features of the pterothoracic segment. The diagram is being viewed laterally.