The Insect Thorax

The Insect Thorax: Complete Guide

The thorax is the main engine room of the insect. It, like the insect abdomen, is built up of a series of concave upper and convex lower integumental plates. In the Thorax, they are known as the ‘Nota’ (singular ‘notum’) and ‘Pleura’ (singular ‘pleuron’), the whole being held together by a tough yet stretchable membrane.

The insect thorax can be conveniently divided into three separate and normally easily visible sections called (from the front), the ‘prothorax’ the ‘mesothorax’ and the ‘metathorax’.

The Insect Body

This leads to the nota and pleura being named ‘Pronotum’, ‘Mesonotum’, ‘Metanotum’ and ‘Propleuron’, ‘Mesopleuron’ and ‘Metapleuron’. Within the whole class insecta, the form and shape of the thorax shows considerable variation – depending on the needs of each species.

In species that are strong fliers, for example, the Mesothorax may become greatly enlarged.

In the adult insects, as well as in the nymphs of those insects with an ‘incomplete metamorphosis’ (those whose life cycle lacks the production of a pupa), each of these sections supports a pair of legs. In the pterygote (winged) insect orders, the first two sections support a pair of wings – except in the Diptera (true flies) where the second or hind wings have become modified into a pair of club-like balancing organs called ‘halteres’.

The typical insect leg, as shown above, consists of six main sections:

• The Coxa, this is the most basal aspect of the insect leg and articulates with the ‘sternites’.
• The Trochanter is usually small and serves as a joint between the ‘coxa’ and the ‘femur’.
• The Femur is usually long and stouter than the other segments and contains the main muscles used in running, jumping and digging.
• The Tibia is also generally long serving to increase the length of the leg, as well as adding an extra joint and thus extra flexibility the underside of the tarsal segments may possess pads.
• The Tarsus is the foot of the insect leg and can consist of between one and five segments.
• The Claws are situated at the end of the ‘tarsus’ and serve to assist the insect in holding onto the substrate or to its prey.

Between the claws may be found a special pad known as an ‘arolium‘ and which acts using suction developed by large numbers of minute tubular hairs to help hold the insect to smooth substrates.

As in all aspects of insect anatomy, there is an amazing diversity of form to be found in the insect leg. The three examples shown below are colour coded to the same pattern as the labled leg above. Can you remember which part is which?

Unlike legs, wings occur only in the adult forms of those insects which possess them. In those insects with ‘incomplete metamorphosis’, small wing buds are visible in the place the wings will occupy in the adult.

Generally speaking insect wings are thin, flat structures consisting of two fine membranes supported by a series of sclerotized veins.

In many small insects, the veins may be absent and in some very small insects (such as the Thrips, which are only one or two millimetres long) the wings consist of a central midrib supporting a series of fine hairs – these look more like feathers.

Wings are not totally stiff but bend and flex in an amazing manner during flight, greatly improving their aerodynamics.

Most species of winged insect have two pairs of wings and – in many of the more advanced orders – these two wings are held together by a variety of mechanisms to form a single, larger, functional wing.

Insect wings are soft and shapeless when the adults first emerge, but are immediately inflated with blood pressure through the veins before they harden and darken as a result of coming into contact with the air.

Though most insect wings appear bare to the naked eye, many are dotted with minute hairs. In some cases such as the ‘Trichoptera’ (Caddisfly) they are completely clothed in fine hairs, or as in the ‘Lepidoptera‘ (Moths and Butterflies) completely covered in tiny scales.

Though insect wings are primarily concerned with flight, they serve a number of purposes. In the ‘Coleoptera‘ (Beetles) and some ‘Hemiptera‘ (True Bugs) the fore wings have become highly sclerotized and act as armour, protecting the insect concerned. While some very small parasitic ‘Hymenoptera‘ use their wings to swim through the water to their hosts.

In Beetles the sclerotised forewings are called elytra, while the somewhat less stiffened forewings of Grasshoppers and Crickets are called tegmen.

The bright colours of many insect wings are there for mating purposes and allow males and females to recognise each other.

In others, the colours perform a role in camouflage, hiding the insect from its predators.

In yet others, they perform a thermodynamic role: the different colours reflecting and absorbing different wave lengths of light, allowing the insect to control its internal temperature. In some insects the wings may perform two or more of these functions simultaneously.

In two orders of insects, one pair of wings has become completely reduced to small balancing organs and they provide no power of lift during flight at all. The most common of these are the Flies (House Flies, Dung Flies etc.), which have the hind wing reduced. Much less commonly seen, but just as interesting, are the Stylops which have the forewing reduced.

In many insects, the hind wings are reduced to some extent and thus smaller than the forewings. In others, the the forewing has become stiffened to form a protective function. The hind wing is often larger than the forewing and therefore needs to be folded up when it is not in use.

Grasshoppers and Crickets fold their wings longitudinally, while in Beetles and Earwigs the wings are folded both longitudinally and laterally. In Staphylinid beetles and Earwigs, the forewings are very short and the degree of folding of the hindwings is truly amazing.

What Next?

Well, I hope this has been an illuminating look into the anatomy of the insect thorax and the legs and wings attached to it!

Perhaps now you’d like to learn a little about insect eyes.