17 Insect Anatomy – The Thorax

Thorax

The thorax runs from the back of the head to the end of the last segment with legs attached.

The head and thorax are connected by a membranous area called the cervix.

The thorax itself consists of three segments, which are easiest to see on the lower, or ventral surface. The prothorax is the first segment, nearest the head, and carries the first pair of legs. The mesothorax is the middle segment, and carries the second pair of legs. The metathorax is the third segment, furthest from the head, and carries the third pair of legs.

The sclerites on the ventral surface of the thorax are referred to as sterna: prosternum, mesosternum, and metasternum. The sclerites on the dorsal surface of the thorax are referred to as nota: pronotum, mesonotum, and metanotum.

Legs

The jointed legs of insects consist of five parts:

• The coxa joins the leg to the body
• A segment called the trochanter provides greater flexibility in leg positioning
• In the grasshopper the hind femur is very long and muscular, for jumping
• The tibia is also lengthened in the grasshopper
• The final section is a multi-segmented tarsus, which is partly obscured by the middle leg in this photo.

When the leg is removed from the body (this is the middle, or mesothoracic leg of the grasshopper), you can see the large muscles that control and attach it to the body.

The segment nearest the body is the coxa, followed by the trochanter, then the femur…

Then the tibia … And, finally, the tarsus.

In the grasshopper, there are three tarsal segments. The tip of last tarsal segment has two claws and a pad, collectively called the pretarsus (not counted as a separate tarsal segment).

Identifying many insects, especially beetles, requires you to count tarsal segments, often on all three pairs of legs.

Wings

The thorax also bears two pairs of wings. The forewings are attached to mesothorax, and the hind wings are attached to the metathorax. The prothorax does not bear any wings. In the grasshopper, the pronotum is enlarged to form a protective shield that extends back over the wing bases.

A number of small sclerites and sutures at the bases allow the wings to articulate with the body with maximum flexibility (more on that in class).

Notice that the front and hind wings differ considerably in texture in the grasshopper. The front wing is tough and leathery serving to protect the hindwing when not flying. The hind wing is membranous, and used for flight (this one was a little mangled, sorry!).