Thorax of insects

The thorax is the middle body region of an insect, situated between the head and the abdomen. It is primarily specialized for locomotion, housing the muscles responsible for wing movement and leg motion. The thorax supports three pairs of legs and, in winged insects, one or two pairs of wings.
Structure of the Thorax
The thorax is divided into three segments:
1. Prothorax:
o The anterior segment.
o Bears the first pair of legs.
o In most insects, it lacks wings (though some groups, like certain beetles, may have adaptations here).
o Usually, the smallest and simplest segment.
2. Mesothorax:
o The middle segment.
o Bears the second pair of legs and, in winged insects, the first pair of wings.
o Contains major flight muscles in most insects.
3. Metathorax:
o The posterior segment.
o Bears the third pair of legs and the second pair of wings (if present).
o Often fused with the mesothorax in highly active fliers like bees and flies, forming the pterothorax.
Components of Each Thoracic Segment
1. Exoskeleton Plates:
o Dorsal (top): Tergum (called the notum in the thorax, e.g., pronotum, mesonotum, metanotum).
o Ventral (bottom): Sternum (e.g., prosternum, mesosternum, metasternum).
o Lateral (sides): Pleura (e.g., propleuron, mesopleuron, metapleuron).
2. Musculature:
o Contains large muscles for wing and leg movement.
o In fliers, the mesothorax and metathorax are heavily muscled.
3. Appendages:
o Legs: Each thoracic segment has a pair of legs.
o Wings: The mesothorax and metathorax may bear wings in winged insects.
Functions of the Thorax
1. Locomotion:
o The thorax drives walking, jumping, flying, or swimming.
o Examples:
 Grasshoppers use powerful metathoracic legs for jumping.
 Flies have robust mesothoracic wings for rapid flight.
2. Flight:
o Winged insects use muscles in the mesothorax and metathorax to power flight.
o Direct flight muscles: Attach directly to the wings (e.g., dragonflies).
o Indirect flight muscles: Cause the thoracic exoskeleton to flex, indirectly moving the wings (e.g., bees, flies).
3. Sensory Role:
o Some thoracic structures (e.g., halteres in flies) aid in balance and navigation during flight.
Special Adaptations in the Thorax
1. Pronotum:
o Enlarged in some insects like beetles and shield bugs, providing protection to the head and thorax.
2. Wing Modifications:
o Elytra in beetles (hardened forewings) originate from the mesothorax.
o Halteres in flies (reduced hindwings) arise from the metathorax.
3. Leg Specializations:
o Raptorial legs in mantids for capturing prey (prothorax).
o Saltatorial legs in grasshoppers for jumping (metathorax).
o Fossorial legs in mole crickets for digging (prothorax).