Structure of Insect Thorax and Appendages

Insect Legs and Their Types

• Insect legs are modified for various functions, such as walking, running, jumping, grasping, and digging
• Segments of a leg:
  • Coxa: basal segment, freely movable
  • Trochanter: small second segment, usually one-segmented, but two-segmented in dragonflies and damselflies
  • Femur: largest and stoutest part of the leg
  • Tibia: long, provided with many spines, helps in footing
  • Tarsus: subdivided into many tarsomeres (1 to 5), basal segment is usually larger and is known as basitarsus
  • Pretarsus: structures beyond the tarsus, including two claws, a median pad (arolium), and two basal pads (pulvilli)

Leg Modifications

• Ambulatorial (Walking) legs:
  • Foreleg and middle leg of grasshopper
  • Femur and tibia are long
• Cursorial (Running) legs:
  • Cockroach
  • Elongate, slimmer legs
  • Femur is not swollen
• Saltatorial (Jumping) legs:
  • Hind legs of grasshopper
  • Can hop (jump, leap, saltate, vault)
  • Trochanter fused with femur
  • Femur enlarged and accommodated with powerful muscles
  • Tibia – long and slender with two rows of spines
• Raptorial (Grasping) legs:
  • Forelegs of preying mantids, giant water bug
  • Hold or grasp their prey in between the leg
  • Coxa is elongate and outstretched
  • Femur and tibia spiny, grooved, work against each other to grasp prey
• Fossorial (Digging/Burrowing) legs:
  • Mole cricket
  • Forelegs burrow or rake through the soil to dislodge soil particles
  • Tibia is rake-like, and femur is stout
  • Tibia has 2-3 strongly pointed tines at the distal part
  • Tarsus has tines like the basitarsus, which works against the tibial tines to shear rootlets

Other Leg Modifications

• Natatorial (Swimming) legs:
  • Hind legs of water beetles and water bugs
  • Segments are much flattened
  • Femur, tibia, and first four tarsomeres are broader and flattened with rows of strong setae (hairs) adapted for rowing
• Scansorial (Clinging) legs:
  • Head louse
  • Tibia is stout, bearing a thumb-like process at one end
  • Tarsus is unsegmented with a single claw
  • Tarsal claw fits against the tibial thumb while clinging to the hairs
• Sticking legs:
  • All three pairs of legs of the housefly
  • Pretarsus has a pair of lateral adhesive pads (pulvilli)
  • Median spine is the empodium
  • Pulvilli have dense mats of tiny glandular hairs (tenent hairs)
  • Smooth surface and roof allow for walking in an upside-down position
• Basket-like legs:
  • Legs of dragonflies and damselflies
  • Legs are just behind the head and anterior in position
  • Spiny and closely placed
  • Seize prey in flight and carry captured insects in a basket
• Clasping legs:
  • Forelegs of male water beetles
  • Tarsus is flattened with adhesive discs
  • Clasp the mate during copulation

Insect Wings and Their Types

• Wing modifications:
  • Tegmina (Tegmen): e.g., the forewings of cockroaches, grasshoppers
    • Leathery wings that protect the membranous hind wings
  • Elytra: e.g., the forewings of beetles, weevils
    • Heavily sclerotized and horny wings that protect the membranous hind wings
  • Hemelytra: e.g., the forewings of heteropteran bugs
    • Wings are more sclerotized basally, but the distal half remains membranous
  • Haltere: e.g., the hind wings of true flies, mosquitoes, male scale insects
    • Wings are modified into a vibrating, sensory appendage
  • Fringed wings: e.g., the wings of thrips
    • Wings are very narrow with a fringe of elongate setae
  • Scaly wings: e.g., the wings of butterflies, moths
    • Wings are clothed with scales
    • Scales are flattened, unicellular modified setae

Wing Coupling

• In primitive winged insects (pterygotes) and in Odonata (dragonflies) and Isoptera (termites), the wings move independently
• In higher pterygotes, the wings of a side operate in union by a wing coupling apparatus
• Types of wing coupling:
  • Hamulate: e.g., honey bees
    • A series of hooks or hamuli (small hooks)
  • Frenate: e.g., fruit-sucking moths
    • A stout bristle (frenulum) arising from the costal margin of the hind wing interlocks with a hook-like bristle (retinaculum) on the subcostal vein of the forewing
  • Amplexiform: e.g., butterflies
    • The forewing and hind wing of either side get coupled by simply overlapping each other
  • Jugate: e.g., Hepialidae
    • The jugal lobe of the forewing lies beneath the costal margin of the hind wing, so that the latter is held between the jugum and the rest of the forewing