Insect Legs and Its Modifications

INSECT LEG AND ITS MODIFICATIONS

            Insects posses three pairs of legs, each in all the three thoracic segments. Insect legs are sub-divided into five basic segments known as Coxa, Trochanter, Femur, Tibia and Tarsus.

v  Coxa: Functional basal segment, attached with thoracic pluerites.

v  Trochanter: Small in size, being articulated to the coxa and forms immovable attachment with femur.

v  Femur: The largest and strongest segment.

v  Tibia: Long slender segment having one or more spurs at its distal end

v  Tarsus: Distal segment attached to the tibia and is subdivided into sub-segments called Tarsomeres.

            The basal tarsomere, often longer than others is named as the basitarsus. The last tarsomere is called pretarsus. And it has a pair of lateral claws (ungues) and pad like median Arolium. In few cases, the pretarsus bears a pair of lobes, or pulvilli, under the claws. The arolium is absent in such cases and instead a spine like empodium is present.

            Insect legs are variously modified and adopted to carry out additional functions depending upon the requirement as follow,

Modification	Description	Example
Ambulatorial	Walking, segments more or less uniform	Fore and middle legs of grasshopper
Cursorial	Running, segments generally long, slender	Cockroaches
Fossorial	Digging, greatly expanded and hardened, tarsi  digitated	Fore legs of mole cricket
Saltatorial	Jumping, enlarged femur and long slender tibia	Hind legs of grasshoppers
Natatorial	Swimming, oar like, tibia and tarsus much flattened and fringed with dense hairs	Water beetle
Scansorial	Clinging, tarsus is one segmented with a single strong, sickle shaped claw	Head louse
Raptorial	Grasping, femur and tibia enlarged with row of spines	Fore leg preying  mantids
Foragial	Pollen gathering, tibia flattened and fringed with long hairs forming the pollen basket (corbicula), basitarsus fringed with row of stiff hairs, the tarsal tip of the notch with a small lobe called auricle and the tibial tip with row of spines called pecten	Hind legs of honey bees

Prolegs: Immature forms bear unsegmented leg like lobes in the abdomen that aid in movement. These are temporary and often referred as false legs or pseudo legs or prolegs. Eg. Lepidopteran caterpillars and few hymenopterans.

            Generally five pairs are present in 3^rd, 4^th, 5^th, 6^th and 10^th abdominal segments. Prolegs are short, fleshy, unsegmented and have a number of tiny incurving hooklets at its tip called crochets, which arise from a basal segment called planta.

Special structures:

v  Suction disc: It is present in fore tarsi of male diving beetle and is used to hold the female during copulation in water.

v  Hind femora of short horned grasshoppers bear peg like short structures with which they rub the forewing and produce sound.

v  Long horned grasshopper and Crickets have oval auditory organ or tympanum at the base of each fore tibia.

v  Chemosensilla is present in the fore tarsi of houseflies and blowflies.

v  Antenna cleaner: In honey bees, fore tarsi have notch depression with hairy surface and a lid called antenna cleaner.