"Leg" - what is it, definition of the term
In arthropods such as ticks, true bugs, lice, and fleas, a limb is a segmented, jointed appendage attached to the thorax and composed of coxa, trochanter, femur, tibia, and tarsus; it provides locomotion, host attachment, and environmental interaction, often terminating in claws or adhesive pads, and bears sensory organs that detect tactile and chemical cues.
Detailed information
Arthropod locomotory appendages consist of a series of hardened plates (sclerites) linked by flexible membranes, each segment bearing muscles, joints, and sensory organs. The basic arrangement includes a basal coxa attaching to the body, followed by a trochanter, femur, tibia, and tarsus, often ending in a claw or adhesive pad.
Ticks possess four pairs of these appendages. The coxae are robust, providing a stable platform for attachment to hosts. The trochanter–femur–tibia segments are elongated, allowing the animal to reach into crevices of the host’s skin. The tarsus ends in a small claw equipped with a hygroscopic pad that enhances grip on moist surfaces. Sensory setae on the dorsal surface detect host vibrations and temperature changes.
Bugs, representing the order Hemiptera, typically have three pairs of walking appendages. The foremost pair may be modified into raptorial structures in predatory species, while the middle and hind pairs retain the standard segmentation. The tibial spines are often serrated, facilitating movement across plant surfaces. The tarsal segments frequently exhibit adhesive pads that secrete a thin fluid, improving adhesion on smooth leaves.
Lice exhibit six legs, each ending in a claw with a hook-like curvature. The coxae are reduced, allowing the legs to fold tightly against the body when the insect is at rest. The femur and tibia are short and stout, providing the force needed to navigate the dense hair of their mammalian hosts. Sensory hairs on the tarsus detect the direction of host grooming movements.
Fleas have five pairs of legs, though the hind pair is smaller and primarily used for jumping. The fore and middle pairs possess elongated femora that store elastic energy, enabling rapid extension during locomotion. The tarsal claws are sharp and curved, allowing penetration of the host’s fur. The tarsal pads are covered with microscopic hairs that increase surface contact, enhancing stability on slippery skin.
Key comparative points:
- Number of pairs: ticks (4), bugs (3), lice (6), fleas (5).
- Modification for host interaction: ticks and lice have claws adapted for gripping hair or skin; fleas have enlarged fore‑ and middle‑pair femora for jumping.
- Sensory structures: all groups possess setae on the tarsus, but ticks additionally have hygroscopic pads, while fleas rely on mechanoreceptors for detecting host vibrations.