Insect Legs

Insect Legs :

One of the most generally known and oft-repeated facts about insects is that they possess three pairs of legs, one pair each on the prothorax, mesothorax, and metathorax. The taxon name Hexapoda came from the Greek hexa, six, and poda, foot. Exceptions to the hexapodous condition are found in the apodous, or legless, insects that have secondarily lost their legs, typically as a result of selection for an obligatory parasitic or sedentary existence. The fore-legs are located on the prothorax, the mid-legs on the mesothorax, and the hind legs on the metathorax.

Structure of leg
Each insectan leg has six major components, listed here from proximal to distal: coxa (plural coxae), trochanter, femur (plural femora), tibia (plural tibiae), tarsus (plural tarsi), pretarsus. Each segment in the insectan leg, unless secondarily lost or fused, is independently movable by muscles inserted on its base.
1. Coax: The coxa is typically short and rather stout, although it varies in shape among taxa. It is set in a coxal cavity and articulates with the thorax at the coxal process of the pleural sulcus (groove).
2. Trochanter: The trochanter is small and freely movable in a vertical direction on the coxa, but it is often rather fixed to the base of the femur.
3. Femur: The femur is usually the largest and strongest segment of the leg. Its size is related to the mass of the tibial extensor muscles within it, varying from a small, thick segment in larval insects to the enormous segment in the hind leg of jumping Orthoptera. The femur often is equipped with spines and other cuticular modifications, especially in predatory insects.
4. Tibia: The tibia typically is long and slender in adult insects. Proximally, it is bent slightly toward the femur, allowing the shaft of the tibia to be flexed close against the femur for more locomotory power in insects such as grasshoppers. It often bears spines for grooming or for engaging the substrate to aid in locomotion. Many insects also have apical or subapical movable spurs on their tibiae.
5. Tarsus: The tarsus is a simple, undivided segment in holometabolous larvae and basal hexapods such as Protura and some Collembola. In collembolans, the tarsus and tibia are fused into a single tibiotarsus. In most insects, tarsus is usually made up of two to five segments called tarsomeres. The ventral surfaces of the tarsomeres often bear pads called tarsal pulvilli that aid movement on smooth surfaces and are especially well developed in some Orthoptera.
6. Pretarsus: The pretarsus, also called the acropod or posttarsus, arises from the distal end of the tarsal segment. In the Protura, Collembola, and larvae of many holometabolous insects, the pretarsus is a simple, clawlike segment. Typically the pretarsus consists of a membranous base, a pair of hollow claws (ungues), and various sclerites and lobes. A saclike, hollow lobe, the arolium, arises between the claws.
Legs joints
The areas of flexion between segments are joints, and the well-sclerotized contact points in the joints are the condyles. The various joints contribute to the mechanical efficiency of the leg.
The articulation between the coxa and the body allows the leg to move forward and rearward, whereas that between the coxa and the trochanter allows the leg to be lifted at the end of the backstroke and depressed at the beginning of the backstroke.
Leg joints are of two types. Monocondylic joints have a single point of articulation, somewhat like a ball-and-socket joint, and usually are situated dorsally. They allow considerable freedom of movement and are characteristic of the legs of larval insects. Dicondylic joints consist of an anterior and a posterior condyle, or a dorsal and ventral condyle in the case of the trochanterofemoral joint. They typically limit movement to that of a hinge. Adult legs usually have dicondylic joints, although the tibiotarsal joint is often monocondylic.

Types of legs

1. Ambulatory legs:  These legs are elongate and slender and are designed for walking.  Leg has clawed at the end and bearing several soft, cushioned pads that enable it to stick to smooth surfaces.
Examples: House Fly (Diptera), Bugs (Hemiptera), leaf beetles (Coleoptera).
2. Cursorial legs:  These legs are modified for running. They have long, thin leg segments.
Examples: Cockroaches (Blattaria), ground and tiger beetles (Coleoptera).
3. Saltatorial legs: Enlarged hind legs adapted for jumping. These legs are characterized by an elongated femur and tibia.
Examples: Grasshoppers, crickets and katydids (Orthoptera).
4. Fossorial legs: Powerful, spadelike forelegs adapted for digging and rapid burrowing.
Examples: Ground dwelling insects, mole crickets (order Orthoptera) and cicada nymphs (order Hemiptera).
5. Natatorial legs: Flattened, fringed legs of aquatic insects modified for swimming. These legs have long setae on the tarsi serve as oars for paddling or swimming.
Examples: Aquatic beetes (order Coleoptera) and bugs (order Hemiptera).
6. Skating legs: Long legs with hydrophobic tarsal hairs and anteapical claws  for skating on the surface of water.
Examples: Found in water striders (Gerridae),
7. Perching legs: The spiny legs are designed for perching or seizing and holding prey captured. These legs are feeble for walking.
Example: Dragonflie (Odonata)
8. Raptorial legs:  Long, stout legs designed to grasping (catching prey).
Examples: Mantids, ambush bugs, giant water bugs and water scorpions (order Hemiptera).
9. Clinging legs: Have a sharp claw on the tip of each leg so that it may better cling to the hair of its host.
Example- Biting louse (Mallophaga)
10. Corbiculate legs: On the hind-leg there are three interesting structures; the pollen basket fringed with hairs, the heavy wax pincers, and the pollen comb at the tip.
Example-Honey bee (Hymenoptera)
Functions of insect’s legs:
(i) Legs play an important defensive role, not only in permitting escape by running, jumping, burrowing, and swimming, but also in ways such as kicking and slashing.
(ii) The spines on the legs of many insects, when used in defense, effectively deter predators and competitors and can inflict considerable damage.
(iii) Insects such as stink bugs and treehoppers deliver powerful kicks at parasitoids and predators that attempt to attack their young.
(iv) All legs are equipped with an extensive arrangement of sensory structures that allow the insect to feel, hear, and taste, providing the insect with its initial assessment of the environment.
(v) Legs also can be used to produce sound for intraspecific communication by drumming them against a substrate, as in some Orthoptera.
(vi) To maintain hygiene, insects spend considerable time preening and grooming their body and appendages with the help of various leg structures as cuticular combs, setal brushes, grooves, and notches.
(vii) The hind leg of honey bees is specially modified to groom pollen from the plumose hairs of the body.
(viii) Legs often are used to hold onto objects. The grasping function is seen, in the pincer-like, spiny raptorial forelegs of many predacious insects.
(ix) Flies that feed on the blood of birds typically have a thumb-like lobe at the base of each of their talon-like claws that helps them grasp feather barbules.
(x) Configurations of pattern and color of legs play a role in camouflage, mimicry, and courtship.
(xiii) Legs of insects perform some highly specialized functions as:
(a) In the Embiidina, the basitarus of each foreleg houses multiple silk glands, and each gland is connected to a seta with an apical pore through which the silk is extruded.
(b) The inflated basitarsus of each leg in phantom crane flies contains a tracheal sac, perhaps aiding buoyancy during the drift like flight.
(c) Some flies have specialized areas on their legs, particularly on the tibia, that possibly produce pheromones.
(d) Insects such as Chironomidae seem to use the legs much as a second set of antennae.
(e) In Protura, which lack antennae, the forelegs probably have assumed an antennal (i.e., sensory) function.
(f) Various ornamentations on insectan legs can serve a courtship or intrasexual combative role, as in some coreid bugs.