(Previous Article: Insect Wing Structure)
Continuing with entomological self-education…
Legs: they define the Hexapods: attached to the prothorax, mesothorax and metathorax of most mature adults in Class Insecta are a pair of legs, giving six legs in total. Once more we will be working with photographs from an as yet unidentified Orthopteran, a katydid (Tettigoniidae) probably in the sub-family Conocephalinae.
The legs have six major parts — from the body outwards we have the coxa, the trochanter, the femur, the tibia, the tarsus and the pretarsus. Often called segments, these leg parts are also known as podites. Each podite (unless lost or fused) is independently moveable with its own muscles. Areas between podites that can flex are called joints, and each joint is either monocondylic (a single point of articulation) or dicondylic (with two points of articulation). Condyles act as fulcrums for the muscles to act on: monocondylic joints have a wide range of movement like a ball joint, while dicondylic joints operate more like hinges, usually allowing movement in one plane only. The muscles are intrinsic, consisting of flexors and extensors, which work against each other (antagonistic) to move the segments at the joints.
1. The basal podite is the coxa, which differs from other segments in that it has extrinsic muscles. The coxa is set in the coxal cavity of the thorax and articulates with the pleuron.
(*In Fig. 1&2 the coxae of the hind legs may be hidden within the coxal cavity, hence labelling of first visible segment as trochanter. The specimen is no longer available for inspection, so any correction/confirmation on this would be appreciated)
2. The trochanter is a small podite, sometimes fixed to the femur. In some fossil insects and in Odonates (dragonflies and damselflies) there can be two trochanters.
3. The femur is the largest and most powerful podite. The size is related to the size of the extensor muscles that lead to the tibia.
4. The tibia is the next largest segment after the femur. Spurs are often found at the distal end of the tibia.
5. Distal to the tibia is the tarsus.The tarsus is formed by several sub-segments known as tarsosmeres, which sometimes carry pads (tarsal pulvilli) that provide traction. The first sub-segment of the tarsus is called the basitarsus, while the remaining tarsomeres (two to four sub-segments) are called the eutarsus.
6. And finally, the apical podite of the leg is the pretarsus, which usually caries two claws called ungues. In many insects there is a pad or lobe (the arolium) between each ungui.
Traditionally, legs have been grouped according to their primary function (Fig. 4):
- Cursorial legs are modified for running fast, such as in cockroaches.
- Saltatorial hind legs are for jumping, as can be seen in Orthopterans such as the conehead in Fig. 1.
- Fossorial fore legs are modified for borrowing and digging, such as in mole crickets.
- Natorial legs are modified for swimming. They are flattened and have bristles for paddling, such as in diving beetles.
- Raptorial fore legs are used for grasping such as seen on Praying Mantids.
- Ambulatory legs, like this bee leg, are used for walking and climbing.
There are may variations in leg structure across the taxa and between males and females, larvae and adults and in the individual insect. There are even some insects which are legless, such as in females in Coccidae, Psychidae and Strepsiptera. Legs vary in function, and are adapted for different means of locomotion, grooming, feeding, defence, and for courtship and mating. Some legs have sensory functions (including hearing!) and others have adhesive pads or even contain silk glands. More on the variations and uses of limbs will be discussed in future articles on the different orders and families.
Coming up next in Entomology 101: The Insect Abdomen
Grimaldi, David and M.S. Engel, Evolution of the Insects, (pp. 125-128) Cambridge University 2005.
Norman F. Johnson and Charles A. Triplehorn, 2004. Borror and DeLong’s Introduction to the Study of Insects (7th edition, pp 11/12) Brooks Cole.
Resh, Vincent H. and R. T. Cardé, Eds. Encyclopedia of Insects, (pp.627-631) Elsevier 2003.