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Leg Joints

 

Ranger Steve Mueller

By Ranger Steve Mueller

 

Femur, tibia, fibula, metatarsals, and tarsals comprise the bones in our legs and feet. Their arrangement allows movement. Other mammals have bones in different arrangements that serve their mobility for greatest survival. Other groups of organisms like birds and frogs have their own special configurations to meet their needs.

Diagram of a typical insect leg.

Insects do not have bones but have specialized leg joints. Insect exoskeletons are on the outside of the body instead of inside like ours. Leg section names are similar but are structurally different. Between the body and femur is a rounded knob called the coxa followed by another small section called the trochanter. The femur is often the largest leg section much like our femur. Though the name is the same, the insect femur is made of a hard chiton comparable to our fingernails that are on the outside to protect inner tissues and muscles. Consider an insect’s skeleton to be like a knight’s armor that protects from the outside.

To move, it is necessary to have flexible connective tissue between various sections of the leg like is used in a knight’s armor. Progressing from the body to leg tip, the leg sections have adaptations that serve the insects life style for survival in its nature niche. The tibia connects the femur with the tarsi. The tibia is comparable to the tibia and fibula of our lower leg.

A series of small leg sections called tarsi beyond the tibia allow flexibility. Most insects have three, four or five aligned in a row. Go outside to look at a large grasshopper or katydid’s leg joints. Some of the large Carolina Grasshoppers with black wings are still active. The last pair of legs on the grasshopper are large, adapted for jumping and are easiest for viewing leg construction. When you try to capture a grasshopper, it becomes obvious how well suited their legs are for escaping danger.

Crickets and long-horned grasshoppers, like the katydids, have an “eardrum” or tympanum at the basal end of their tibia. We have been enjoying the raucous sound penetrating the blackness of night. It is essential for noise making insects to hear the sexual calls at night for successful breeding. Instead of hearing in their heads like us, they hear in their legs.

There are more specialized leg structures than described here so consider visiting the library, the web, and spend time outdoors exploring. One very important feature not mentioned is tarsal claws. Most insects have small claws that aid gripping surfaces. When an insect stands on your arm, you often feel the claws grip.

The front legs of the praying mantis have a long femur and tibia lined with stiff spines that allow it to grip insect prey firmly. Its coxa that connects the femur to the body is long instead of small and round. It allows greater mobility for capturing prey. Each insect species has unique adaptations that meet its lifestyle. If you have been grabbed by a mantis, its spines might have penetrated your skin and even caused some bleeding.

Inside the exoskeleton leg, is where the muscles are attached. Our leg muscles extend across joints so when contracted they cause the leg to bend. If both ends were attached to the same bone, contraction would not result in movement. Insects are similar in muscle attachment except their muscles are inside the hollow exoskeleton but they stretch across joints. You will not find an insect with bulging muscles because they are hidden inside.

Beetles have interesting legs. They are sometimes quite easy to observe in fall because they frequently stand on flower heads for extended periods. Visit a goldenrod or New England Aster to watch. Notice how the front pair of legs reaches forward and the second and third pair of legs extend backwards. Some insects only use four legs (two pairs) when walking so watch to discover them. Observe leg movement and notice if the front and back legs on one side are used with the middle leg on the opposite side when walking.

Some insects like the blister beetle ooze a substance from their leg joints when disturbed. The fluid can cause skin blisters. There are thousands of insects with interesting leg joints. Take time to observe nature’s wonders.

Natural history questions or topic suggestions can be directed to Ranger Steve (Mueller) at odybrook@chartermi.net – Ody Brook Nature Sanctuary, 13010 Northland Dr. Cedar Springs, MI 49319 or call 616-696-1753.

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