JANUARY
2005

 

 

 

By
Tom Turpin
 
Professor of
Entomology
Purdue University

 

 

 

 

 

01-13-05

Not All Insect Legs Are Made for Walking

What is the essence of an insect? If your mind immediately conjures up thoughts of some exotic sweet-smelling perfume, you are on the wrong path. I suppose the essence of an insect might be an odor. After all, some insects do stink. Those Asian ladybugs that invade our homes immediately come to mind.

But that's not the essence we're talking about. This essence is what dictionaries call a prime character, something important to the highest degree. In other words, what are the structures that make an insect an insect?

The best way to answer this question is by asking people to draw a picture of an insect. Even better, ask children to draw insects. Such renderings almost always include three major body sections. Those sections are officially called the head, thorax and abdomen. Most drawings include wings, legs, eyes, antennae and a mouth. Some kids add a stinger to their renderings.

It is the obvious characteristics of an insect that most people recognize. These are the same ones that constitute the essence of an insect, as well as the characteristics that scientists use to define and classify insects.

Wings are so prominent that many order names of insects say something about those structures. For instance, butterflies and moths have scales on their wings. These insects are classified in an order name made by combining Lepidos (scale) with ptera (wing) to form Lepidoptera. Flies lack one pair of wings and are classified in the order that means two wings -- Diptera.

>But not all adult insects possess wings. Some, like fleas and lice, are wingless. In addition, all immature insects are wingless. Wings are acquired only after the immature turns into the adult stage. 

Unlike wings, legs are present on many immature insects. Only fly immatures, called maggots, are legless. The legs, six on all insects, are so obvious that the name hexapod was once used for this group of animals. The title of this column, "On Six Legs," reflects that characteristic and number.

The basic insect leg is made for running. In fact, it is very similar in structure to legs used for running in many other animals, including humans. Each insect leg consists of five parts. Think about a human leg. Our foot is made up a batch of small bones called tarsae. So is the insect foot. The difference is that the human foot ends up in toes, the insect foot in claws. 

So the toe bone is connected to the shin bone called the tibia. Same in the insect. The shin bone is connected to the thigh bone, the femur. Same in the insect. The thigh bone is connected to hip bone. Here's where the human leg and insect leg differ somewhat. The human thigh bone articulates with the pelvic saddle. The insect leg has two additional segments called the trochanter and coax, which join the leg to the insect exoskeleton.

There are a lot of modifications to the basic running design of the insect leg. Some legs are adapted for leaping. Think grasshoppers here. Such legs are called saltatorial which is a word based on the Latin word meaning to dance. Some insect legs are called fossorial and function for digging. These legs are found on scarab beetles that bury manure balls in the soil and mole crickets that, as their name suggests, burrow in the soil.

Like the birds of prey that are called raptors, some insects have raptorial legs. These legs are made for grasping. Praying mantids have raptorial legs. As most people are aware, the raptorial legs of mantids are the front pair of legs. The other four legs are of the walking type.

So the essence of an insect is that it has six legs. But they are not always made for walking -- some are for digging, grabbing, jumping and even singing!

 

Writer: Tom Turpin
Editor: Olivia Maddox