Thursday, October 16, 2014

Insect legs: thorax or abdomen?

We received in the mail a question from a first-grade class:
Dear UMass entomologists,
We would like to know, Do insect legs always attach to the thorax, or are there are any insect legs that attach to the abdomen?

Thanks, Mrs. Hasbrouck's class, for inspiring our first blog post!

The short answer is, All true insect legs attach to the thorax. 

Insects, or "class Insecta" if you are feeling sciencey, have three pairs of legs. Each of these three pairs of legs attach to one of the three thoracic segments: the forelegs attach to the prothorax, the middle legs to the mesothorax, and the hindlegs to the metathorax. As arthropods (meaning "jointed leg", just like "arthritis" means "joint inflammation"), insects have legs that are divided into segments. The coxa (plural: "coxae") attaches the leg to the rest of the body. The trochanter links the coxa to the long femur, which is usually enlarged in jumping insects, like the Orthopteran crickets (Gryllidae) and grasshoppers (Caeliferae). Next comes the tibia and finally the tarsus, which is usually divided into several segments, ending with the "tarsal claw".

What about caterpillars? Don't they have lots of legs?

Yes, the larvae of moths do have more than six legs. However, these are not considered "true legs", but rather "prolegs". Prolegs may attach to the abdomen, but their structure is different from the true legs--they lack joints and characteristic 5 segments, and prolegs are generally not retained in insect adults.
For a great overview of prolegs with colorful images, visit this Wikipedia page: