Re: Why does a caterpillar have so many legs?

Dear Deeptha,

Thank you for your question about the locomotion of caterpillars. First I will describe the classification of different invertebrate animals, and then I will explain why different forms of locomotion work for different animals.

The classification of life forms is called taxonomy. I will quickly review animal taxonomy for you here. All animals are in the kingdom Metazoa. The kingdom Metazoa is divided into many Phyla. These Phyla include the Chordates (which are vertebrates like birds, humans, and most fish), the Molluscs (which include clams, snails, and even squid) the Annelids (which contain the segmented worms, such as the common earth worm), and the largest phylum, the Arthropods (which includes insects, spiders, lobsters and centipedes).

Confused? Quickly look over the Tree of Life's phylogenetic tree of the Metazoa. The branches that come off of this sideways tree indicate how closely related the different Phyla are.

Each Phylum is made up different Classes, and each Class is made up of many Orders. For example, in the Phylum Arthropoda, some of the Classes are Centipedes, Millipedes, Crustaceans, and Hexapods. Hexapods (which means "six legged") have many Orders including Coleoptera (beetles) Hymenoptera (ants, wasps and bees) Isoptera (termites) Diptera (flies) and Lepidoptera (butterflies, moths and skippers).

Many of the insects have complicated life cycles. They pass through immature stages that look completely different than the adult form. Such insects include all of the Endopterygote Orders of insects. Adult flies come from maggots, adult beetles come from grubs, and adult butterflies, moths and skippers all develop from caterpillars.

Worms do not have such a complicated life cycle. They simply grow longer and longer without changing their basic shape. Worms are really quite different from caterpillars. This is because the goal of a caterpillar is to become a butterfly or moth, while the goal of a young worm is simply to live in a similar manner through adulthood. Worms do have tiny grippers on their bottoms that help them undulate through soil and leaf litter. Worms eat decaying vegetation, and build nests and mate all in the soil and leaf litter on the ground. They never have need to climb up onto a plant.

Caterpillars must eat a huge amount of material to gain the energy to transform into the winged adult form. The caterpillars most often live on the plants that they eat, and it is important for them not to fall off. The specialized appendages of the caterpillar that you are calling legs are referred to as pseudopods. (Pseudopod means fake foot, which means that the appendage looks the same as the adult leg, but is, in fact, a different structure.) These pseudopods grip onto the host plant and prevent the caterpillar from falling off as it eats its way through the plant. If the caterpillar falls off the plant it might be eaten by a predator or simply starve if it can't find its way back onto the plant.

The placement of the pseudopods also helps the locomotion of the caterpillar. By having pseudopods on the ends of the caterpillar, the animal can safely inch its way along the stems and leaves of a plant. This is in contrast to many worms which undulate along the ground without lifting their bodies off of the ground for any significant period of time. Worms and caterpillars have different methods of locomotion that fit their different life styles.

Invertebrates are fascinating animals because of their wide variety of behaviors. The methods that different animals use to move through their environments (and the body shapes they've evolved to help them) is one such interesting subject. If you have anymore questions, please ask us again!


Sincerely,

John Carlson
MAD Scientist