Most land invertebrates (along with crabs, shrimp, crayfish, and others) are arthropods — invertebrates with jointed legs. Most of us recognize arthropods when we see them, but here are some basic ways to start grouping them.
Centipedes, millipedes, and sowbugs all have elongated bodies with many similar segments and many legs.
Spiders, ticks, scorpions, and daddy longlegs all have eight walking legs — often with extra appendages near the head for manipulating food.
Insects — a group staggering in numbers and diversity — all have six legs, and their bodies are divided into three portions: head, thorax, and abdomen. The adults often have one or two pairs of wings, and one pair of antennae. Several insect groups undergo a striking change (metamorphosis) between juvenile forms and winged, adult forms.
There may be as many as 10 million species of insects alive on earth today, and they probably constitute more than 90 percent all animal species. Scientists are busy cataloging and describing new insect species.
Insects are divided into orders, or major lineages, such as “the dragonflies and damselflies” and the “grasshoppers, crickets, and katydids.”
Other land invertebrates include earthworms, slugs. and snails. Slugs and snails are molluscs, related to clams, limpets, conchs, and octopuses. Earthworms are a large, diverse group of annelids (segmented worms) that have enormous impacts on soil ecology.