Insect, Spider, and Kin Facts

Insect, Spider, and Kin Facts

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.

yellow garden spider in its web
Yellow garden spider

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.

The four largest orders of arthropods

The beetles, which you can quickly recognize by the two shell-like covers (actually, they’re a modified first pair of wings) that meet in a straight line down the back.

The butterflies and moths, which all have tiny scales on their wings. They're so popular, they have their own section in MDC's online field guide.

The flies, which all have only one pair of wings. This group includes houseflies, mosquitoes, midges, and gnats.

The ants, bees, and wasps have two pairs of wings (queen ants have wings though workers do not) and a selection of other characteristics, often with the ability to sting and a range of colonial behaviors.

Amazing diversity, immeasurably important
  • Many are crop pests, but insects are also crucial for pollinating nearly every flowering plant on earth.
  • They are the food source for thousands of vertebrates: Fish, amphibians, reptiles, birds, and mammals rely on them, directly or indirectly.
  • Many humans eat insects, too; if not directly, then indirectly by consuming animals that eat insects.
  • Insects parasitize nearly every type of animal, including humans; mosquitoes are dreaded worldwide for the diseases they carry. A few spiders in our state deliver venomous bites to people.
  • Spiders, ladybugs, lacewings, mantids, robber flies, assassin bugs, dragonflies, and many others provide natural pest control.
  • Termites destroy wooden structures.
  • Bees make honey; fruit flies are used in genetics research; hellgrammites and crickets are used a fishing bait; silkworms make silk.