When you think about all the animals on Earth, vertebrates (animals with backbones) are only a small part of the story. Invertebrates are animals that do NOT have a backbone, and they make up about 97% of all animal species on Earth! That's right — there are far more kinds of animals without backbones than with them. Invertebrates include insects, spiders, worms, jellyfish, octopus, clams, starfish, and many more.
Insects are the largest group of invertebrates. They have six legs, three body parts (head, thorax, abdomen), and most have wings. There are over 1 million known species of insects! Ants, bees, butterflies, beetles, grasshoppers, and flies are all insects. Insects have an exoskeleton — a hard outer shell that protects their body instead of an internal skeleton. Some insects go through complete metamorphosis (egg → larva → pupa → adult), while others go through incomplete metamorphosis (egg → nymph → adult).
Arachnids are another invertebrate group. They have eight legs and two body parts (cephalothorax and abdomen). Spiders, scorpions, ticks, and mites are arachnids. Unlike insects, arachnids do NOT have antennae or wings. Most spiders produce silk to build webs for catching prey. Only a tiny fraction of spider species are dangerous to humans — most are actually helpful because they eat pests!
Other invertebrates include crustaceans (crabs, lobsters, shrimp) with hard shells, mollusks (snails, clams, octopus, squid) with soft bodies, and annelids (earthworms) with segmented bodies. Echinoderms (starfish, sea urchins, sand dollars) have a water vascular system that helps them move and catch food. Cnidarians (jellyfish, corals, sea anemones) have stinging cells called cnidocytes that they use to catch prey.
1. What percentage of animal species are invertebrates?
2. How many legs do insects have? How many legs do arachnids have?
3. What is an exoskeleton and how is it different from a backbone?
4. Explain the difference between complete and incomplete metamorphosis. Give an example of each.
Butterfly Life Cycle — Complete Metamorphosis
Frog Life Cycle — Metamorphosis
| Feature | Vertebrates | Invertebrates |
|---|---|---|
| Backbone | Yes | No |
| Body type | ||
| Skeleton | ||
| Examples | ||
| Number of species | ||
| Size range |
Write YES if it's an insect, NO if it's not. Explain why.
| Animal | Insect? (YES/NO) | Why? |
|---|---|---|
| Butterfly | ||
| Spider | ||
| Beetle | ||
| Scorpion | ||
| Ant | ||
| Centipede | ||
| Grasshopper | ||
| Tick |
Draw each stage of the butterfly life cycle. Label each stage.
Materials: An earthworm (ask an adult to help!), a magnifying glass, a damp paper towel, a plastic container with holes in the lid.
Steps:
1. Place the damp paper towel in the container and gently set the earthworm on top.
2. Observe the earthworm for 5 minutes. Draw what you see.
3. Notice how it moves — does it use legs? How does it move without legs?
4. Gently touch the earthworm and note its reaction.
5. After observing, return the earthworm to damp soil outside.
Draw your earthworm and label what you can see:
Observations:
1. How does the earthworm move? ______________________________________________
2. What did you notice about its skin? __________________________________________
3. Earthworms are ________________ (vertebrate/invertebrate). Why? ______________
If invertebrates make up 97% of all animal species, why do people usually think about vertebrates more? Do you think invertebrates are important? Explain.