The most successful animals
Insects and arachnids belong to a huge group called arthropods, which means "jointed feet." Arthropods make up more than 80% of all known animal species. There are over a million named species of insects alone, and possibly millions more still undiscovered.
All arthropods have:
- an exoskeleton (a hard outer skeleton made of chitin)
- a segmented body
- jointed legs
- no backbone (they are invertebrates)
Because the exoskeleton doesn’t grow, arthropods must shed it to get bigger. This is called molting.
Insects vs. arachnids
| Feature | Insect | Arachnid |
|---|---|---|
| Body parts | 3: head, thorax, abdomen | 2: cephalothorax (head and chest together) and abdomen |
| Legs | 6 | 8 |
| Antennae | 2 | None |
| Wings | Most adults have wings | Never |
| Eyes | Usually compound eyes | Simple eyes |
| Examples | Ants, bees, beetles, butterflies, flies | Spiders, scorpions, ticks, mites |
A quick test: count the legs! Six legs = insect. Eight legs = arachnid.
Insects
Body plan
| Part | What’s there |
|---|---|
| Head | Eyes, antennae, mouthparts |
| Thorax | Legs and wings |
| Abdomen | Digestive and reproductive organs; breathing holes (spiracles) |
Compound eyes are made of thousands of tiny lenses. They are excellent at detecting movement, which is why flies are so hard to swat.
Mouthparts
- Chewing (grasshoppers, beetles)
- Sucking through a tube (butterflies)
- Piercing and sucking (mosquitoes)
- Sponging (house flies)
Metamorphosis
| Type | Stages | Examples |
|---|---|---|
| Complete metamorphosis | Egg → larva → pupa → adult | Butterflies, beetles, bees, flies |
| Incomplete metamorphosis | Egg → nymph → adult | Grasshoppers, dragonflies, cockroaches |
Social insects
Ants, honeybees, wasps and termites live in colonies with a queen, workers and other roles. Honeybees "dance" to tell each other where flowers are. Some ant colonies have millions of members.
Arachnids
Spiders
- All spiders make silk, though not all build webs. Some hunt, jump or ambush prey.
- Spider silk is, by weight, stronger than steel.
- Most spiders have fangs and venom to subdue prey, but very few are dangerous to people. In Canada, the black widow is the main venomous spider of concern, and bites are rare.
Other arachnids
| Arachnid | Notes |
|---|---|
| Scorpions | Pincers and a venomous stinger; glow under UV light |
| Ticks | Feed on blood; can spread Lyme disease |
| Mites | Tiny; some live in soil, others on animals or even in your eyelashes |
| Harvestmen ("daddy longlegs") | One body section; no silk or venom glands |
Why they matter
| Role | Example |
|---|---|
| Pollinators | Bees, butterflies and flies pollinate many crops and wildflowers |
| Food | Insects feed birds, fish, frogs and people (in many cultures) |
| Decomposers | Beetles and fly larvae break down dead matter |
| Pest control | Spiders and ladybugs eat crop-damaging insects |
| Products | Honey, beeswax, silk |
| Disease carriers | Mosquitoes spread malaria; ticks spread Lyme disease |
Insects in decline
Many insect populations are falling because of habitat loss, pesticides, climate change and light pollution. Monarch butterflies, which migrate from Canada to Mexico, are listed as endangered in Canada. Planting milkweed and native flowers helps.
Common mistakes
- Calling spiders insects.
- Thinking all spiders are dangerous. Most are harmless and helpful.
- Thinking daddy longlegs are spiders. They’re a different kind of arachnid.
Try it yourself
- Find five small creatures outdoors (look under logs, with an adult). Count legs and body parts to sort them into insects, arachnids or neither.
- Draw and label an insect and a spider side by side.
- Raise caterpillars (or watch a mealworm turn into a beetle) and record each stage of metamorphosis.
Quick recap
- Insects and arachnids are arthropods with exoskeletons and jointed legs.
- Insects have 3 body parts, 6 legs and antennae; arachnids have 2 parts and 8 legs.
- Many insects go through complete or incomplete metamorphosis.
- Spiders make silk; ticks, mites and scorpions are also arachnids.
- These animals pollinate, recycle and feed ecosystems.