Compare insects and arachnids
Count leg pairs and distinguish body divisions and antennae.
Core and Extended; Biology 0610, 2026–2028. Use the stated specimen features and evidence. Diagrams are schematic. Written practice is self-checked
Before you begin
- Read the stated observations before choosing a group.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which feature is shared by arthropods?
What you will learn
- Recognise arthropod features.
- Separate insects and arachnids using several structures.
Arthropods have jointed appendages
Arthropods have an external skeleton, a segmented body and jointed legs. Insects and arachnids are two groups within them. These shared features do not mean every arthropod has six legs.
A typical adult insect has three main body divisions: head, thorax and abdomen. It has three pairs of legs attached to the thorax and one pair of antennae. Wings may be present but are not required for every insect.
Count pairs correctly
A typical arachnid has two main body regions, four pairs of legs and no antennae. A spider with eight individual legs therefore has four pairs; do not confuse eight legs with eight pairs.
Use the whole observed adult specimen. A missing or hidden leg can make the visible count incomplete. When observations are uncertain, body divisions and antennae can provide additional evidence rather than forcing a conclusion from an incomplete photograph.
Pause and explain
Which adult combination supports an insect?
Worked example
An adult arthropod has eight legs, two body regions and no antennae. Classify it.
Show the worked solution
- Eight individual legs make four pairs.
- Two body regions support arachnids.
- Absence of antennae supplies further evidence.
Answer Arachnid.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Convert six individual legs to pairs and identify the typical arthropod group.
Give me a hint
Two legs make one pair.
Compare my reasoning
- 6 ÷ 2 = 3 pairs.
- Three pairs support an adult insect.
- Check body divisions and antennae as well.
3 pairs; insect on the stated adult features.
Look for these in your work
- Correct count.
- Name insect with supporting features.
Compare a spider and a beetle in leg pairs and antenna pairs.
Give me a hint
Use typical adult features.
Compare my reasoning
- Spider: four leg pairs and no antennae.
- Beetle: three leg pairs and one antenna pair.
- Both have jointed legs and external skeletons.
Spider 4/0; beetle 3/1, for leg pairs/antenna pairs.
Look for these in your work
- Both leg counts correct.
- Both antenna counts correct.
Only five legs are visible in a photograph, but the specimen has a head, thorax, abdomen and one antenna pair. Explain a careful conclusion.
Give me a hint
A leg may be hidden or missing.
Compare my reasoning
- Five visible legs need not be the complete normal count.
- The other features support an insect.
- Report the count limitation and seek a fuller view.
Insect is supported by other features; verify the incomplete visible leg count.
Look for these in your work
- Do not invent an odd-leg group.
- State the observation limit.
Common mistakes
- Count individual legs and pairs separately.
- Wing absence does not exclude an insect.
What can you explain now?
A wingless adult has six legs and one antenna pair. Is an insect still plausible?
Compare with the explanation
Yes.
Wings are not universal in insects; the leg and antenna features support insects.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Compare an insect and arachnid without relying on wings.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.