Q6.
An earthworm (annelida) and a beetle (arthropoda), both have segmented bodies but the beetle has a hard external skeleton. How does the beetle's external skeleton help it survive?
Answer
The exoskeleton is what lets the beetle live out in the open, dry world that the earthworm cannot enter. Both animals are segmented, so segmentation is not the difference — the rigid, waxy external covering is.
| Earthworm (Annelida) | Beetle (Arthropoda) | |
|---|---|---|
| Body | Segmented, soft, cylindrical | Segmented, with segments specialised into head, thorax, abdomen |
| Skeleton | None — a fluid-filled body cavity acts as a hydrostatic skeleton | Hard exoskeleton |
| Water loss | High — skin must stay moist; it also breathes through the skin | Low — waxy covering seals the body |
| Where it can live | Only in moist soil; dies in the open sun | Dry and exposed environments, including deserts |
| Appendages | None — moves by muscle waves and bristles | Jointed legs and wings |
The three things it does, and why each matters:
- Protection. The beetle's hardened forewings and body plates are armour against predators and against being crushed among stones and bark.
- It reduces water loss. This is the crucial one. The earthworm's skin must stay wet because it exchanges gases through it, so the worm is confined to damp soil and comes out only at night or after rain. The beetle's waxy covering stops evaporation, and it breathes instead through small openings that it can close. That is why beetles are found in the driest places on the Earth.
- It supports powerful muscles. Muscles pull on the inside of the rigid case, so the beetle gets strong, precise leverage — for walking, digging, gripping and flying. The earthworm can only push against fluid.
Tip for the exam: the question asks how it helps the beetle survive. Answer with the three functions and then name the consequence — arthropods are the most numerous and widespread animal group precisely because the exoskeleton solved drying out, the biggest single obstacle to life on land.