Read Arthropods by Their Specialised Systems
Arthropods share a segmented body and jointed appendages but different subgroups have distinctive respiratory, excretory and sensory structures.
Living-system questions are solved by matching a structure or organism to its function, process and stated condition.
Recognise arachnid and insect features
Scorpions and some other arachnids breathe using book lungs. In insects, prognathous mouthparts point forward, hypognathous downward and opisthognathous backward. Grasshopper is hypognathous, earwig prognathous and cicada opisthognathous.
Compare other systems
Arthropod excretory organs vary by group: insects commonly have Malpighian tubules, arachnids coxal glands and crustaceans green/antennal glands. Statocysts serve as balancing organs in some arthropods. Cockroaches are dioecious and have open circulation.
Use metamorphosis as an insect clue
Dragonflies (Odonata) are predatory insects with two functional pairs of wings and incomplete, or hemimetabolous, metamorphosis: the aquatic nymph develops directly into the adult without a pupal stage. Wasps, caddisflies and moths undergo complete, holometabolous metamorphosis and therefore do not match this combination.
Map a developmental mutation on a tree
Map a derived developmental change to the earliest node shared by every lineage showing it. The Ubx change suppresses abdominal walking legs in true insects, but springtails branch before the true-insect crown group. The mutation therefore belongs at node S, after springtails split and before fly, mosquito, butterfly, moth and beetle diversify. Choose D.
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- ✓Book lungs: arachnids such as scorpions.
- ✓Head orientation: forward/prognathous; down/hypognathous; back/opisthognathous.
- ✓Excretion varies: Malpighian, coxal or antennal glands.
- ✓Cockroach: open circulation and separate sexes.
- ✓Dragonflies: two functional wing pairs and incomplete metamorphosis.
- ✓A derived mutation maps to the last common ancestor of all taxa with the derived trait.
Do not assign one excretory organ to every arthropod class.
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