IGCSE Biology 0610: Classifying Invertebrates into Arthropod Groups

An entomologist collects four invertebrates from a garden and records the following features of each.

Specimen 1: body divided into a head, thorax and abdomen; three pairs of jointed legs, all attached to the thorax; one pair of antennae; two pairs of wings.

Specimen 2: body divided into only two regions, a cephalothorax and an abdomen; four pairs of jointed legs; no antennae.

Specimen 3: hard exoskeleton; five pairs of jointed legs; two pairs of antennae; found under a stone at the edge of a garden pond.

Specimen 4: long, thin body made up of many similar segments; most segments bear one pair of jointed legs; one pair of antennae.

(a) State the feature that all four specimens share which shows that each belongs to the arthropod group of invertebrates. [1]

(b) State the group, within the arthropod group, to which specimen 1 belongs. [1]

(d) State the group, within the arthropod group, to which specimen 3 belongs. [1]

(e) State the group, within the arthropod group, to which specimen 4 belongs. [1]

Part (a): The shared arthropod feature

All four specimens have jointed legs attached to a body covered by a hard exoskeleton, and a body made of segments. These are the features that place an invertebrate in the arthropod group, whatever its exact body plan.

Part (b): Specimen 1

Specimen 1 has a body divided into three regions, a head, thorax and abdomen, with three pairs of jointed legs attached to the thorax, one pair of antennae, and two pairs of wings. A three-part body, three pairs of legs and one pair of antennae are the features of an insect.

Part (c): Specimen 2

Specimen 2 has a body divided into only two regions, a cephalothorax and an abdomen, with four pairs of jointed legs and no antennae. A two-part body, four pairs of legs and the absence of antennae are the features of an arachnid.

Part (d): Specimen 3

Specimen 3 has a hard exoskeleton, five pairs of jointed legs and, importantly, two pairs of antennae, and it was found near a pond. Two pairs of antennae, together with several pairs of legs and a typically aquatic or damp habitat, are the features of a crustacean.

Part (e): Specimen 4

Specimen 4 has a long, thin body made of many similar segments, with most segments bearing one pair of jointed legs, and one pair of antennae. A long, segmented body with one (or two) pairs of legs repeated along most segments is the feature of a myriapod.

Final answers

(a) All four specimens have jointed legs and a hard exoskeleton.
(b) Specimen 1: insect
(c) Specimen 2: arachnid
(d) Specimen 3: crustacean
(e) Specimen 4: myriapod