Rupa and Sachin observed an animal in their school garden. Rupa called it an insect while Sachin identified it as an earthworm. Which one of the following characteristics confirms that it is an insect ?
- (a)The animal had jointed legs.
- (b)Body of the animal had very little segmentation.
- (c)Body of the animal was cylindrical.
- (d)Body of the animal was bilaterally symmetrical.
Correct — A, The animal had jointed legs. Jointed legs are the defining feature of the arthropods, the group insects belong to; the word arthropod is built from the Greek for jointed foot, and the standard description says as much — these animals are bilaterally symmetrical and segmented, and they have jointed legs. An earthworm is an annelid and has no legs at all, jointed or otherwise, so the moment jointed legs are observed the identification is settled in Rupa's favour. The other three characteristics are the reason the disagreement was possible in the first place, because insects and earthworms share every one of them. Both are bilaterally symmetrical, both have a cylindrical body plan, and both are segmented — an earthworm conspicuously so, with its segments lined up one behind another from head to tail, and an insect divided into head, thorax and abdomen. Only the appendages tell them apart at a glance.
- (b)Body of the animal had very little segmentation. — Backwards on both counts. Insects are segmented, and reduced segmentation is a feature of the molluscs rather than a mark of insect identity.
- (c)Body of the animal was cylindrical. — Shared by both candidates and by many other invertebrates. An earthworm is about as cylindrical as an animal gets, so this observation cannot decide between them.
- (d)Body of the animal was bilaterally symmetrical. — True of insects, earthworms, and most other animals besides. Bilateral symmetry appears in every group above the coelenterates and separates nothing here.
Arthropoda is probably the largest group of animals. Its members are bilaterally symmetrical and segmented, have an open circulatory system in which the body cavity is filled with blood, and carry jointed legs; prawns, butterflies, houseflies, spiders, scorpions and crabs are all arthropods. Annelida, to which the earthworm belongs, shares bilateral symmetry and segmentation and adds a true body cavity, but its segments are alike and bear no jointed appendages. Insects form one class within the arthropods, distinguished by three body regions and three pairs of legs.
The item is a lesson in how classification actually works: a character is useful only if it is present in one group and absent in the other. Three of the options name characters shared by insects and earthworms, and a candidate who checks each against both animals rather than against insects alone will discard them quickly. Two extras are worth carrying. Not every arthropod is an insect — spiders, scorpions, ticks and mites are arachnids with eight legs, while insects have six — so jointed legs establish the phylum and the leg count narrows it to the class. And the segmentation of an earthworm is of a different kind from that of an insect: the annelid repeats near-identical rings along its length, whereas the insect groups its segments into a head, a thorax and an abdomen with different jobs.
- Arthropods have jointed legs, and the name itself means jointed legs; they are bilaterally symmetrical and segmented, with an open circulatory system.
- Prawns, butterflies, houseflies, spiders, scorpions and crabs are all arthropods, and Arthropoda is probably the largest animal group.
- Annelids such as the earthworm are bilaterally symmetrical, triploblastic and segmented, with the segments lined up from head to tail, but have no jointed appendages.
- Insects have three body regions — head, thorax and abdomen — and three pairs of legs; arachnids such as spiders and ticks have four pairs.
- Bilateral symmetry and a cylindrical body are shared by insects and earthworms, so neither can separate them.
- Choosing a character that both candidate animals share, such as bilateral symmetry or a cylindrical body.
- Treating every jointed-legged animal as an insect; spiders and ticks are arachnids.
- Assuming that segmentation rules out an insect, when insects are segmented too.
As an identify-the-decisive-character item like this one, as a phylum-to-feature match, or as a which-animal-does-not-belong question within a group.
Ticks and mites are actually
- (a) Arachnids
- (b) Crustaceans
- (c) Insects
- (d) Myriapods
Answer(a) Arachnids
The next distinction after this one. Jointed legs place an animal among the arthropods, but the number of pairs decides the class — and ticks and mites, with four pairs, are arachnids rather than insects.
Ecologists have so far found and certified a large number of species in the plant and animal kingdoms. In terms of members, the largest group identified so far is among the
- (a) Fungi
- (b) Plants
- (c) Insects
- (d) Bacteria
Answer(c) Insects
The scale of the group this question is identifying. Arthropods are described as probably the largest group of animals, and insects supply most of that total — over a million described species.
- practice — not a real PYQ
Which one of the following features is found in arthropods but not in annelids?
- (a)Bilateral symmetry
- (b)Segmentation
- (c)Jointed appendages
- (d)A true body cavity
Answer(c) Jointed appendages — the other three are present in both groups.
- practice — not a real PYQ
Spiders and scorpions differ from insects mainly in having
- (a)no segmentation
- (b)four pairs of legs instead of three
- (c)a closed circulatory system
- (d)no jointed legs
Answer(b) four pairs of legs instead of three — both are arthropods, but spiders and scorpions are arachnids.