CARA'S TIPSPractical 2: insects, mosquito life stages, tapeworm, Ascaris and earthworm
WAECNECO
What the syllabus asks
WAEC: invertebrate specimens (Arthropoda: cockroach, grasshopper, housefly, mosquito life stages; Platyhelminthes: tapeworm; Nematoda: Ascaris; Annelida: earthworm): identification, classification, observable features, adaptations, life cycles, economic importance, keys, drawing.
NECO: invertebrate specimens (Arthropoda: cockroach, grasshopper, housefly, mosquito life stages; Platyhelminthes: tapeworm; Nematoda: Ascaris; Annelida: earthworm): identification, classification, observable features, adaptations, life cycles, economic importance, keys, drawing.
What you will learn
- Identify and classify common invertebrate specimens.
- State observable features and relate them to adaptations.
- Distinguish Anopheles, Culex and Aedes at every life stage.
- Compare mouthparts, legs and wings of insects.
- Distinguish tapeworm, Ascaris and earthworm.
- Construct a dichotomous key from observable features.
- Draw invertebrate specimens correctly with magnification.
Key points
- Insects: 3 body parts, 3 pairs of legs on thorax, 1 pair of antennae.
- Cockroach (tegmina, cerci) and grasshopper (jumping legs, tympanum): incomplete metamorphosis.
- Housefly and mosquito: one pair of wings and halteres; complete metamorphosis.
- Anopheles: floats, no siphon, angled resting adult, long female palps; malaria vector.
- Tapeworm (flat, scolex, proglottids), Ascaris (round, unsegmented nematode), earthworm (segments, clitellum).
- Keys use observable contrasting features.
- Draw legs on the thorax; label; title with view; magnification.
Start here: Invertebrate specimens: the arthropod body plan
Invertebrates appear in almost every biology practical paper, either as preserved specimens in bottles, as dried pinned insects, or as fresh animals collected by the school. The commonest are the cockroach, grasshopper, housefly and mosquito (insects), the earthworm (an annelid), and the parasitic worms tapeworm and Ascaris. Snails, millipedes, spiders and crabs appear less often.
Arthropods first
Phylum Arthropoda (“jointed feet”) is the largest animal phylum. Every arthropod has:
- a segmented body covered by a hard exoskeleton of chitin, shed during growth (moulting or ecdysis);
- jointed appendages (legs, antennae, mouthparts);
- bilateral symmetry and an open blood system (not observable, but often asked in theory).
Insects: Class Insecta
On the specimen you can see the three features that define an insect:
- The body is divided into head, thorax and abdomen.
- There are three pairs of jointed legs, all on the thorax.
- There is one pair of antennae on the head, and usually one or two pairs of wings on the thorax.
Spiders (Arachnida) have four pairs of legs, two body parts (cephalothorax and abdomen) and no antennae; millipedes (Diplopoda) have two pairs of legs on most segments. These look-alikes are favourite “give one reason why specimen X is not an insect” questions.
| Feature | Insect | Spider | Millipede |
|---|---|---|---|
| Body parts | head, thorax, abdomen | cephalothorax, abdomen | head + many similar segments |
| Legs | 3 pairs | 4 pairs | 2 pairs per segment |
| Antennae | 1 pair | none | 1 pair (short) |
| Wings | usually present | none | none |
Metamorphosis
Insects change form as they grow. Incomplete metamorphosis (egg → nymph → adult) occurs in the cockroach and grasshopper: the nymph looks like a small wingless adult and moults several times. Complete metamorphosis (egg → larva → pupa → adult) occurs in the housefly, mosquito, butterfly and beetle: the larva looks completely different from the adult and changes inside a pupa. A table asking you to tick “complete” or “incomplete” for each specimen is common.
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Practice questions
- thorax
- head
- abdomen
- antennae
Show answer
A: thorax
Insects have three pairs of jointed legs, all on the thorax.
- cephalothorax and abdomen
- head and trunk
- segments only
- head, thorax and abdomen
Show answer
D: head, thorax and abdomen
Three regions.
- piercing mouthparts
- siphoning mouthparts
- biting and chewing mouthparts
- sponging mouthparts
Show answer
C: biting and chewing mouthparts
Omnivorous scavenger.
- no mouthparts
- sponging mouthparts
- biting mouthparts
- piercing mouthparts
Show answer
B: sponging mouthparts
Labella mop liquids.
- one
- two
- three
- none
Show answer
A: one
Hind wings reduced to halteres.
Key words
- Exoskeleton
- Hard outer covering of chitin in arthropods.
- Metamorphosis
- Change of form during development (complete or incomplete).
- Nymph
- Young stage of an insect with incomplete metamorphosis, resembling the adult.
- Tegmina
- Leathery forewings of cockroaches and grasshoppers.
- Halteres
- Knobbed balancers that replace the hindwings in flies.
- Labella
- Spongy pads at the tip of the housefly proboscis.
- Scolex
- Head of a tapeworm, bearing suckers and sometimes hooks.
- Proglottid
- One segment-like unit of a tapeworm body.
- Clitellum
- Glandular saddle of the earthworm that secretes the cocoon.
- Setae
- Bristles on earthworm segments, used for grip.
- Vector
- Organism that carries a pathogen from one host to another.
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