⚡ CHEAT SHEET

Ch.5: Insects & Their Relatives

What the exam tests on arthropod classes, insect anatomy, classification ranking, and the three types of metamorphosis.

🎯 Top 5 Traps

What ALL arthropods share = APPENDAGES IN PAIRS — not 6 legs. Spiders have 8 legs; centipedes have many; pillbugs have many. The shared arthropod traits are: segmented body, paired appendages, and exoskeleton.

Legs and wings attach to the THORAX — not the head, not the abdomen. The insect body has THREE regions: HEAD (antennae), THORAX (3 pairs of legs + usually wings), ABDOMEN (digestive + reproductive systems). Easy to confuse "thorax" with general body or to misplace wings on the head.

Classification ranking: CLASS → ORDER → FAMILY → GENUS → SPECIES. Common error: putting genus and species in the wrong order, or putting order before class. Genus comes BEFORE species (genus is the broader category — multiple species per genus).

WINGED insects are ALWAYS ADULTS. Nymphs are WINGLESS — even in gradual metamorphosis where adults DO have wings. The chapter is explicit: "Winged insects are always adults." If you see wings, it's an adult. Cockroach nymph: no wings. Cockroach adult: wings.

Metamorphosis classifications by common pest: Cockroach = GRADUAL. Ant = COMPLETE. Silverfish = SIMPLE. These three common structural pests illustrate all three metamorphosis types — a frequent matching test. Memorize: silverfish (simple), cockroach (gradual), ant (complete).

🔢 Key Facts & Structure Counts

Term / Count

Definition / Description

Arthropoda

The largest phylum in the animal kingdom. Includes spiders, mites, ticks, millipedes, centipedes, crabs, shrimp, AND insects. All arthropods share three traits: segmented body, paired appendages, exoskeleton.

Exoskeleton

Hard or tough external covering. Holds the body together and gives it shape — the arthropod equivalent of a mammal's bony internal skeleton. Cannot grow continuously — see Molting.

Molting

Arthropods grow in stages by forming a new soft exoskeleton under the old one and shedding the old. New exoskeleton is white at first, hardens and darkens in a few hours.

Metamorphosis

"Change in form." How the insect class is divided into developmental groups. Three types: simple, gradual, complete.

4 arthropod classes

Arachnida (spiders, mites, scorpions, daddy longlegs); Crustacea (crabs, lobsters, shrimp, pillbugs, sowbugs); Myriapoda (millipedes + centipedes); Insecta (insects)

3 insect body regions

HEAD (1 pair of antennae), THORAX (3 pairs of legs + usually wings), ABDOMEN (digestive system + reproductive organs)

3 metamorphosis types

Simple (no drastic change — silverfish); Gradual (nymphs lack wings — 14 orders incl. cockroaches); Complete (4-stage egg-larva-pupa-adult — 9 orders containing the majority of insect species)

5 classification ranks

CLASS → ORDER → FAMILY → GENUS → SPECIES (broader to narrower). Scientific names = Genus species (capitalized + lowercase, italicized — e.g., Musca domestica)

4 stages of complete metamorphosis

Egg → Larva (grubs/maggots/caterpillars — feed and grow) → Pupa (inactive; body rearrangement) → Adult (reproduction)

14 orders / 9 orders

Gradual metamorphosis = 14 orders (incl. cockroaches, crickets, grasshoppers, box elder bugs, earwigs). Complete metamorphosis = 9 orders containing MORE species than all other animals combined.

🔀 Easily Confused

Pair / Group

Distinguishing Feature

Arachnida vs Insecta

Arachnida: 4 pairs of legs (8); 2 body regions (mouthparts/legs + reproduction/digestion); piercing mouthparts. Insecta: 3 pairs of legs (6); 3 body regions (head/thorax/abdomen); 1 pair of antennae; usually wings.

Millipedes vs Centipedes (both Myriapoda)

Millipede: cylindrical body; 2 pairs of legs PER segment; short antennae. Centipede: more flattened; 1 pair of legs PER segment; long antennae and hind legs (house centipede has very long legs).

Crustacea — what counts

Includes aquatic crabs, lobsters, shrimp, AND land-dwelling pillbugs and sowbugs. Pillbugs/sowbugs are NOT insects — they are crustaceans.

Simple vs Gradual vs Complete metamorphosis

Simple: no drastic change; hatch and grow by molting; FEW orders (silverfish). Gradual: nymphs partially resemble adults; nymphs WINGLESS, adults WINGED; 14 orders. Complete: 4-stage egg-larva-pupa-adult; nine orders but MOST insect species.

Gradual metamorphosis pest examples

Cockroaches, crickets, grasshoppers, box elder bugs, earwigs. Nymphs and adults often found together; usually eat the same food.

Complete metamorphosis pest examples

Beetles, moths and butterflies, flies, fleas, AND stinging insects (ants, bees, wasps). Larva and adult often live in DIFFERENT habitats and eat DIFFERENT food — this drives stage-specific control.

Genus vs Species naming

Scientific name = TWO words: GENUS first (capitalized), SPECIES second (lowercase). Both italicized or underlined. Example: Musca domestica (housefly). Genus is the broader grouping; species is the specific kind.

🕷️ Four Arthropod Classes Compared

Class

Body Regions / Legs

Examples & Key Feature

Arachnida

2 body regions; 4 pairs of legs (8)

Spiders, mites, scorpions, daddy longlegs. Mouthparts with two prominent piercing tips. NO antennae.

Crustacea

Variable; mostly aquatic but includes terrestrial pillbugs/sowbugs

Crabs, lobsters, shrimp (aquatic); pillbugs, sowbugs (land).

Myriapoda

Many segments; legs per segment varies

Millipedes (2 pairs legs/segment, cylindrical, short antennae); Centipedes (1 pair legs/segment, flattened, long antennae and hind legs).

Insecta

3 body regions (head/thorax/abdomen); 3 pairs of legs (6) + usually wings

All insects. Head bears 1 pair antennae; thorax bears legs + wings; abdomen contains digestive + reproductive organs. The most species-rich class.

🦋 Three Types of Metamorphosis

Type

Life Stages / Key Feature

Pest Examples / Mgmt Implication

Simple

Egg → Immature → Adult. NO drastic change. Hatch and grow by molting. Few orders.

Silverfish. Same habitat throughout life — single management approach.

Gradual

Egg → Nymph (wingless) → Adult (winged). Nymphs partially resemble adults but lack wings. 14 orders.

Cockroaches, crickets, grasshoppers, box elder bugs, earwigs. Nymphs + adults found together; usually same food. Single management approach often works.

Complete

Egg → Larva (feeds, grows) → Pupa (inactive; body rearrangement) → Adult (reproduces). 4 stages. 9 orders but MOST insect species.

Beetles, moths/butterflies, flies, fleas, ants, bees, wasps. Larva + adult often DIFFERENT habitats and food — STAGE-SPECIFIC management required (different traps/baits/treatments per stage).

💡 Memory Hooks

Arthropod traits: "Segments, pairs, exoskeleton." All arthropods share these three. NOT "six legs" — that's insect-specific.

Body regions: "Head antennae, thorax legs+wings, abdomen guts+gonads." Three regions, three jobs.

Classification: "Class, Order, Family, Genus, Species — Cool Old Frogs Get Slimy." Broader to narrower.

Wings = adult: "If it has wings, it's an adult." Even in gradual metamorphosis. Wingless = nymph or larva.

Three pests, three types: "Silverfish simple, cockroach gradual, ant complete." A clean case of each type from common structural pests.

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