Chapter 4: Arthropods: Insects, Spiders, and Crustaceans

Phylum Arthropoda is the most species-rich phylum on Earth, encompassing over one million described species — roughly 80% of all known animal species. Their success is built on a powerful combination of an exoskeleton, jointed appendages, segmentation, and extraordinary adaptability. Arthropods dominate every terrestrial and aquatic habitat.

Unifying Features of Arthropods

• Chitinous exoskeleton: provides protection and muscle attachment, but requires periodic shedding (molting/ecdysis)
• Jointed appendages: modified for walking, swimming, feeding, sensing, and reproduction
• Tagmatization: body segments fused into functional regions (e.g., head/thorax/abdomen in insects)
• Open circulatory system: hemolymph bathes organs directly

Ecdysis (Molting)

Because the exoskeleton cannot grow, arthropods shed it periodically. The animal secretes a new, soft cuticle, then pumps hemolymph to expand before the new shell hardens. This vulnerable period is exploited by predators.

Comparing Major Arthropod Classes

Feature	Insecta	Arachnida	Crustacea
Body regions	Head, thorax, abdomen	Cephalothorax, abdomen	Head, thorax, abdomen
Leg pairs	3	4	5+
Antennae	1 pair	None	2 pairs
Wings	Often present	Absent	Absent
Habitat	Mostly terrestrial	Mostly terrestrial	Mostly aquatic
Examples	Beetles, bees, flies	Spiders, scorpions, mites	Crabs, lobsters, shrimp

Insect Metamorphosis

Complete Metamorphosis (Holometabolism)

Egg → Larva → Pupa → Adult Examples: butterflies (Lepidoptera), beetles (Coleoptera), flies (Diptera), bees (Hymenoptera) The larva and adult occupy different ecological niches, reducing intraspecific competition.

Incomplete Metamorphosis (Hemimetabolism)

Egg → Nymph (multiple instars) → Adult Examples: grasshoppers (Orthoptera), cockroaches (Blattodea), dragonflies (Odonata) Nymphs resemble small adults and share the same habitat.

Social Insects

Some of the most complex animal societies are found in insects:

Honeybees (Apis mellifera):

• Eusociality: reproductive division of labor (queen, workers, drones)
• Waggle dance: encodes direction and distance of food sources relative to the sun
• Pheromone communication: alarm substances, queen mandibular pheromone

Ants (Formicidae):

• Supercolonies of Formica yessensis may contain 45,000 queens and 306 million workers
• Leafcutter ants (Atta) cultivate fungal gardens — the oldest known agriculture

Termites (Isoptera):

• Build mounds with sophisticated temperature regulation
• Digest cellulose via gut symbiont microorganisms

Key Checklist

• I can compare insects, arachnids, and crustaceans by body regions, leg pairs, and antennae
• I can distinguish complete (holometabolous) and incomplete (hemimetabolous) metamorphosis with examples
• I can describe the waggle dance of honeybees and explain what information it conveys