Arthropoda

5 major lines of arthropod evolution:1. Trilobitamorpha2. Cheliceriformes

a. Merostomatab. Arachnida

3. Crustacea4. Hexapoda

a. Entognathab. Insecta

5. Myriapoda

Sub phylum Hexapoda

• Diverse group (~1000 families)– Twice as diverse as all other animal taxa

combined

• Co-evolved with angiosperms– Flower morphology / pollinators– Herbivory / 2°plant compounds/resistance and

specialization– Parasitoidism (20% of insects are parasitoids)

Hexapoda

• Mostly terrestrial

• In every habitat except the subtidal marine habitat – competition from crustacea?

• 3 tagma: head, thorax, abdomen– 3 pairs of legs, and usually 2 pairs of wings on

thorax– no abdominal appendages

Major contributors to insect success Arthropod body plan• Small size• Resistance to desiccation

– Trachea– Waterproof cuticle, egg shell– Malphigian tubules

• Flight– Rapid dispersal– Escape from predators– Access to distant food, mates

• Holometaboly: – Larvae and adults occupy separate niches (most speciose

groups are all holometabolous: coleoptera, lepidoptera, hymenoptera, diptera)

Holometabula

Hemimetabolous developmente.g. Odonata, Orthoptera, BlattodeaMantodea, Hemipteroids-Incomplete metamorphosis-Wing pads present in pre-adult-Larvae often resemble small adults

Holometabolous developmente.g. coleoptera, lepidoptera, diptera, hymenoptera-Complete metamorphosis-Inactive pupal stage-Larvae do not

-resemble parents-eat the same things as parents-live in the same habitat as parents

Big picture

• Hugely important ecologically as pollinators (2/3 of all flowering plants), detritivores, herbivores

• Important economically and socially (to humans) as disease vectors (fleas, lice, bedbugs, biting flies), crop pests (and also as pollinators and biological control agents)

Hexapoda

2 classes:

1. Entognatha (mouth parts recessed),

2. Insecta (mouthparts are ectognathous = exposed and projecting from head capsule)

• From aquatic crustacean ancestor – Probably freshwater

• Winglessness (apterygota) is primitive condition

Hexapod origins

“Pterygota”

• Flight evolved 300-400 mya

• Wings = outpocketings of exoskeleton

• Origins of wings? Multiple hypotheses:– Used for thermoregulatory purposes then later

co-opted for flight?– Used to stabilize body during jumping?– Modified from external gills? (genetic

similarities with crustacean gill structures)

Requirements for flight

• Musculature– Striated– Attached to strong exoskeleton with flexible joints

• Small body size• Impervious to water loss• Efficient internal physiology for gas exchange

(trachea), nutrient storage and distribution• Well-developed and integrated sensory

organs to regulate rapid movement

• Low rate of wing-flapping – Odonata, ephemeroptera, orthoptera,

lepidoptera– Flapping rate limited by firing rate of neurons

• High rate of wing-flapping – Diptera, hymenoptera, coleoptera– Use elastico-mechanical properties of

exoskeleton to stimulate stretch receptors to produce self-sustaining flight