Exoskeleton and Nervous System

Exoskelton

Single layer of living cells: the epidermis

Cuticle

• Lifeless like our hair

• Can’t grow

Epidermis

Living cells

Endocuticle

Somewhat flexible

Exocuticle

Hardened layer

Epicuticle

Waterproofing

Advantages of the exoskeleton

• "Suit of armor”

• Prevents water movement

• Protection from physical damage

• Barrier to pathogens and predators

• Resists UV rays

• Structure for muscle attachment

Disadvantages of the exoskeleton

• Can’t sense much about surroundings

• Can’t grow inside armor

• Must molt

  • Dangerous

  • Exposed physically and chemically

  • Water loss

  • Takes a while

  • Can’t grow too big

Molting

• Done by immature insects

• Stages are numbered and called instars

• Done 3-20 times

• Most adults don’t do this

How to molt

• Old cuticle splits and arthropod wiggles out of it

• Hardenes and darkens

• Whole process usually requires a few hours

Saving Resources

Some arthropods eat their shed exoskeleton

When to molt

Hormone called ecdysone is secreted

How to know what stage to molt into

• Regulated by juvenile hormone

• Present when changing into nymph/larvae

• Less is present when changing into a pupa

• None is present when changing to an adult

Insect Growth Regulators

Insecticides that mimic juvenile hormone and prevent insect from turning into an adult

Olfactory Receptors

• Smell receptors

• Detect airborne molecules like odors and pheromones

• Hairs on antennae with lots of openings

Gustatory Receptors

• Taste receptors

• Hairs that detect molecules by contact with one opening

• On mouthparts and feet

Sense of Touch

Mechanoreceptor hairs bend when they touch something

Sense of Hearing

• Mechanoreceptor hairs feel vibrations

• Some insects have a tympanum

Vision

• Insects have compound eyes

• Each made up of many lenses (up to 20,000)

• Some insects have better resolution than others because they have more ommatidia

Ommatidia

Individual lenses of the compound eye

Nervous System

• Collects and transmits sensory information

• Controls responses

Anatomy of a Neuron

• Dendrites

• Axon

Dendrites

Receive messages

Axon

Sends message to next neuron

Synapse

Gap between neurons

Neurotransmitter

Molecules released from an axon that carry the message across the synapse to the next neuron’s dendrites

Acetylcholine

Tells muscles to contract

Acetylcholinesterase

Enzyme that breaks down acetylcholine so messages stop getting sent

Chemical Insecticides

• Nerve poisons

• Mimic acetylcholine

• Interferes with acetylcholinesterase

• Causes continual muscle contraction, tremors, convulsions, loss of coordination, or death

Sarin

• Military nerve gas

• Irreversibly inactivates acetylcholinesterase so there’s no “off switch” for muscles

• Death occurs due to asphyxia since can’t control the muscles involved in breathing

• Fatal at very low doses

Arrangement of Insect Nervous System

• Have a vental nerve cord

• Opposite of mammals

Ganglia

• Mini-brains found in each body segment

• Contain clusters of neuron that control muscles of their segment

Nervous System Organization in Arthropods

• Decentralized nervous system

• Brain and ganglia controlling body functions and behavior