Arthropods- Insects

Major Characteristics of Arthropods

• Terrestrial, freshwater, and even some marine

• Abundant

Reasons for success

• Evolution of segmented and compartmentalized bodies

• Coevolution with plants (350 million years)

• Miniaturization

• Invention of flight

• Broad feeding strategies

Good Ecological Roles

• Important role in food webs

• Flower pollination: Colony collapse disorder

Bad Ecological Roles

• Consume 1/3 of potential annual harvest

• Vectors of human disease

• Termites: $1.5 billion in annual damages in US

• Fire ants: Nearly $1 billion annually in California alone

Morphology Insects

• Body composed of head, thorax, and abdomen

• Uniramous legs

• Gas exchange by spiracles and trachea

• Malpighian tubules

Feeding Adaptations

• Every possible strategy

  • Herbivory

  • Carnivory

  • Scavenging (scatophagous)

  • Commensalism

  • Parasitism

  • Etc.

• Mouth modified to fit feeding style

Gills

What can’t be exposed to air or evaporation would be too high?

Larger insects

Rely in tracheal system

Tracheal system

• Completely independent of circulatory system

• Extensive tubular invaginations of body wall

• Open externally via spiracles on thorax and abdomen

Grasshoppers

• have unidirectional flow of air

• Inspire through spiracles in thorax

• Expire through spiracles in abdomen

Gas exchange

• Exchange in trachea occurs by diffusion down pressure gradients

  • Very efficient

    • Diffusion through air is 300,000X more efficient than through water and 1,000,000X more efficient than through tissues!

• Air passes into progressively finer-branched tracheoles (similar to tree branch)

• Tracheole ends embeded in muscle tissues and ends are fluid filled

• Final gas exchange by diffusion through fluid

Mechanism of gas exchange:

• metabolic byproducts build up in cell

•  fluids from tracheoles enter tissues by osmosis, creating low pressure in tubes

•  gases (O2 and CO2) now flow down pressure gradient

Malpighian tubules (Excretion)

• Blind end tubes that extend from gut into hemocoel

• Actively pump K+ and other ions into tubules to create osmotic pressure

  • Causes water to move out of hemocoel into tubules via osmosis

  • Water then passed to gut

  • Water and ions then filter back out of gut into body cavity, leaving behind precipitated potassium urate crystals that are packaged into feces

Nervous System

• Similar to crustaceans

  • Ventral nerve cords largely fused

  • Fusion of ganglia

    • Primitive condition is less fusion – one pair of ganglia per segment

      • Ganglia coordinate function of body segment they represent

    • Brain is fusion of 3 pairs of ganglia

    • In most advanced insects, all ganglia are fused into single body-ganglion (e.g., house fly)

Image forming vision

• Number of ommatidia determines quality of vision

  • Greatest number (10,000/eye) in predatory species or those that are nocturnal

  • Fewest number in colonial workers or other species that primarily use chemical communication (1/eye; worker ants)

No optic nerve

• Each ommatidium of eyes connects directly to optic lobe of brain

  • Different than in crustaceans where ommatidia all integrated with an optic nerve.

Hearing

• Species-specific form of communication (mating)

• Phonoreceptors

Tympanic organs

• form from fusion of parts of tracheal system and body wall

  • Responds to vibrations similar to cochlea of human ear

  • Important especially in species that make sound (crickets, cicadas)

Johnston’s organ

movement of hair on antenna

Auditory hairs

occur on larvae; sensitive to particle velocity

Pilifer

only in certain moths and allows to hear frequency used by echolocating bats.

Sensilla

• sensory cilia or bristles

  • Concentrated around antennae, mouth, and legs

  • Tactile mostly, but also chemosensory

Proprioceptors

Stretch across joints to monitor movement and body position

Reproductive strategies

• Dioecious

• Internal fertilization

• Mostly oviparous (eggs hatch outside body)

• Primitive state is direct development

  • More advanced have indirect development

• Some have haplodipoidy

  • Some ants, bees, wasps, termites

  • Female are diploid and result from sexual reproduction

  • Males are haploid and result from parthenogenesis

Hemimetabolous

Incomplete development

Holometabolous

Complete Development

Leaf Cutter Ants

• Dimorphism

  • Huge soldier ants protect colony

  • Large leaf cutter ants cut and carry leaves

  • Small guard ants (‘minims’) ride on leaves

• Phorid flies are parasitoids on ants

Burying Beetle

They find a carcass of a mouse. The female will lay her eggs around the dirt. She slows down the deterioration of the carcass and sees how big the carcass is. The larvae grow and goes inside the carcass. If she laid too much eggs, she will cannibalize the eggs to regain energy.