Exam II Rescheduled

• Lecture Exam II will be given on Thursday, April 27, instead of Thursday, April 20.
• Format (types and approximate number of questions) will be the same as in Lecture Exam I.
• I will be available for questions or discussion in the sunny part of the Student Union (outside McDonald’s) from 11:00-1:00 on Thursday, April 20.

Uniramia -
The Conquest of Land and Air

• A Quick Review:
• Arthropod Bauplan
• Arthropod relationships
• Mandibles
• Anterior segments

Outline:

• Challenges and general solutions to life on land
• Gravity
• Locomotion
• Respiration
• Water balance
• Sensory systems
• Reproduction
• Uniramian synapomorphies
• Diversity in the subphylum Uniramia
• Myriapoda (Class Chilopoda, Class Diplopoda)
• Class Insecta
  • Synapomorphies of the Insecta
  • Fossil history
  • Relationships and evolutionary innovations
  • "Apterygota" -- paraphyletic wingless insect groups
  • Pterygota -- monophyletic winged insect groups
    • 2 types of Pterygota (Paleoptera, Neoptera)
    • 3 theories for the origin of wings
    • How do wings work?
  • Factors involved in insect success
  • Small body size
  • Short generation time
  • Complex behavior
  • Mouthparts
  • Powered flight
  • Complex life cycles with complete metamorphosis

A Quick Review:
The Arthropod Bauplan

• General characteristics
• Segmentation/Metamerism/Tagmosis
• Open circulatory system
• Well developed
  • Appendages
  • Respiratory systems (trachea or gills)
  • Sensory organs
• Metamorphosis common

Arthropod relationships

A closer look at mandibles

• Shared by Crustacea and Uniramia
• From third segment
• Differ in details
• Crustacea - "Gnathobasic"
• Uniramia - "Whole arm"
• Jaw a single element
• Chew with end rather than base

Anterior segments (head = 1-5)

See handout

Uniramia
The conquest of land and air

Uniramia - "crustaceans of the land" (we sometimes call crustaceans "insects of the sea")

• And then some ...
• In virtually all terrestrial habitats
• Underground
• Surfaces
• Boring
• Flying
• Secondarily aquatic
• Etc., etc.

Challenges and general solutions to life on land - Gravity

• Challenges
• Body not supported by medium
  • Energetic cost of supporting body in a relatively light medium
• Large body is size difficult to attain
• Develop area vs. mass
• General solutions
• Strong, light exoskeleton
  • Tubular design resists buckling
• Exoskeleton a composite (epicuticle - thin, flexible; exocuticle - scleratized; endocuticle - laminated)
  • Layering makes it both strong and flexible
• Tagmosis and reduction of segments
  • Extra stiffening (to withstand muscle pull)
• Problems of exoskeleton
• Requires molting in order to grow
  • Animal left vulnerable to predators, gravity, etc.
  • Physiologically costly, significant mortality
• Limits maximum size
  • Exokeleton gets too heavy and molting gets too costly

Challenges and general solutions to life on land - Locomotion

• Challenges
• Need for high speed
• Need for flexible pattern of limb movement
• Need for better balance
• Need for support
• General solutions
• Reduction in limb number
  • Reduces problem of foot placement
  • Reduces recovery time
  • Allows longer strides
  • Not found in all terrestrial arthropods, as we'll see
• Larger brain
  • More complex control of locomotion

Challenges and general solutions to life on land - Respiration

• Challenges
• Air high in O2 and it's easy to move in air (little resistance), but ...
• Gills and similar structures tend to collapse due to surface tension
• Water loss at respiratory surfaces
• General solutions
• Oxygen exchange occurs in semi-enclosed chambers:
  • Book lungs in arachnids
  • Air enter spiracles (small opening reduces water loss)
  • Gases exchanged across elaborate, layered book lung
  • Tracheae
  • Air enters spiracles
  • Tracheae ramify throughout body
  • Diffusion or sometimes ventilation (pumping)
  • Convergent structures in Uniramia, some Arachnida, some Crustacea (isopods)

Challenges and general solutions to life on land - Water balance

• Challenges
• Evaporative water loss is a big problem in air, especially at high temperatures (air also has greater temperature variation than water)
• Water potentially lost via:
  • Surface
  • Respiratory organs
  • Defecation
  • Excretion
• General solutions
• Stay in humid environments (behavioral regulation)
• Waxy epicuticle (insects)
• Improved excretory systems
  • Malpighian tubules: secrete uric acid (water removed as uric acid molecule formed; uric acid associated with least amount of H2O lost of any nitrogenous wastes--NH3, urea, uric acid)

Challenges and general solutions to life on land - Sensory systems

• Challenges
• Sensory medium very different
  • Light/odors more easily transmitted in air than water
  • Sound less easily transmitted in air than in water
• General solutions
• Increased emphasis on vision on land

Challenges and general solutions to life on land - Reproduction

• Challenges
• No planktonic environment on land
• Outcrossing
  • Harder to disperse gametes
  • Harder to disperse young
  • Potentially harder to find mates
• General solutions
• Loss of planktonic larvae
• Direct sperm transfer
  • Arachnids - pedipalps used to transfer sperm
  • Uniramia - various structures facilitate sperm transfer
  • Requires copulation, often sophistocated courtship and mating behaviors -- sets up possibility of sexual selection (many crustaceans also have copulation and complex social behaviors, but some marine invertebrates broadcast eggs and sperm into the sea water without mating)
• General solutions
• Loss of planktonic Larvae
• Direct sperm transfer
  • Arachnids - pedipalps
  • Uniramia - various structures
  • Requires copulation, enhances possibility of sexual selection
• Flight - facilitates finding mates, complicated courtships, enhances possibility of sexual selection

Diversity of the Phylum Uniramia

• Millions of species (some experts say 30 million, concensus closer to 10 million known plus unkown species, about 1 million described species)
• Two major groups
• Myriapoda
• Insecta
• Appendages uniramous
• The vast majority of uniramians are insects

Uniramian Synapomorphies

• Whole limb jaws
• Tracheal gas exchange
• Convergent* with those in arachnids and isopods
• Malpighian tubules
• Convergent* with those in arachnids
• Loss of second appendage
• Antennae in crustaceans
• Legs or pedipalps in trilobites and chelicerates
  
• *independently evolved in terrestrial forms

Anterior segments (head = 1-5)

(see handout)

Myriapoda (Chilopoda, Diplopoda)

• Characteristics
• Two tagmata - Head, trunk
• Appendages on most trunk segments
• No waxy cuticle
• Synapomorphies
• Repugnatorial glands
• Have ocelli, lost compound eyes (or never had them?)

Class Chilopoda

• Centipedes - 2,500 species
• First maxilliped modified as venomous fang
• Active predators
• Can be fast!
• Retain large number of body segments

Class Diplopoda

• Millipedes - 10,000 species
• Two pairs of legs on each segment (probably results from fused segments)
• Very large number of legs
• Record - 752!
• Numerous, ventrally placed legs good for burrowing
• Gnathochilarium
• Retain large number of body segments

Class Insecta

• Incredibly diverse
• At least 10,000,000 known and unknown species! -- more than any other group of animals on Earth
• Primitively, numerous segments with legs
• But more advanced, specialized groups have reduced segments and legs
• Synapomorphies
• Three tagmata
  • Head (5 segments)
  • Thorax (3 segments)
  • Abdomen (11 segments)
• Loss of abdomenal appendages
  • Present in some embryos
• Waxy cuticle
• Fossil history
• Appear about 400 mya
• Late Paleozoic radiation
• Two important points:
• Many groups have become extinct or reduced in prominence
• Large proportion of modern insect families are found in a relatively few, extraordinarily successful orders

  • Coleoptera (beetles)
  • Diptera (flies)
  • Leptidoptera (moths and butterflies)
  • Hymenoptera (ants, wasps, bees)

Class Insecta - Success

• Why have insects been so successful on land?
• More specifically,
• Why were insects so successful in initially invading land?
• Why have some insect groups been so successful compared to other insects (and animals in general)?
• In addition to their small body size, we'll focus on two key innovations that allowed them to successfully invade and radiate on land:
• 1) The origin and evolution of wings
• 2) New patterns of development
• First, we'll look at the relationships of the major groups of insects in order to understand their evolutionary innovations

Class Insecta - Relationships

• Cladogram: 2 sister groups:
• "Apterygota"
  • Primitively wingless
  • Direct development
• Pterygota
  • Within the winged insects, 2 sister groups:
  • Paleoptera (primitive wing form)
  • Neoptera (advanced wing form)
    • Within the Neoptera, 2 sister groups:
      *Hemimetabolous development
      *Holometabolous development

Class Insecta - "Apterygota"

• Paraphyletic
• Collembola - springtails
• Thysanura - silverfish
• No wings
• Primitively absent
• Direct development
• Young appear like small adults (no larva or pupa, no metamorphosis)

Class Insecta - Pterygota

• Monophyletic
• Single origin of wings
• The vast majority of insects are pterygotes
• 2 types of pterygotes
• Paleoptera
• Neoptera
• 2 types of development
• Hemimetabolous
• Holometabolous

Two types of Pterygotes

• Paleoptera
• Wings do not fold, are held out or upward
• E.g., dragonflies, mayflies
• Neoptera
• Wings fold flat over back
• Includes most insects
• E.g., houseflies, beetles, grasshoppers
• In Siphonaptera (lice, fleas -- related to flies) - wings are secondarily lost

Two types of development

• Hemimetabolous development
  • Primitive
  • But retained in many modern pterygote orders
  • Fertilized egg develops into a larva that undergoes partial metamorphosis, but larva hatches as a juvenile, and passes through a number of molts to gradually assume adult form
• Holometabolous development
• Derived - occurs in the 5 terminal, specialized lineages characterized by extraordinary success
• Fertilized egg develops into a larva that has a completely different morphology and ecology from the adult (larva specialized for feeding and growth)
• Larva develops into a pupa (chrysalis) - quiescent stage in which body is completely reorganized = complete metamorphosis

Class Insecta - Pterygota

• 1) Hemimetabolous development
• 2) Holometabolous development
• In derived, specialized, successful lineages
• Larva specialized for feeding and growth
• Pupa undergoes complete metamorphosis
• Adult emerges in a completely new, final form and never molts again; ecology is completely different from larva. Adult often specialized for reproduction: for mobility and dispersal, for finding mates and places to reproduce, may even be unable to feed.

Evolution of wings

• How did wings evolve?
• What good is half a wing (i.e., how did wings start to evolve, how could a tiny flap be an advantage)?
• Three general theories of the origin of wings
• 1) Paranotal origin
• 2) Branchial origin
• 3) Heat collector origin
• All attempt to present uses for incipient structures

Evolution of wings - Paranotal origin

• Wings originally developed from lateral extensions of thorax to stabilize landing from a jump
• Lateral extensions may have become longer to allow gliding during a jump from one plant to another, then powered flying was eventually developed
• Pro:
• Lateral extensions from various thoracic segments (e.g., paranotal lobe) are seen in some fossil insects
• Con:
• Would need to evolve:
  • Hinge mechanism
  • Muscles for control

Evolution of wings - Branchial origin

• Wings arose from thoracic gills that evolved for swimming, aeration
• Paddle-like structures of emergent insects may facilitate gliding (or stabilizing jumps) from plant to plant, then powered flying eventually developed
• Pros:
• Various fossil and contemporary aquatic insects are known to have flap-like gills for swimming or aeration
• Muscles and articulations are present
• Cons:
• Tough to imagine flight evolving in water (although some nymphs were terrestrial or semi-terrestrial, as in modern dragonflies)
• Many known gills arise from abdomen rather than thorax

Evolution of wings - Heat collectors

• Extensions of the body originally evolved to collect heat from the environment
• Lateral extensions may have become longer to allow gliding during a jump from one plant to another, then flying was eventually developed
• Pros:
• Even incipient wings would have collected heat when surface was exposed to sun
• Lateral extensions of the thorax are known in fossil insects
• Cons:
• Would need to evolve:
  • Hinge mechanism
  • Muscles for control

How are wings moved?

• Muscles work antagonistically (flexor, extensor)
• Direct vs. indirect operation of wings
• Direct: contraction of muscles pulls wing up directly or down (power stroke) directly
• Indirect: Muscle pulls a secondary structure (e.g., dorsal surface of thorax) that moves the wing relative to a fulcrum (side of thorax)
• Upstroke always indirect (contracting muscle pulls dorsal thorax down relative to fulcrum, flipping wing up)
• Downstroke either direct or indirect:
• Direct - e.g., dragonflies (Odonata)
  • Angular contracting muscle is inserted outside the fulcrum, pulls wings down directly
  • Direct control of wings is primitive within the Pterygota

How are wings moved?

• Upstroke always indirect
• Downstroke either direct or indirect:
• Direct - e.g., Odonata (dragonflies)
  • Contracting muscle inserted outside the fulcrum
  • Primitive in Pterygota
• Indirect - e.g., Diptera (flies)
• Longitudinal (anterior-posterior) muscles contract , bending dorsal thorax up (above fulcrum), pulling wings down
  (upstroke indirect as above - dorso-ventral muscles contract to pull dorsal thorax down below fulcrum, flipping wings up)
• Thorax is used as an elastic vibrator, assisting power stroke (down)
• Hinge/fulcrum area allows greater flexibility of wing (figure 8 movement of wings allows hovering and greater maneuverability)
• Myogenic control in fast insects
  • Muscles contract due to intrinsic rhythm rather than depending on external nervous impulse for each contraction
• Indirect control is derived in specialized pterygotes

Factors in insect success

• Small body size
• Microhabitats (more different kinds of small habitats)
• Reduced support problems
• Usually relatively low dispersal associated with small body size (unless carried by wind or migratory) -- leads to isolation, divergence, speciation

Factors in insect success

• Small body size
• Short generation time
• Rapid colonizaton of new habitats (if dispersal is not too great, occasional new colonists can establish themselves and diverging genotypes won’t be swamped out by new dispersing individuals of the old genotype)_
• Can exploit temporary habitats
• Quickly adapt, potential for rapid genetic change because of short generation time

Factors in insect success

• Small body size
• Short generation time
• Complex behavior
• Learning
• Complicated courtship and mating rituals allow the development of sexual selection
• Sociality - aggregate in groups
• Eusociality - cooperative, altruistic behavior (where individual may pass on more genes by helping another, e.g., a relative with shared genes, than by reproducing him/herself)

Factors in insect success

• Small body size
• Short generation time
• Complex behavior
• Adaptable mouthparts
• From chewing to sucking, biting, piercing, sponging, etc.
• Exploit wide variety of food, including parasitism

Factors in insect success

• Small body size
• Short generation time
• Complex behavior
• Adaptable mouthparts
• Powered flight
• Can find patchy, protected foods
• Can find and compete for mates
• Allows aggregation of individuals, migrations

Factors in insect success

• Small body size
• Short generation time
• Complex behavior
• Adaptable mouthparts
• Powered flight
• Complex life cycle: complete metamorphosis
• Gradation of life history patterns: Direct -> Hemimetaboly -> Holometaboly
• Larvae focus on acquiring resources, adults focus on reproduction
• Larvae, adults don't compete for food
• Larvae, adults often live in separate habitats, not exposed to same predators - difficult for predator to eliminate them

Factors in insect success

• Holometabolic groups - The majority of modern insects! The most spectacular recent evolutionary radiations!
• Coleoptera
• Hymenoptera
• Diptera
• Lepidoptera