Entomology 10

Skipper butterfly feeding on Aster flowerhead

Pollination: Sexual Encounters of the Floral Kind

I. Flowering plant biology-

1. Seed pod explodes- initiating plant life
2. Flowers- Purpose: Sex (Time-lapse of anthesis events)
3. Most flowers have male & female sexes
   1. Sex parts-
      • Female: Stigmas (receptive)
      • Male: Anthers with pollen
   2. Selfing easy with both sexes in one flower, but cross-pollination needed for genetic variation and adaptation
   3. How do plants achieve cross-pollination when literally rooted?

II. Hammer-head orchid & thynnid wasps in Australia-

1. Thynnid wasp female (wingless) emerges from soil
   1. Will return to soil to paralyze & lay eggs in beetle grubs
   2. Climbs plant, opens mandibles to release "perfume"
      • [Pheromone] attractive to winged male
   3. Males emerge a couple of weeks before female [Protandrous]
   4. Male picks up female- carries her to flower where he feeds
   5. Mating starts while in flight
   6. Female obtains food and sperm through male abdomen
2. Orchid exploits wasp mating behavior
   1. Flower shape and odor mimic female wasp
   2. Male wasp attracted to dummy female, picks up pollen sacs
   3. Male wasp must visit 2nd orchid for cross-pollination to occur
   4. After female wasps emerge, male wasps no longer fooled by orchids
      • [Timing is critical for orchid]
      • [Narrow window between male and female wasp emergences]

III. Adaptations of flowers for insects & other animal pollinators-

1. Grasses- pollen carried by wind [Anemophily] (Also many trees)
   1. Wasteful of pollen. Why not use insects as "go-betweens?"
2. Advertising-
   1. Energy expensive, but pays off
   2. Shapes, designs, patterns (some in U-V that we cannot see)
3. Rewards-
   1. After attracting visitors, must pay them
      [to ensure visit to 2nd flower of same kind]
   2. Pollen (protein)
      [Energy for insect eggs & offspring]
   3. Nectar (rich in sugars)
      [Flight fuel- Adult energy source]
   4. Wax (used by stingless bees to build nests)
4. Deception [false offerings] by plants-
   1. Ichneumon wasp [male] attempts to mate with flower
      [Pseudocopulation]
      [Mate mimicry]
   2. Lethal relationship-
      African water lily exploits syrphid flies [see below]
5. Some insects cheat
   [Nectar Thievery]-
   1. Bumble bees:
      • long-tongued
        [Legitimate visitors]
      • short-tongued, bite holes near bottom
        [Thieves]
   2. Honey bees feed at holes made by short-tongued bumble bees,
      get nectar without pollinating
      [Secondary thieves]
   3. Floral defenses against thieves-
      • Teasel has moat that serves as ant trap
      • Ginger has extra-floral nectar that attracts ants as guards

IV. Exploitation by flower for self interest (lethal to visitor)-

1. African water lily and syrphid flies-
   1. Day 1- Pool of nectar-like fluid, toxic to flies
      Anthers "Smooth as silk," fly slips into pool, drowns
      Pollen [from visit to day 2 flowers] washes to stigma
   2. Day 2- Anthers shed pollen, flies feed on pollen "lollipops"

V. Flower specializations-

1. Competition molds flowers for better reproduction
   [Natural Selection]
2. Night flowers- scented for moths or bats
3. Buzz pollination- Sophisticated (sonicated- buzzing by bees)
4. Digitalis spikes- lure bees to bottom flowers first
   • Ensures all flowers pollinated in turn
5. Figwort- specialized for wasps, but ants can steal nectar
6. Trigger plant [Australia]- Zaps pollen onto visitors' back
   • Size of visitor important
7. Dwarf owls clover intertwined, easy access by crawling ants
8. Most, but not all animal pollinators are insects & have wings
   • Pygmy possum among few mammal pollinators (non-flying)
9. Wings make system more effective
10. Bird flowers-
    • Kangaroo paws- Adapted to put pollen on foreheads of birds
    • Strelitzia- Slippery perch, puts pollen on bird's belly
    • Protea (Africa)- large, tall ones visited by birds
      small, low ones visited by mice
11. Australian honey possum- attracted to Eucalyptus- lots of nectar
12. Central America- lots of nectar rich flowers & hummingbirds
13. Flowers have evolved structures, colors, and odors as signals to
    attract effective "go-betweens" [Pollinators]
    [and rewards to keep them coming]

VI. Kidnapping of visitors by flowers [Trap flowers]-

1. Blow flies- lay eggs on rotting flesh which maggots devour
2. Arum lily- Elaborate structure in chamber below floral entrance
   • Stamens [male] above spikes and female flowers below
3. Flower has stench of rotting meat [also meat colored]
4. "Perfume" attracts female blow flies seeking places to lay eggs
5. Hairs on surface force flies one-way [into flower]
6. Captive flies feed on nectar and even lay eggs in chamber;
   but maggots doomed to die of starvation [No food]
7. Day 2- male flowers open, shower pollen on flies
8. Day 3- hairs wither and flies exit to seek carrion or another lily

VII. Flowers living in water face different problems [Hydrophily]

1. Vallisneria (Ribbon weed)
2. Male flowers float on surface at whim of wind & water currents
3. Female and male flowers on separate plants
4. Female flowers- move to surface on ribbon-like stalks
   rest in small depressions on water surface
5. Many male flowers intercepted by fish as they drift up to surface
6. Male flowers open, like little boats with anthers protruding
   • When within an inch or so of female flowers, irreversibly drawn into surrounding depression, pollen contacts stigma.

VIII. Sun-tracking flowers of Arctic-

1. Dryas, Arctic rose, in Greenland
2. Parabolic arrangement of petals directs suns rays to center
3. Warms anthers [male] and stigma [female]
4. Insects find 10° warmer than surrounding air
5. Insects feed on pollen & seek next warm flower
6. Flowers track sun so rays always directed into flower

IX. Orchids visited by male bees seeking odor rewards-

1. Tropical rain forests, no gentle breezes to move pollen
2. Orchids numerous, many with only one pollinator that can operate elaborate pollination mechanisms
3. Coryanthes, bucket orchid, complex structure
   1. Pollinated by male green orchid bee
   2. Orchid opens early morning
      • Drips fluid from 2 glands into bucket
      • Has odor attractive to male bees
   3. Male bees scrape waxy perfumed surfaces
      [Store in hind legs and modify] odors to use to attract female bees
   4. Male bee falls into bucket of liquid, must escape via tunnel
   5. Pollen sacs attached to bee during struggle through exit
   6. As bee goes through 2nd flower, pollen sacs go into stigma
   7. Risky! If bee disappears, orchid would vanish

X. Conclusion-

1. Flowers designed to produce seeds to perpetuate flowers
2. As important as human or animal sex to life on Earth

Readings in Gullan, P.J. and Cranston, P. S. (1994). The Insects: An Outline of Entomology:

• Read: pp. 266-270 (10.3)

Other background reading:

Return to the Course Syllabus