Many complex relationships have arisen between plants
and animals, in particular, flowering plants and arthropods.
Some interactions between plants and arthropods are mutually
beneficial, while others are beneficial to one partner
but detrimental (and sometimes even lethal) to the
other. Plants have therefore developed numerous modifications
that help them attract the arthropods that help them and
others that fend off arthropods that do them harm. These
adaptations may determine whether a plant is able to
survive and successfully reproduce. Plants that successfully
reproduce will pass advantageous traits on to their
Some of these attributes are chemical. Plants manufacture
a vast array of chemicals that are not directly involved
in the day-to-day business of trapping solar energy
and converting it into the chemical energy that drives
plant growth. These chemicals give plants many of the
characteristic colors, scents, and flavors that distinguish
one plant species from another. Some pigments and aromatic
compounds are instrumental in luring arthropods or other
animals to disperse pollen or seeds. Other compounds
are primarily defensive in nature. They protect vulnerable
plant tissues from predation via repellent smells,
contact irritation, or direct toxicity.
Plants also have advantageous mechanical modifications.
Flowering plants often provide prospective pollinators
with nutritious rewards, including nectar, that represent
a significant investment of energy. Flowers are often
designed so that the reward is available only to the
arthropod or animal that is most likely to deliver its
pollen to the stigma of an appropriate flower but won't
be wasted on visitors unlikely to aid in pollination.
Plants also have mechanical attributes that aid in
defense, including spines, dense coverings of hairs,
or tough tissues that are difficult to penetrate or
digest. Latex (sticky white or colored sap) can be
considered both a chemical and a mechanical defense
Plants are by nature sedentary. Flowering plants are
the dominant form of plant life in many parts of Earth,
and their success has been in large part due to modifications
that have enabled them to use animals to gain reproductive
mobility. Flowers are often characterized by groups
of characteristics (pollination syndromes) that enable
us to propose a hypothesis about the optimal method
of pollination for a plant species. The pollination
syndromes provide clues, but careful field observations
are always required. These are some of the questions
to ask when you observe flowers, followed by brief
descriptions of some of the common pollination syndromes.
- Is the flower large and showy or small and inconspicuous?
- Is the flower open during the day or during the night?
- Is the flower aromatic? If so, what does it smell
like, and when is the aroma strongest?
- What color is the flower?
- What shape is the flower? Does it look like a cup,
an urn, a bell, a funnel, or a tube? Does it have any
"lips" or landing platforms for arthropods?
- Does the flower produce nectar? If so, can you tell
where it's produced or where it accumulates? Are there
any markings on the flower that might guide an arthropod
to the nectar?
- Are the anthers hidden down within the flower, or
are they in an exposed position?
- Does it look as if the anthers touch the stigma? Could
the flower self-pollinate?
- If you see an arthropod visiting a flower, can you tell
if it contacts pollen? Does the pollen stick to a particular
part of its body? Can you see where the arthropod goes
after it leaves the flower?
Common Pollination Syndromes
Wind: Wind-pollinated flowers often appear before the
leaves develop. The flowers are exposed, reduced, and
inconspicuous. The anthers and stigmas are exposed.
The anthers often dangle from their filaments and produce
large amounts of pollen; the stigmas are often long
and feathery; the flowers lack an odor.
Beetles: Beetle-pollinated flowers are usually flat
or shaped like a shallow bowl (easily accessible).
The anthers and stigma are exposed. The pollen or nectar
is easily accessible. The flowers have a dull greenish
or off-white color and a strong fruity or putrid odor.
Carrion and Dung Flies: The anthers and stigma are hidden.
The flowers lack nectar; have a dull greenish, brownish,
or purplish color; and have a strong odor of decaying
protein. (Note that other flies visit different types
of flowers, and several fly families actually mimic
Bees: Bee-pollinated flowers usually have an intricate
shape, and strength and dexterity are often required
to enter. The anthers and stigma are usually hidden.
The flowers produce moderate amounts of nectar that
is hidden (but there may be patterned nectar guides,
visible in either ambient or ultraviolet light). The
flowers are blue, violet, or yellow, with a weak but
Butterflies and Moths: Butterfly and moth-pollinated
flowers are often tubular or funnel-shaped, with nectar
at the base. The length of the tube may be correlated
with the length of the arthropods proboscis. The flower
often has some sort of landing platform. Butterfly
flowers can be yellow, blue, violet, or red and often
lack a strong odor. Moth flowers are nocturnal, often
with a white or drab color and a sweet scent.
Bats: Bat-pollinated flowers are nocturnal. They have
an easily accessible position away from the leaves
or may appear before the leaves develop. The flowers
may be either large and cup-shaped or "brushlike,"
with many exposed stamens. They produce large amounts
of both pollen and nectar, and they have a dull color
and a strong, sometimes unpleasant odor.
Hummingbirds: Hummingbird-pollinated flowers are tubular,
and the length of the tube may be precisely correlated
with the length of the pollinator bill. They produce
large amounts of nectar available at the base of the
tube, have a red color, and lack an odor.
Most plants maintain some sort of chemical arsenal against
any predators or parasites, including arthropods, fungi,
and bacteria. The line between toxic and medicinal
is often slender (depending on dose, mode of preparation,
or mode of ingestion), and about 40 percent of the
drugs in our modern pharmacopoeia are based on compounds
originally identified in plants. Insects are, however,
notorious in their ability to circumnavigate chemical
defenses. Those that do develop the capacity to detoxify
or sequester plant toxins for their own defense may
gain access to both a safe haven and an unexploited
food source. They may ultimately require the presence
of supposed "defensive" compounds to stimulate
feeding or reproductive behavior. These arthropods are
often considered specialists, because they feed on
a single plant species or a group of related plant
species. Other arthropods are considered generalists,
because they apparently tolerate a wide range of plant
chemicals and feed on many different plant species.
The association between monarchs and their milkweed
(Asciepias) host plants is one of the most thoroughly
investigated system of arthropod/plant interactions. Many
milkweeds produce toxic compounds (cardiac glycosides)
with a bitter taste. Monarch larvae are able not only
to feed upon these plants, but they sequester the toxins
that are passed on to the chrysalids, the adults, and
even the eggs of the next generation. The sequestered
toxins provide the monarchs with a degree of defense
against predators. Blue jays that have not yet encountered
monarchs will attempt to eat them. The dose of cardiac
glycoside that stimulates vomiting is about half the
lethal dose, so the blue jay survives, but it is loath
to sample another monarch. Because monarchs are conspicuously
colored, the blue jay presumably associates its unfortunate
gustatory experience with the characteristic coloration.
Thereafter, the bird will avoid not only a monarch
but also any arthropod that even resembles a monarch.
These are some questions to ask when you see an arthropod
on a plant:
- Does the arthropod have a beneficial, neutral, or detrimental
impact on the plant?
- Is the arthropod collecting pollen or any other substance
from the plant?
- Is the arthropod eating part of the plant, resting on
the plant, or looking for prey on the plant?
- If the arthropod is eating the plant, what part of the
plant is it attacking (flower, leaf, stem, fruit, seed,
- Do you see any relationship between the mouthparts
of the arthropod and how it exploits the plant?
- Is the arthropod an exposed feeder (on the surface of
the plant) or a concealed feeder (within the plant
- Is the arthropod a specialist (found on a single plant
species or related plant species) or a generalist (found
on many different plant species)?
- Does the plant have any sort of latex (sticky white
or colored sap)?
- Does the plant have any characteristic odor?
- Does the plant have any mechanical defenses? If so,
how does the arthropod avoid them?
- How might the arthropod defend itself from predators?
- Is the arthropod camouflaged, or does it have warning
coloration (that might alert a predator to its toxicity
or bad flavor)?