The two most speciose groups of multicellular eukaryotes today are
the angiosperm plants and the insects. Angiosperms are distinguished
from other kinds of plants by the presence of the reproductive
structure known as the flower. Earlier types of seed-bearing plants had
cones for seed production; still earlier forms reproduced not with
seeds but with spores. Angiosperms may have originated during the early
part of the Mesozoic era, but during much of that time period the
landscape was dominated by more plesiomorphic plants, including large
ferns, horsetails, and various gymnosperms. These plants formed the
extensive coal forests during the Mississippian and Pennsylvanian
periods of the Paleozoic era, which are thus collectively known as the
"Carboniferous" period. The angiosperms first appear in the fossil
record early in the Cretaceous period. They blossomed (pun intended!)
in a great adaptive radiation during the Cretaceous, soon became more
diverse and numerous than the earlier forms, and have remained the
dominant plant group since that time.
The insect fossil record is actually quite complete for a group whose
members are in general small animals, and for one that is primarily
terrestrial, where fossil formation is a rare occurrence. Insects first
appear during the Devonian period of the mid-Paleozoic era (about 390
million years ago). Most of the major orders of insects are present in
Paleozoic strata, and only a few orders became extinct at the Permian -
Mesozoic boundary. The rate of increase in the diversity of insects was
exponential during the early part of the Mesozoic, then slowed during
the Cretaceous. Some groups, especially the beetles, the lepidopterans,
the flies, and the hymenopterans, again proliferated during the
Cenozoic. These groups are the most diverse of the Recent insect fauna.
The traditional idea concerning the more or less concomitant adaptive
radiations of angiosperms and insects is a coevolutionary one. The
argument goes like this: One of the driving selective forces on plants
is the ability to transport gametes (pollen) from one individual to
another, so as to avoid selfing or inbreeding. Pre-angiosperm plants
relied on wind to carry their pollen, and this process necessitated the
evolution of sticky female parts to trap the male pollen. This process
was inherently inefficient, as there was probably lots of
heterospecific (other species) pollen about, and little pollen from
your own species. Wind pollination thus probably limited the "seed set"
of these primitive plants. Enter the insects. The first true insects,
as this story goes, were probably like modern beetles; they were
terrestrial and could fly, and probably ate plant parts. Stems and
leaves were probably not very good to eat, but pollen and seeds were.
Thus the first insects were predators on the reproductive parts of the
ancestors of angiosperms. They would fly from plant to plant, eating
the pollen and looking for seed-producing tissues. Some of these
insects would be trapped in the sticky material intended to trap
wind-borne pollen grains; in this case, the insects might also have
been carrying pollen, so the pollen would be trapped too.
Alternatively, the flying insects might carry pollen from
pollen-producing structures of one plant to seed-producing structures
of another plant, and deposit pollen on the sticky female structures
inadvertently. This scenario sets up the possibility that some plants
could preferentially attract insects that were carrying pollen, and
thus could increase their seed set (reproductive success). Those plants
that were most successful in attracting ancestral pollinators became
the angiosperms, and the rapid adaptive radiation of the plants was
accompanied by equally rapid diversification of insects.
There are a number of difficulties with this story. First, the
diversification of the insects began at least 100 million years before
angiosperms appear in the fossil record, and even if we suppose that
angiosperms had originated much earlier, they could not have been a
major component of the flora or we would have fossils of them. Second,
the Mesozoic adaptive radiation of the insects was most intense in the
Triassic and Jurassic, and began to slack off during the Cretaceous,
just when the angiosperms began to take off. Third, the insect taxa
that are the most important pollinators today began their adaptive
radiations in the early Mesozoic, well in advance of angiosperm
ascendancy. However, these groups (especially the flies, lepidopterans,
and hymenopterans) exhibit a marked increase in diversity during the
late Mesozoic, and have continued to diversify through the Cenozoic. It
is probable that at least during their relatively recent evolutionary
history, members of these orders coevolved with angiosperms.
In spite of the debate about the phylogeny and diversification of
angiosperms and insects, it is clear that today there are many examples
of complex and specialized relationships between plants and their
pollinators that could only be the result of prolonged and tight
coevolution. Some of the more spectacular examples include orchids and
several species of euglossine bees, arum-lilies and flies, figs and fig
wasps, and yuccas and yucca-moths. Not all ecological relationships are
as striking, but most angiosperms that are pollinated by insects (or
vertebrates) show some degree of adaptation to specific pollinators,
which in turn have their own specializations. These "flower syndromes"
are discussed in more detail below.
Flowers are the genitalia of the angiosperms. The basic function of a
flower is to accomplish transfer of gametes from one individual to
another, often using an animal for transportation. Flowers consist of
four types of structures (sepals, petals, stamens, carpels) borne on a
modified stem called the receptacle. There are usually three, four,
five, or more of each type of structure in each flower. Two of these
structures are sterile and two are fertile reproductive structures. The
basal structures are the sepals and the petals, which are sterile.
These are the structures that most of us think of when we think of
flowers. The main function of sepals and petals (collectively called
perianth segments) is to attract pollinators. The apical, fertile
structures are the stamens and the carpels. The stamens are the male,
pollen-producing structures. These usually consist of a filament which
bears an anther, where the pollen is produced. The carpels are the
female reproductive structures. A carpel has a distal style with a
pollen-receiving structure at the tip (the stigma), and a proximal
ovary, where the egg (oocyte) develops. In general, the stamens and the
carpels of any particular flower are not mature at the same time to
prevent selfing.
Not all flowers have all four of the structures. Some flowers have lost
or modified perianth segments. In some cases the perianth segments may
be replaced by specialized leaves called bracts, which function to
attract pollinators as do the perianth segments. Flowers with both male
parts (stamens) and female parts (carpels) are called "perfect"
flowers; flowers with either stamens or carpels but not both are
"imperfect". Some plant species have separate male flowers and female
flowers on the same individual plant; these plant species are called
"monoecious". Other plant species have the two types of flowers
separated onto different individual plants (there are male plants and
female plants); these are called "dioecious".
With the basic flower structure as a starting point, flowers have
diversified into a host of morphological types. Some flowers are
relatively simple and little different from the basic pattern, with all
of the parts, and with more or less radial symmetry (actinomorphic).
Many other plants have flowers that have lost some of the parts, or
have become bilaterally symmetric (zygomorphic). Still others have
developed not individual flowers but clusters of flowers called
inflorescences. There are many types and arrangements of
inflorescences. In extreme cases (e.g. the composites) inflorescences
are so specialized that they resemble individual flowers.
In spite of the almost bewildering array of flower types, there are
some patterns in their diversity. Pollination or flower "syndromes"
reflect the nature of the most common pollinators of flowers. For
instance, flowers with a deep corolla tube (a tube formed by the
perianth segments) and radial symmetry are often pollinated by
lepidopterans, because these insects have a long proboscis. Daytime
foraging butterflies visit colorful flowers, especially yellows and
pinks, while nocturnal moths visit pale flowers with strong scents.
Beetles are not usually important pollinators, but they visit flowers
(or inflorescences) that are wide and flat, that are drab or white, and
that have a strong pungent odor. Plants that are pollinated by flies
may have flowers that act as traps or that produce noxious odors
(because flies are attracted to decay and filth), but other
fly-pollinated flowers are colorful and fragrant. Bees usually visit
flowers that are relatively shallow, that often are zygomorphic, that
are yellow, blue, or white, and that have a strong, sweet aroma. Many
flowers that are pollinated by diurnal insects have color patterns that
reflect ultraviolet light, which is visible to some insects but not to
vertebrates. Finally, hummingbirds pollinate flowers that have a deep
corolla and that are bright red or orange (vertebrates see reds better
than blues), but that have little if any odor (because birds have a
poor sense of smell).
Pollinators are as diverse in their behavior as flowers are in their
morphology. The main types of pollinators, the flies, the
lepidopterans, and the hymenopterans, are phylogenetically distantly
related, are very diverse (with total numbers of species in the
hundreds of thousands) and thus have very different ways of foraging
and different needs that they fulfill by visiting flowers. Then there
are the beetles, which are not very effective pollinators (from the
plant's point of view), but are instead predators of plant parts, and
thus have a very different effect on the plants than the other insect
visitors. Leps and flies do not provision developing offspring, so are
interested only in food for themselves. As adult insects (and ones with
relatively short lifespans) the leps and flies do not need protein for
growth, so visit flowers to collect only nectar. Nectar provides sugars
which are the source of energy for flight. Bees, in contrast, collect
pollen in addition to nectar, because the pollen is the source of
protein for developing larvae (which the worker bees provision). The
nectar is processed to make honey, which is the source of sugar
(energy) for both larvae and adult workers.
Our purposes for this lab exercise are to a) observe the diversity of
pollinators that visit a small set of composite flower species during
the fall, b) to compare the foraging behavior of some of these
pollinators, and c) to compare pollinators' foraging behavior on
different plant types. We will develop hypotheses about pollinator
foraging behavior after we have observed some of these species in the
field.
Comparisons that have been fruitful in the past:
Time spent per flower for different species of pollinators [honeybees and skippers (Hesperiidae) work well].
Number of flowers visited per unit time for different species of pollinators.
The same species of pollinator on different flower types (this is
done
in the fall so there are lots of fall-flowering composites).