Insect Pollinated flowers
Pollination – the transfer of pollen grains from the male anther of one flower to the female stigma of another flower. In Britain most pollen transport is carried out by insects. To learn more about pollination click HERE.
Flowers use colour and scent to attract insects and then offer landing sites and food which may take the form of nectar or pollen. Nectar is 60% sugar in water. It is used as an in flight fuel or stored for later use. Pollen is a food for some insects and their larvae. Bees forage for pollen or nectar and carry collected pollen in baskets back to the hive. This pollen is not available for pollination so flowers try to limit it by their complexity and shape.
Co-evolution of flowers and pollinators.
The first insect pollinated flowers to evolve were simple, are called ‘primitive flowers’ and offered nectar or pollen to all insects. However, there is a trade-off between how much nectar should be given to the insect and how much pollen the insect carries to another flower of the same species. As flowers evolved, nectar became concealed and pollen rationed. At the same time insects evolved to have longer tongues or be more agile. As a result insect pollinated flowers vary in colour, scent, shape and complexity to match the pollinators for which they are adapted. For more information on the pollinators themselves click HERE.
‘Primitive’ flowers have easily accessible pollen and nectar and are pollinated by a wide range of insects. They are generally radial and star shaped and often flat or saucer-shaped so that insects can land easily and don’t have to hover. The buttercup shown here has prominent pollen-releasing anthers and nectar easily reached at the base of the petals which form an open bowl. This type of flower is visited by many different insects but pollen they carry may end up on an entirely different species and so be wasted.
Some simple flowers such as the poppy and hypericum do not provide nectar at all but bees still access the flowers to collect pollen needed for larval growth. Bumblebees speed up the collection of pollen from the anthers by vibrating their wing muscles in a process called buzz pollination. Magnolias, considered to be one of the earliest flowers to evolve, have deep cup-shaped flowers which offer protection and pollen food to beetles.
Flowers of the Daisy family. Another way that flowers may attract insects is to have a mass of small flowers arranged close together to form a large landing site called a ‘head’ . All the flowers may be the same or the head may be more complex as in the The Daisy (Compositae) family. Instead of one large complex flower the insect is offered a ‘head’ of hundreds of closely packed tubular disc flowers (called florets) arising at the same flat level, surrounded by a ring of ray florets to make the flower conspicuous. The example shown here is the Sneezeweed Helenium autumnale a popular garden plant.
The disc florets offer pollen at their tips and nectar at the base of each tube. Typically florets are 6mm deep so pollen and nectar are readily available to a wide range of pollinators but pollen is only available from some of the florets on any given visit. The flower signals which of the florets is offering nectar by its colour – in this case they are yellow with pollen. In accessing the nectar the bee in this photo will collect pollen on its belly. The brown florets will open later in the season. Other flowers such as scabious and teasel have compact ‘heads’.
More complex flowers are adapted to attract and be pollinated by more specialise long-tonged insects such as bees and moths and are more precise in the use of pollen. These flowers conceal nectar in different ways and ensure that pollen is carried between flowers of the same species. Extreme forms of complex flowers are found in the orchids, which carry out deception on a grand scale. Shown here is the Bee Orchid. First noted by Darwin, considerable research since has revealed that not only does the orchid look like a bee it also emits species-specific pheromones that mimic the female bee.
The male bee is thus attracted to the orchid to mate (unsuccessfully) with what it perceives to be a female and as a result carries away the pollinia (a pollen mass). This is a classic example of sexual mimicry which is common throughout the Orchid family. In some orchid species the pollinia is carried on the forehead of the bee and precisely positioned so that when the insect tries to mate with or take nectar from another orchid of the same species the pollinia sticks to the new flower’s stigma to result in cross pollination.
Radially symmetrical flowers look the same when rotated about the centre of the flower, like the butter cup. They appear to be simple but many have floral tubes in which nectar is placed so that it can only be reached by insects with a specific tongue length. These flowers ( e.g Wallflower, Red Campion and Phlox) have a long narrow tube below a flat ‘landing disc’ and are pollinated by long-tongued bees and butterflies. Primrose and Periwinkle also have a flat ‘landing disc’ but a more slender tube and because the flowers are small they are pollinated by hovering bee flies. Rose family flowers have nectar in a cup-shaped receptacle located below the anthers but the receptacle is not very deep. As a result many insects can reach the nectar but in doing so they pick up pollen.
In Pinks (Dianthus) the sepals form a slender tube and these flowers are pollinated by butterflies. Similarly night-flying moths pollinate white and bladder campion which have slender tubes.
Many radially symmetrical flowers are bell shaped, hang down and have nectar at the base of the bell. This nectar can only be accessed by long-tongued, agile bees like the bumblebee which hang upside down and in the process pick up pollen. Examples are Harebell, Comfrey, Bluebell, Hellebore, Deadly Nightshade, Bell Heather and Fritillaries.
Bilaterally symmetrical flowers are divided into two equal halves along a single vertical plane. Usually they have 2 upper petals and 3 lower petals with the central petal being larger than the other two. This encourages the insect to take up a particular position as it lands. An example is the snapdragon which has an upper and lower lip but mirror imaged down a vertical line like a face. The flowers are pollinated by bumblebees. Normally the mouth is completely closed and can only be pushed open by large insects such as bumblebees, attracted by nectar secreted at the base of the stamens (see photo).
The anthers touch the back of the bee as it enters and pollen is transferred. When it enters another flower the pollen is removed by its receptive stigma. There are many similar flowers such as the White Dead Nettle, Yellow Archangel, Toadflax, Betony and Acanthus. Other flowers such as the Foxglove and Penstemon have open tubes with a landing zone at the entrance and nectar at the top of the tube forcing the insect to crawl up the tube and become coated with pollen.
Bilaterally symmetrical flower’s point of contact. The style/stamens may come into contact with the under side of the insect or the upper side. The pea flower, which is shown here has 2 lower petals linked together called the keel. When the bee lands on the keel it forces the 2 petals apart releasing the stamens and stigma explosively so that 5 of the stamens hit the underside of the bee. The photo shows the flower after the bee has left (with pollen on its belly). In Delphiniums and Columbines nectar is at the end of a spur and the transfer of pollen is to the belly of the bee as it stretches its long tongue to access it. Violets and Orchids also have nectar in spurs but in these flowers the pollen is attached to the head or proboscis of the insect as it reaches for the nectar.
In the Flag Iris the nectar is concealed in a tube just above the ovary but to reach it the insect must follow brown nectar-guides. As the insect moves forward it touches the overhanging stigma and transfers pollen to it. As it goes further in it brushes against an anther and collects fresh pollen. The Lavender flower has nectar secreted at the base of its ovary which is nearly 8mm below the flower entrance. Bumble bees can access the nectar without entering the flower but honey bees have to insert their heads into the tube. So bumblebees can collect nectar faster than honey bees. Dead nettles and Sages in the Mint family, members of the Figwort family and members of the Plantain family (Snapdragons and Toadflaxes) all transfer pollen to the back the insect.
Scent emitting flowers. Some flowers are adapted for pollination by night-flying moths – sweet-smelling honeysuckle by hawk-moths and red valerian by butterflies and humming bird hawk moths. Shown here is the Greater Butterfly Orchid Platanthera chlorantha which is fragrant at night and pollinated by night-flying Noctuid moths. White flowers such as white and bladder campion and yellow flowers such as evening primrose are strongly scented to attract night-flying moths.
Colour of flowers. In the tropics bees and humming birds pollinate flowers. Bees and hummingbirds see colours differently. Bees are sensitive to ultraviolet, blue and green wavelengths but hummingbirds are additionally sensitive to red wavelengths. In the USA 80% of penstemon species flowers are blue , white or purple and are pollinated by bees, 20% are red, pink or yellow and are pollinated by hummingbirds. The fact that bees can also see light in the ultraviolet range means that what the bee sees and what we see may be completely different. Some flowers have landing patterns which emit light in the UV range which appear vivid to bees. Other flowers have petals that reflect a mixture of UV and a colour such as yellow and these appear appear purple to the bee.