Flower shape and insect visitors
If you spend any time at all in a garden, you will notice that flowers can have very different shapes. The shape of a flower is important because it limits how an insect can get to different parts of the flower.
Some flowers are arranged so all parts are easily accessible. These “open-access” flowers are visited by a wide range of insects. Because insects can get at the pollen and/or nectar from pretty much any angle, the flower has little control over where pollen is rubbed onto an insect or where on the next flower the pollen rubs off. A flower that is shaped so an insect can only get at pollen or nectar in a certain way restricts where pollen is placed, both on the insect an on the next flower. These “restricted-access” flowers may not get the variety of insect visitors that open-access flowers do, but the pollen is more accurately targeted to the stigma of the next plant, so the plant does not need as many visitors for successful pollination.
Halteres – these drumstick-like bits replace the second pair of wings in Diptera (flies)
All insects have the same basic body plan, with changes in body shape for different lifestyles. Pollinators come in a range of shapes and sizes. There is a rough link between insect size and flower size, with larger insects visiting larger flowers, since small flowers do not provide a large enough landing platform. Some flowers get round this by grouping their flowers. The Umbelliferae (carrot family) group their flowers in umbels (think ‘like an umbrella’ and you have the shape in your mind). Larger insects use the whole umbel as a landing platform. Some flies don’t even have to land, they simply lower their mouthparts (which look like a drinking straw) into the flower and drink nectar while they hover. Flower shape and insect shape can be very closely linked – especially for some restricted-access flowers.
The most effective pollinators are insects that move large amounts of pollen from the anthers to stigmas of many different flowers of the same species.
Three things affect how well this happens:
- Pollinator size and shape
- Pollinator behaviour
- Flower arrangement and timing.
These are closely connected, since flowers and their pollinators have co-evolved. Only some interesting examples are given below, but read Barth (Insects and Flowers – the Biology of a Partnership) for lots more interesting examples.
Pollinator size and shape
Large insects will transfer more pollen simply because there is more body surface for pollen to stick to, but if an insect is too large, the flower cannot physically support the insect. Some plants get around this problem by grouping their flowers, so the insect can treat the group as a landing platform.
Pollen from insect-pollinated plants often has bits that stick out and this helps the pollen get trapped between hairs on the insect’s body. Honeybees and bumblebees groom themselves and move the collected pollen into ‘pollen baskets’. Female native bees also collect pollen in the same way, but because they do not pack the pollen down as well, it is more easily dislodged when they visit another flower. The difference in the amount of pollen transferred by exotic and native bees is still being investigated.
Tongue length is important, because some flowers hide their nectar deep inside the flower. Only insects with long tongues can get to this nectar. Other insects may collect only pollen, but since they are using the pollen as a food source, less of it will be transferred between flowers.
The honeybee, Apis mellifera, is flower constant, which means that on any foraging trip, it focuses on only one kind of flower. Pollen is transferred only between flowers of the same species and this is one of the features that make honeybees so popular for commercial pollination of crops.
Sometimes insects will ‘rob’ a flower of its nectar by cutting a hole in the side of the flower for easy access to the nectaries. Bumblebees, Bombus spp., will do this with kowhai (Sophora microphylla) or banana passionfruit (Passiflora mollissima) flowers. The hole made by the bumblebee will often be revisited by the bumblebee and also by other insects. An interesting observation is that sometimes, bumblebees will visit a flower that they have robbed for nectar in the ‘usual’ way. Why they do this is not fully understood – perhaps there is less nectar at that time, or they are simply collecting pollen for the hive.
Flower arrangement and timing
Simple ‘open-access’ flowers can be visited by almost any insect. The sheer number of visitors means some pollen will end up on a receptive stigma, but the flower has no control over how an insect moves within the flower or where the insect flies to next. ‘Restricted-access’ flowers are shaped so insects can only enter them in a way that results in pollen being placed where it will rub off onto the stigma of the next flower. Pollen is more accurately targeted, but only if the next flower visited belongs to a different plant of the same species.
Some plants manage to time production of flowers with anthers and stigmas that function at different times. For example, foxgloves have their flowers arranged in a spike, with the oldest flowers at the bottom. The anthers mature first and the stigma later. Bumblebees will start feeding in the lower flowers, depositing any pollen on their bodies onto the stigmas of these flowers. As the bumblebees move up the flower spike, they get dusted with pollen from the anthers in the younger flowers. Once the bumblebee reaches unopened flowers that do not produce nectar, they will move onto the next flower spike, again transferring pollen between plants.