Take a photograph of a flower, examine it closely – or enlarge it on a computer screen – and you will invariably find an insect lurking somewhere in the picture. This is not altogether surprising when we learn that two-thirds of flowers are pollinated by insects. To achieve this, flowers have learnt – OK evolved! – how to bribe, cajole, or trick insects into carrying out this function. Plants don’t walk, so they need animals to carry out a vital function for them; they need them to carry their sperm (in the form of pollen) to another individual where it can fuse with the eggs (ovules) of the other plant. Sex, or to give it another name: reproductive out-crossing!
Some plants self-fertilise – i.e. sperm fuses with eggs from the same flower. Dandelions are among such plants – which can produce seeds without having to be fertilised – although they can also be sexual as well, relying on being fertilised by pollen carried from one dandelion to another by insects. The photo (below) shows a dandelion head composed of dozens of tiny florets, each with pistols bearing pollen, which can be picked up by visiting insects, e.g. bees.
Some plants – the anemophilous ones, lovely word – rely on the wind to carry their pollen to another individual; but if a plant is to rely on an insect to vector its pollen, then it is going to have to have a strategy to achieve this. In practice, thousands of different strategies have evolved over time. If a flower is going to rely an insect to help it reproduce, it needs to do a number of things. First of all it needs to get the attention of the insects; most insect pollinated flowers are large and brightly coloured. Next it needs to offer some sort of inducement, usually in the form of nectar, although the pollen itself is a reward for many visitors.
A variety of different insects – e.g. bees, wasps, ants, butterflies, beetles and so on – may visit a given flower. Some may be feeding on nectar (butterflies and moths); some might be defending their sap-sucking aphids (ants); some might just be sheltering or hiding in the petals; and some may be eating the plant; but the pollinator species which is best for the plant is the one that helps it to reproduce successfully (i.e. it helps to increase the plant’s fecundity). These are the insects that the flower will evolve to attract. But not all flowers are specialists in this regard; some may be visited by a variety of pollinating bees and butterflies during the day, and by moths during the night. I wonder they ever get any sleep!
The hummingbird hawk-moth (Macroglossum stellatarum) – which has a very long proboscis – has been seen visiting the wild, Fringed Pink, Dianthus monspessulanus (1, 2). This plant has very long-tubed flowers (below) and emits a strong evening fragrance to attract the moths. It may however, not be the only flower species competing for the moth’s attention! The most attractive, and sweet-smelling flowers – to a hawk-moth’s nose that is! – will presumably get visited the most. Those flowers will be the ones which are the most fecund in the next generation, and the attractiveness to hawk-moths will continue to evolve. It is interesting that we humans also find the smell of such plants appealing; after all they are not trying to attract us! I guess it demonstrates the universality of the chemistry involved; it involves a compound called linalool, which is a common attractant for nocturnal hawk-moths (3).
Sea daffodil (Pancratium maritimum) is another flower which relies on hawk-moths for pollination (4).
Spring Squill (Scilla verna) is a plant that may be ant pollinated. Ants can sometimes be seen feeding on the ovaries (below); they can contribute to pollination by transferring pollen from one flower to another, but they can also be nectar thieves, just feeding on the nectar without carrying out any pollination services in return! (5).
An alternative strategy is of course to trick the insect into carrying out the needs of the plant. Some orchids do this by looking like bees or flies, but fascinating though this is, we will not follow this further here. Flowers providing rewards of nectar need to ensure that it is not wasted and that the pollen is successfully attached to the insects, for onward transport. All manner of devices and structures are used to make sure that the pollen is first attached – by sticking, brushing or hooking – and then successfully detached and delivered to the receptive female organ: the sticky tip of the pistil, the stigma. Some plants have even evolved ways of selecting the right sort of pollinator for their needs! For example, the guard hairs (below) on the Foxglove flower, Digitalis purpurea, have it is thought, evolved to exclude small bees, which presumably are not strong enough to push past them! The flowers are effectively selecting large bees to pollinate them, especially ones with long-tongues which can reach deep inside the flower to get the nectar (6). It is beneficial to both the bees and the flower to keep the relationship between themselves; it is a mutualistic arrangement which has evolved to suit both parties: the bees get to feed on the nectar, and the flower gets its pollen spread around in an efficient and effective manner. It would not benefit either of them if little upstarts got in and stole the pollen!
Both the ecology of pollination, and the evolution of the relationships between plants and insects, are vast and much studied subjects. All I want to do here is to illustrate by means of a few photographs, how easy it is to observe some aspects of this biological phenomenon. Most cameras allow close up photograph now, and the results are often surprisingly good – even with a relatively inexpensive camera – if one is prepared to be patient, and capture a detailed image. It is not always obvious that an insect is in the picture! Many of the photos shown here were just ones I had taken of a flower. Only afterwards, when examining the image on a computer screen, did I notice the insect! Most of the images shown here have been heavily cropped (i.e. by selecting the centre of the photo) to obtain the close up I wanted. I have also included some nice close up images of flowers without insects because I just like the shapes and appearances of these flowers. There is no end to what can be done just by taking a camera out into the garden or countryside (at the right time of year!).
Finally, just a note of caution. Hopefully this blog will have shown how dependent flowers and pollinators are on each other. Anything which affects the pollinators – and bees and butterflies are suffering in our modern world – will affect the plants too. This is particularly true in the case of specialised species, which are dependent on – i.e. adapted to – a particular type of pollinator species. The bottom line is, if the pollinator goes, the flower goes too. It’s an interdependent world and we need to take better care of it.
- Willemstein, Sjoert Cornelis. An evolutionary basis for pollination ecology. Vol. 10. Brill Archive, 1987.
- Miyake, Takashi, Ryohei Yamaoka, and Tetsukazu Yahara. “Floral scents of hawkmoth-pollinated flowers in Japan.” Journal of Plant Research 111.2 (1998): 199-205.