Biological adaptations of flowers

For a plant to survive from one generation to the next, it should do all it can to help its pollinator to thrive and produce healthy offspring. This needs to be done without reducing its' own ability to produce viable seed. Nectar is rich in sugars, a high energy food for insects. In addition to these sugars, butterfly flowers provide a relatively high proportion of amino-acids (1). Amino acids are the building blocks of cells, and insects which take advantage of this, where available, probably increase their chances of survival and reproduction. To the flower this would make the insect more likely to be a pollinator, generation after generation. This has most significance amongst insect species which will feed mainly on a few types of flower.

Communication with insects

Apart from colour, plants have many ways of attracting potential pollinators including scent, reflectance, size, outline, surface texture, temperature and motion. Although colour is thought to be the most important factor for insect pollinators. Plants which do not depend on insect or bird pollination are unlikely to have showy or scented flowers (3).

Colour

In order to attract the potential pollinator to that particular blossom, availability of nectar has to be advertised to the butterfly. This is displayed in the colour of the petals. The colour of the nectar guide of Aesculus hippocastanum changes from yellow to red when nectar is no longer in production. This happens after the flower has been pollinated and the ovules have begun to develop. The flowers are then less regularly visited by insects(1, 3). Once pollination has taken place the flower may wilt and discolour rapidly as it is of no further use. The exception being if it is a small flower which is a part of an inflorescence (3).

Nectar Guides

Further help is available in the form of nectar guides, also referred to as "pollen guides" or "honey guides". These are usually a visual contrast, either in colours which we can see or against ultraviolet. These guides may not function to attract the insect, but act as guides for closing in on the target once the flower has been chosen (1).

The lines towards the nectar may be a structural adaptation as in the Thistle (Cirsium vulgare - see Fig. 1), or more commonly, the lines pointing to the nectar are a pattern on the petals, leading towards the centre from all angles. While others such as Dog Violet (Viola riviniana - Fig. 3, right), have their guide lines as marks on the petals which are visible from the direction of approach. The flowers of the Dog violet hang down, and so there are guides on the lower petals are where the insect lands, no guides are needed on the upper petals.

Fig. 3 Dog Violet - Viola riviniana

Bullseyes

One of the best known flowers with the bulls eye effect in ultraviolet is the black-eyed Susan, Rudbeckia hirta , which contains compounds absorbing strongly between 340nm and 380nm (2). The petals of the Black-eyed Susan, a large daisy-like flower, appear plain yellow to humans while appearing to have a very dark centre to insects.

The use of ultraviolet by flowers

Apart from pollen guides, plants will use ultraviolet to their benefit in other ways, such as ultraviolet pollen, ultraviolet nectar, fluorescent pollen or fluorescent nectar. They can also use a contrasting background to make the flowers stand out against a different level of ultraviolet reflection from leaves or leaf hairs. Ultraviolet light is also of use, to insects, for the identification of plants. This is most apparent where many plants which appear similar to humans, grow together (e.g. many composites). Yet these are presumably distinguishable to insects.

It has also been proposed that there may be differences in ultraviolet reflectance as a flower matures, to prevent competition within a species (4). Some flowers have been recorded as having fluorescent nectar, however, butterfly pollinated flowers have not been found to use this tactic. The significance of fluorescent nectar is still under debate, but occurs regularly enough to assume that it is not present merely by chance, and so must have some function (5, 6).

Most blue light receptors of plants are reactive to long wave ultraviolet, to around 370nm. In general, ultraviolet is not thought to have a significant effect on plants. Adaptive features, to protect against damage from ultraviolet radiation, are likely to be present in plants growing at high altitudes. Ultraviolet levels are raised at high altitudes because atmospheric scattering and absorption has had less distance to reduce the ultraviolet content of solar radiation (7).

Fig.4 Himalayan balsam (Policeman's helmet) - Impatiens glandulifera.

Visual system of insects

Bees are very widely studied insects with regard to their visual system. They can detect three colours ultraviolet, blue and yellow (8), no bees investigated can see red(9). This has been interpreted in the trichromatic theory. A pentachromatic visual system (i.e. the eyes contain 5 different types of cell which react to different bands of light), has been reported in Papilio xuthus, the Japanese yellow swallowtail butterfly (10). Butterflies vary widely in their sensitivity of light, and are considered to have the widest visual range of any animal (11). Atrophaneura alcinous has a visual range from at least 400nm(See Appendix 1, Note 1) to 700 nm, while Heliconis sara has a range from 310nm to 650nm (12).

The ability to see red is rare in the insect kingdom but appears to be quite common in butterflies (11), and is now known to be an essential part in the release of courtship behaviour of some species(12, 13). It had previously been thought to be that red, orange and even yellow colouration of butterflies served merely as a warning for potential predators (11).

Butterflies seem to prefer yellow in their feeding behaviour (8, 13), with Pieris and Eristalis spp having a preference for the yellow-flowered morph of Raphanus raphanistrum (wild radish), over the white coloured morph of the same plant (14) (See Appendix 1, Note 2).

Sensitivity to the polarization of light has been reported in some cells of a few butterflies (15). It occurs mostly in the upper lenses of the compound eye, and is most likely used for spatial orientation in overcast conditions.

Ultraviolet patterns and behavioural effects of colour on butterflies

Fig.5 Butterfly vision simulation of Cleopatra - Goneopteryx cleopatra.

Not all butterflies feed on nectar, there are some species which specialize in feeding on tree sap and/or fruits, so they will have adapted to that way of life, and are unlikely to have the same visual responses as nectar feeders (1).

The male and the female of a species of butterfly are often very different, this is a necessity for quick recognition(4, 16, 17). Quick recognition during courtship is important, as they normally rely on a rapid, erratic escape flight to defend against predation. The courtship, however, involves an ascending flight with a conspecific, this is conspicuous and predation would tend to select against it.

To the human eye many butterflies appear the same, but the butterflies themselves can often identify each other quite easily from ultraviolet markings. For example the males and females of Eurema lisa , the small sulphur butterfly, differ only in the ultraviolet region, the males being strongly ultraviolet reflective and the females unreflective in ultraviolet. The need for significant differences in appearance exists only in butterflies which are palatable to birds. Those which are not palatable have a much reduced risk of being attacked and so can spend much more time in identification and courtship (18).

In addition to this sexual selection can occur within species, the males choosing the female on the basis of age. The younger females have less ultraviolet than the older individuals, and it is the younger ones which are preferred. The male preference for younger females in Pieris rapae is due to the female laying approximately half their lifetime production of eggs in the first quarter of their life (19).

The ultraviolet patches on some butterflies are directionally iridescent (17), as a result they appear to flicker in flight. This flickering is thought to have an important role in butterfly behaviour and communication (4).

Butterflies tend to avoid the colour green in their feeding behaviour, possibly being effectively invisible (3), but are attracted to it during egg laying. The next generation need to be placed near a good source of food as caterpillars have a voracious appetite. The green photoreceptors are instead used for the detection of movement (10).