The complex world of the relationship between flowers and insects.

Bumble bee visiting the flowers.

Source

Introduction

It is now a well known fact that flowers and insects are reliant on each other. The plight of the world bees is a threat to world food { see my article of the same name}, has been in the news of late, as their numbers have worryingly declined, along with a number of other important pollinators.

Here in this article I review the history of this complex relationship between insects and plants and give a basic explanation of specific relationships. Wild flora,along with many cultivated crops of all kinds, rely on insects for pollination, without whose actions the fruits would fail, the seeds would not develop and thus in a matter of time many species would become extinct.

Geranium sylvaticum

Creative Commons Attribution Share Alike 3.0 unported license. Geranium sylvaticum was the first plant  that Sprengel studied.
Creative Commons Attribution Share Alike 3.0 unported license. Geranium sylvaticum was the first plant that Sprengel studied. | Source

Insects and flowers

Sprengel seems to have been the first to note the intricate relationship between flowers and insects. His observations of the flower Geranium sylvaticum led him to research this relationship.In the year 1787, he noted that the corolla {flower petals etc} was covered with a number of delicate hairs and stated " The wise author of nature would not have created a single hair in vain. He therefore reasoned the hairs were for a practical purpose and satisfied himself that these hairs protected the honey from rain.

Thus his attention to plant details became more intense and he studied several other species with great care. He soon discovered, to his surprise, how the different constituents of the plant could be explained by their relationship to insects.

The visits of insects are of great importance to the plants in transferring pollen from the stamens { male organ of the flower consisting [usually} of a stalk{filaments} and a pollen-bearing portion -anther} to the pistil { a more or less tubular structure usually in the center of the flower that contains carpels consisting of the ovary, stigma and usually a style}. In many plants the stamens and pistils are located in different flowers. Even when they are contained in the same flower, self-fertilization is rendered very difficult, or impossible. Sometimes the relative position of the stamens and pistil are not aligned sometimes by them coming to maturity at different times. Under these circumstances the transference of pollen from the stamen to the pistil needs to occur in another way.

In some species of flora the pollen is carried by the wind , in a few species by birds. However, the majority of plants rely heavily on insects for the transference to occur. in such flowers the whole organisation of the flower is adapted for this purpose.

Although the sweet scent and bright colours may attract insects to the flower it is for the honey they come. The lines and circles on the corolla guide them to where the honey is secreted, and there are a number of contrivances flowers have developed in order to bring the insects down to the right spot.

Darwen advanced the knowledge

Sprengel's observations were merely the tip of the iceberg as regards the relationship between insects and flowers. It was Darwen that advanced the knowledge of the relationship. It was he that discovered that it was not merely the transportation of pollen from the stigma to the pistil, but it was transferred from the stigma of one flower to the pistil of another.

Whilst we have known for centuries the importance of insects to plants, it is only relatively recently that we have realised how important, indeed how necessary, insects are to flowers. Sir John Lubbock states " Flowers are in many cases necessary to the insects, on the other hand insects are still more indispensable to the very existence of flowers"

Insects, have in many cases, adapted with a view to obtain honey and pollen from flowers. In turn flowers have produced scent, honey and colour and even their distinctive forms to the action of insects. This interaction between the two have led to a gradual modification of both insects and flowers. Charles Darwen's theory was based on the following observations.

1- That no two animals or plants are identical in all respects.

2- That the offspring tend to inherit the peculiarities of their parents.

3- That of those that come into existence, only a certain number reach maturity

4- That those which are, on the whole, best adapted to the circumstances in which they are placed, are most likely to leave decendants.

Applying these observations to plants it seems obvious that those which are brightly coloured, scented, have honey or are large will attract the greater number of insects, and so have advantage in the struggle for life and most likely continue their race.

Psuedomyrma Ants on Acacia

Source

Studying the relationship

The study of this relationship between insects and flowers has been carried out over many, many years. here I share an example which is on record fro Mr. Belt.-" A species of Acacia if unprotected would possibly be stripped of its leaves by a leaf cutting ant. The Acacia bears hollow thorns, and each leaflet produces honey in a crater-formed gland at the base, and a small, sweet, pera shaped body at the tip. In consequence it is inhabited by myriads of a small ant Psuedomryma bicolor, which nests in the hollow thorns, thus it finds meat,drink and lodgings all provided for it. These small ants are continually roaming the plant, and constitute a most effective body guard, not only driving off the leaf-cutter ants but also rendering the leaves less liable to be eaten by herbivorous mammals. this is an example of plants and insects benefiting each other.

However, the principle service which insects perform for plants is that of transferring the pollen from one flower to another. It is believed that it was Darwen himself that first demonstrated that if a flower was fertilized from pollen of another flower from a different plant, the seedlings produced were much stronger than those produced by their own pollen.

Sir John Lubbock refers to this when in the 1870's, he stated that he had witnessed several experiments which revealed the ' quite striking differences'. Six cross pollinated and six self -fertilized seeds of Ipmoea purpurea were grown in pairs on opposite sides of the same pots. The former reached the height of seven feet while the others were on average only five feet four inches. The first also flowered more profusely.

The complex working of a flower as regards fertilization was also studied by Fritz Muller who recorded the following observations. Some species in which pollen, if placed on the stigma of the same flower, " has not only no more effect than so much inorganic dust" But, which is perhaps, even more extraordinary, in others, he states that pollen placed on the stigma of its own flower acted like a poison. This he noticed in several species, the flowers failed and fell off, the pollen-grains themselves, and the stigma in contact with them, shrivelled up, turned brown, and decayed, while other flowers on the same branch, which were not so treated,retained their freshness.

Wasp on ivy flowers

Source

Acer negundo has no secreted honey

Source

Strategy for attracting the insects

Bright colours and strong scents attract the attention of insects but something further is required if the flower is to achieve its aim of pollen transference. no matter how pretty or colourful the flower, or indeed how sweet scented, it would not be enough to lure the insects inside their corollas unless something more substantial is on offer to them. this of course is the honey and pollen. { although it must be mentioned that some plants beguile insects by holding expectations of honey which does not really exist, just as some animals repel their enemies by resembling other species which are dangerous or disagreeable to taste}.

Night flowers are generally pale yellow or white these colours making them more conspicuous when daylight has given way to darkness. night flowers such as Silene nutans {White campion} and Hesperis matronalis { Dame's violet } for example have little or no scent during the day, but are sweet smelling by the time evening commences.

Most Rosaceae are fertilized by insects and possess nectaries, but as Delpino pointed out the genus Poterium {now Sanguisorba} is anemophilous { wind fertilized and posses no honey.} Wind pollinated plants are a different subject for a different article room being confined in this article for insect pollination. The Maples are almost all fertilized by insects and produce honey, however, Acer negundo is wind pollinated and has no honey, again this occurs in other genera.


The honey is secreted, sometimes in one part of the flower, sometimes by another, and a great variation can be encountered in this respect even within the same Order of plants. For example the honey glands in Ranunculaceae {buttercup family} the glands are in the calyx, on the petals in buttercups and Hellebore and according to Muller , on the stamens in Pulsatilla and on the ovary in the genus Caltha.

To protect themselves from unwanted and insects which deliver no benefits to them some plants are protected by downward pointing hairs, others by viscid glandular hairs, others by extreme smoothness on their surface. In other plants have flowers closed by barriers, which only leave sufficient space for the slender proboscis of the bee, while others again, such as the Foxglove are closed 'boxes' which bees alone are able to enter.

Many flowers close their petals during rain, which is an advantageous way of protecting honey and pollen from being washed away or spoiled. Yet as the more observant will testify some plants close their flowers at certain times of the day, even in the brightest weather. This habit of ' sleeping' is indeed a curious one. in animals we can understand it, they are tired and need rest. By why do flowers ' sleep' ?

The daisy opens at sunrise and closes at sunset. The dandelion is open from about seven am and closes around five pm. Arenaria rubra from nine am until three pm. The Goats beard a member of the Dandelion family opens its flowers early in the morning and they close again at mid day regardless of the weather. it is quite obvious that flowers which are pollinated by night flying insects such as moths would gain little advantage by being open in the hours of daylight. Conversely those that are pollinated by diurnal insects such as butterflies and bees would not gain any benefit by being open during the hours of darkness.


Foxglove flowers

The bell-shaped flowers of the Foxglove are designed to allow entry for bees only.
The bell-shaped flowers of the Foxglove are designed to allow entry for bees only.

Hedge Bindweed flower visited by beetles

Source

The 'sleep' and insects

Sir John Lubbock states on the subject of flowers 'sleeping'-- " I would venture to suggest, then, that the closing of the flowers may have reference to the habits of insects, and, it mat be observed also in support of this, that wind pollinated flowers do not 'sleep' and that some of those flowers that attract insects by smell emit their scent at particular hours."

M.Boissier {les Nectaries page 166} conveys that he observed some species of Sempervivum [tectorum,mantanum and refexum} growing abundantly on rocks, which secrete honey in the morning's only. These plants were much frequented by bees up until midday, but quite deserted in the afternoon.

Aristotle mentioned in his writings his observation of bees which confine themselves in each foray to one single species of plant. This can still be observed in our gardens today.


Stock is sweet scented at night

Constituents of Arum maculatum

Note the spadix bottom tight hand corner
Note the spadix bottom tight hand corner | Source

Arum's strategy for insect pollination

Plants have adopted many means of utilizing insects to transfer their pollen. The Common Arum is a good example to illustrate this point. This well know plant has a large leaf that encloses a central pillar called a spadix, which supports a number of stigma's near the base, and a number of anthers somewhat higher.

One would think that there would be nothing simpler than the natural dropping of the pollen from the anthers would fertilize the pistil. However, this does not occur. The stigma's mature before the anthers and by the time the pollen is shed they have become incapable of fertilization. Thus it is impossible for the plant can become self fertilized.

Because the pollen drops to the bottom of the 'tube' it is well sheltered from the wind and rain and nothing short of an hurricane could dislodge it. hence the chances of the wind blowing the pollen to the spadix of another plant is nigh impossible. Insects are attracted by the showy central spadix whether this is for food or shelter they enter the tube while the stigma's are mature and find themselves imprisoned by the fringe of hairs. These hairs are so positioned that entrance is allowed but returning is not.

However, after a while the period of maturity of the stigma's ceases, and each secretes a drop of honey in this way repaying the insect for its captivity. The anthers then ripen and shed their pollen which falls and sticks to the insect. The hairs that prevent their exit gradually shrivel up and let the insect free, which carry the pollen with them, so that those that visit another plant can hardly fail to deposit some of it on the stigma's. Sometimes small flies can be seen swarming around a single Arum plant.

This association between insects and plants is a complex one, and, the method of adaptation on the part of the plant and that of the insects that visit the flowers is a very interesting one. In future articles I will be looking at the specific species and their associated reliance of insects, starting with the Ranunculaceae order of plants that includes the buttercups, hellebore's, marsh marigold's, anemones and delphiniums.

Hoverfly on ivy flower

Source

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6 comments

D.A.L. profile image

D.A.L. 3 years ago from Lancashire north west England Author

avvianovice,

you are so right about the natural world is very remarkable. Thank you for visit and for comments. Best wishes to you.


aviannovice profile image

aviannovice 3 years ago from Stillwater, OK

Flowers and insects do have an interestingly symbiotic nature. Many times in my life have I just sat and watched how insects and plants depend on one another. The same existence trickles down to us with fruits and vegetables. The natural world jus so remarkable to me.


D.A.L. profile image

D.A.L. 3 years ago from Lancashire north west England Author

DDE,

Hello Devika thank you for visiting. Insects are fascinating creatures and I am happy to have shared this with you. Best wishes to you.


DDE profile image

DDE 3 years ago from Dubrovnik, Croatia

The complex world of the relationship between flowers and insects very interesting and this summer my garden had many insects I enjoyed watching them and also wondered now I know more about the insects from this extraordinary hub


D.A.L. profile image

D.A.L. 3 years ago from Lancashire north west England Author

MarieAlana1,

Hi, Thank you so much for your inspirational comments they are appreciated. Best wishes to you.


MarieAlana1 profile image

MarieAlana1 3 years ago from Ohio

Wow! This is fascinating! These pictures are absolutely wonderful! Great job with this hub.

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