EVOLUTION AND INTERDEPENDENCE BETWEEN INSECT AND PLANTS
The earliest forms of plants that is the algae produced asexually and sexually (depends on water to transport the sex cells) 4. Moss represents the earliest forms of life to invade the land but encountered the same problem as the algae in terms of transportation of sex cells. The moss occupied the wet areas and didn’t have very specialised structures such as the roots4. As time progressed so as to be able exploit the land plants had to develop roots to be able to obtain water from the ground. Club mosses, horsetails and ferns were among the first vascular plants could stand on the ground before seeded plants appeared, some were tall plants up to 30m long.1,2,3,4 All of them had root to exploit and could be able to stand giving rise to the first forests. As forests were evolving also were the animals evolving to be able to occupy the habitats which include the insects group that had developed three segments of body head, thorax and abdomen, breathed through the trachea, had 3 jointed pairs of legs on the thorax. Insects have closer evolutionary ties with the crustaceans than millipedes even though grouped closer to millipedes.4
Originally insects climbed on vegetation but later developed wings, this helps them increase the surface area and for thermoregulation4. With the development of wings the insects were therefore entering into another phase to assist in plant reproduction. So also the plants were also evolving to avoid being victims of herbivores and desiccation especially the ones growing on the ground. Tall plants didn’t have problem a problem with the spore distribution spores as they would be blown by the wind but the sexual part with the transportation of sex cells by water meant that the sexual forms would remain shorter and closer to the ground exposing them to desiccation and being eaten by herbivores4. Ferns, horsetails and club mosses experienced this problem as they produced sexually through a structure called thallus which has sex cells on the lower surface where there is moisture all the time. After the female cell is fertilised it grows into a huge spore bearing plants4.
350 millions ago cycads evolved a different kind of reproduction by spores growing larger while still being attached onto the parent. They develop into conical-shaped structure with eggs. These spores called pollen are blown by wind and land on the egg bearing cones have a pollen tube which has secreted some fluid which aids it to swim to the eggs and fertilisation occurs. As this was happening various insects were evolving with modifications on wings formation. This was an advantage the plants were to make use for the transport of male reproductive cells (pollen) which are required to reach the female cells for fertilisation to occur and so new plants to develop. Wind dispersal of pollen was utilised by some plants like conifers but requires production of a lot of pollen4.
When insect were looking for food they could transport the pollen effectively; this was the point the plants exploited to have their pollen distributed. By alluring the insect with food the plants could have their pollen carried and this would mean less pollen production as compared to wind dispersal. This happened 100 million years ago with magnolias evolving flowers. Egg cells are clustered in the centre, each protected by a green coat with a receptive spike on the top called a stigma with which it receives pollen and is necessary for fertilization. Grouped around the egg cells with their stigmas are stamens which produce the pollen. This means the male and the female parts of the flowers are together but this posses the problem of cross pollination. This was solved by the egg and pollen cells maturing at different times. In order to attract the insects the structure is surrounded by brightly coloured petals and we have a complete flower. Beetles had learnt to feed on cycad’s pollen and were the first to venture to other plants like the magnolias and water lilies. They moved from one flower to the other collecting pollen meals but ended being covered by excess pollen which they delivered to the next flower visited4.
Different specialised methods of attracting insects for fertilisation purposes have evolved they include production of scented chemicals. Stepelia plants solicited flies for this purpose by evolution of flowers that mimicked the scent of rotting flesh and also had skin that looked like that of dead animal and generates heat to mimic warmth produced by decomposing flesh. Flies transport pollen and also lay eggs on these flowers. Others like the orchids evolved the sexual impersonation characteristic with one species having flowers that look like the female wasp even the eyes, antenna and wings and produce sex hormones produced by the female wasp. As the male “copulates” the flower, is covered by pollen and moves to the next flower to deposit pollen. 4
Other like the Yucca plant and the moth have a symbiotic relationship the flowering having pollen transported and the moth provide with food for caterpillar and a place to lay eggs4.
1. Wikipedia contributors. Horsetail [Internet]. Wikipedia, The Free Encyclopedia; 2006 May 4, 08:51 UTC [cited 2006 May 4]. Available from: http://en.wikipedia.org/w/index.php?title=Horsetail&oldid=51499558.
2. Wikipedia contributors. Lycopodium [Internet]. Wikipedia, The Free Encyclopedia; 2006 Apr 30, 17:41 UTC [cited 2006 May 4]. Available from: http://en.wikipedia.org/w/index.php?title=Lycopodium&oldid=50913472.
3. Wikipedia contributors. Fern [Internet]. Wikipedia, The Free Encyclopedia; 2006 May 2, 09:08 UTC [cited 2006 May 4]. Available from: http://en.wikipedia.org/w/index.php?title=Fern&oldid=51183313.
4. Knight, R. BCB Biodiversity chapter 2 The first Forests (Cited 2006 May 4) http://planet.uwc.ac.za/nisl/biodiversity/Chapter2/page_40.htm
VINCENT MUCHAI WAIRIMU
Biodiversity and Conservation Biology
University of the
Private Bag X17 Bellville