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What Factors are responsible for the success of Insects?

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Introduction

What Factors are responsible for the success of Insects? Introduction "To a near approximation all organisms can be considered to be insects" (Gullan & Cranston, 1994). Estimates of species diversity varies, from less than 5 million to as many as 80 million species, with a number between 30 and 80 million being most likely, around half of global species diversity (Gullan & Cranston, 1994). Recent studies have estimated that 20 to 50 million insect species still remain to be described in the tropics alone (Brusca & Brusca, 1990) and the total number of insects on earth at any moment has been calculated at 1019 individuals (Berenbaum, 1995). Without a doubt, insects are the most successful group of organisms on the earth, both in terms of species diversity and number of individuals. But the radiation of insects is predominantly a terrestrial phenomenon. Limitations of the basic insect design have restricted the radiation of insect species into the marine environment, although many freshwater species do exist. Insect body design also constrains their possible size. Therefore, whilst insects have been successful in terms of diversity and overall numbers, the body plan that has made them so successful has prevented radiation into the sea or the evolution of larger individuals. But the smallness of their size is one of the major reasons for their success. Size and Body Plan Insects range in size from small midges less than 0.1 mg is size (some 'fairy flies' are smaller than a one-celled Protozoan) to scarabeid beetles which weigh 30 g - as much as a mouse. Hence they are generally small. Fossil evidence shows that insects were once much larger: a Permian dragonfly, Meganeuropsis americana, had a wingspan of 71 cm; but such insects are now extinct, probably because they were unable to compete with other, smaller rivals. Insect size is limited by a number of factors, most of which can be traced back to the surface area/volume ratio. ...read more.

Middle

They have proved remarkably able to do this. Vertebrates, on the other hand, are larger and can control their internal environment with a high degree of precision that makes them much more independent of the external environment. Because they are longer lived, they generally adapt to change by some degree of learning (Gullan & Cranston, 1994). Insects, however, normally adapt to their environment through genetic change (for example the evolution of insecticide resistance as a result of the application of insecticides). Most insect species are genetically highly heterogeneous (i.e. a lot of variation exists in the species). Therefore, when a change occurs some individuals will be better adapted than others will, and, as a result, the species will survive the change - it is persistent. Speciation may often result if the surviving groups are isolated spatially or temporarily. These characteristics of insects are another potential diversifying influence that may account for the species richness of the insects (Gullan & Cranston, 1994). Why do insect species have a high level of genetic heterogeneity? The answer to this question is based on their reproductive cycles. Insects reproduce sexually between different male or female individuals and this produces far more variation than asexual reproduction would. But insects also reproduce prolifically. For example, a termite queen may produce hundreds of thousands of eggs in her lifetime: creating a colony of up to a million individuals. The generation time of most species is often very short: many insects reach maturity within a matter of days (compared with 14 years for a human). In Drosophila a generation (egg - adult - egg) can take as little as two weeks, that is 25 generations a year with each female producing 100 eggs. So starting from a single pair, and assuming all offspring survive and reproduce, 1041 flies could be produced in a single year. This is enough to form a sphere with a diameter equal to the distance from the earth to the sun (Borror et al, 1976). ...read more.

Conclusion

The reason why is clear. Sexual reproduction is just as important for plants as it is for animals, because it generates considerable variation. But plants are unable to pass gametes from one individual to another: they rely either on abiotic factors or animals for pollination. Undoubtedly the latter are far more reliable, and insects, because they can fly and because they are small enough to enter flowers, have become the primary agents of pollination in flowering plants. Evidence has shown that plants have evolved to adapt to the insects' limitations, rather than the other way around. Angiosperms provide nectar as an incentive for the insect to continue to visit flowers of the same species, thereby bringing about pollination. Coevolution over many millions of years has produced the close relationships that are seen between some angiosperms and insects - particularly in the orchid family, which is one of the most diverse groups of angiosperms. Conclusion The success of insects relies on many factors, including the small size of individuals, their short generation time, sensory and neuro-motor sophistication, evolutionary interactions with plants and other organisms, metamorphosis and production of winged adults. Of these the coevolution with angiosperms was probably the most important, and this relied upon the foundation of features that already existed within the insects when the angiosperms arose. For example, the small size of insects makes them ideal for entering flowers (larger animals would need larger flowers that would take more resources to produce). Their short generation time allowed rapid evolution alongside that of the angiosperms. With advanced sensory organs, particularly of sight and smell, insects could detect the complex flowers and scents plants evolved to attract specific individuals. Metamorphosis of insects was necessary because the angiosperms do not produce flowers all year round. The insect life cycle allows for this, so that while the plants are not flowering the insects survive as larvae and pupae waiting for the next year when the plants flowered again. Lastly, of course, wings were an essential adaptation to reach the flowers and to transport the pollen over the distance from plant to plant. ...read more.

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