Biological success, as measured in terms of Darwinian fitness, refers to the ability of an organism to survive and reproduce successfully. Differential reproductive success is the key to a species' evolution over time. Individuals with the most favorable traits in a particular environment will be able to survive and leave more offspring. This is deemed "survival of the fittest" and is due to a mechanism called natural selection. Charles Darwin described natural selection as the means by which organisms evolve. Based on this description, organisms with some adaptive advantage will be the most successful. In the animal kingdom, a myriad of examples exist of organisms with interesting and unique adaptive traits.
One group of animals, though stands out the arthropods, specifically, the insects. There are believed to be a billion billion (10^18) arthropods on Earth. Two thirds of ALL species known are arthropods. Amazingly, more species of insects exist than ALL other organisms combined. (1) Insects exist in virtually all habitats on Earth whether terrestrial, aquatic, or in the air. This being said, since biological success is a numbers game, the insects win hands down.
Why are insects so successful, you may ask? Favorable adaptations abound in this group, flight being the most significant. The ability to take flight allows insects to exploit more resources in more locations, while at the same time escaping predators and finding mates. The wings of an insect are actually not appendages, but instead, are extensions of the cuticle, which make up the exoskeleton of arthropods. The exoskeleton is composed of a rigid polysaccharide known as chitin that provides a protective covering for these organisms. Extensions of the cuticle formed at some point in the evolutionary past of insects that allowed them to take flight. This left the true appendages free to specialize in order to utilize a wide variety of food sources, to forage, to act as a means of locomotion, to collect sensory input, to mate, or to defend the insect from predators.
Another interesting adaptation seen in most insects is metamorphosis. Metamorphosis involves having several developmental stages through which the organism moves during its life cycle. The most familiar form of metamorphosis in insects is called complete metamorphosis and involves eggs hatching into a larval or worm-like form that later becomes encapsulated in a cocoon or pupa. The adults emerge from the pupal stage with a quite different appearance from that seen in the larvae. The metamorphosis seen from caterpillar to butterfly provides a common example of this phenomenon. Having different life stages allows insects to again utilize resources in such as way as to maximize success. The main job of the larval forms of insects is to feed and larvae have very different food requirements than the adult forms. Adults are more interested in the reproduction and propagation of the species. There are some adult insects who lack mouth parts entirely so specialized is there reproductive purpose.
Camouflage, mimicry, and amazing defensive capabilities are other significant advantageous and adaptive traits. Camouflage, also known as cryptic coloration, provides a classic example of natural selection in insects. The peppered moth studied by H.B. Kettlewell, is famous. In this study, Kettlewell found that the dark form of the moth became favored due to environmental pollution resulting in an evolutionary shift in the population. Other interesting examples of camouflage can be seen in the mantids. Not only do mantids show cryptic coloration, but also they have a wide variety of structural adaptations. They have evolved cuticle projections that resemble flowers as seen in the Malaysian flower mantid, sticks as in an African form, and leaves as in the Peruvian leaf mantid. (1&2) Mimicry also increases the fitness of many species of insects. Mimicry involves a nontoxic, possibly palatable species resembling the appearance of a toxic, unpalatable species. A common example of this is the Monarch Butterfly and its mimic the Viceroy Butterfly. By mimicking a toxic species, the Viceroy increases its fitness and its ability to pass on this favorable adaptation to its offspring. Another interesting aspect of insect defense is the use of chemical "warfare". One of the best-known insects for this is the Bombardier Beetle. This beetle stores two chemicals that when mixed generate excessive heat and a foul-smelling spray that is aimed at a likely predator. Darwin, himself, was an avid beetle collector and was believed to have wrangled a Bombardier beetle. (3)
Whether it be flight, a protective exoskeleton, specialized appendages, differing developmental life stages, or camouflage, the many adaptations of insects have allowed them to fill the niches our Earth has to offer. With vast species diversity, a huge variety of habitats, and unrivaled numbers, the insects are the most biologically successful group of animals on Earth. (1)
1) Campbell, Neil and Reece, Jane. Biology, 7th edition. Benjamin Cummings, San Francisco, 2005.