Insects have become so successful because they have adapted fully to the terrestrial environment, and the exoskeleton is one of their major adaptations that has made this possible. It is a great invention; sturdy but lightweight, it provides protection against bumps and collisions for small bodies, it allows for muscle attachment and the leverage of jointed limbs, it prevents drying out and it forms the substance of the insect wing, another of the major reasons behind their success.
However, any exoskeleton has one major disadvantage, which is another reason why insects are small. An exoskeleton cannot grow along with the organism; it must be shed to increase in size. So all arthropods periodically shed their exoskeleton during a process called ecdysis. The new cuticle is initially flexible – to allow the organism to expand to a new size – and then hardens. For a large insect this represents a problem because the pull of gravity is so great that a soft, newly moulted insect runs the risk of collapsing in on itself before its cuticle can harden (Berenbaum, 1995).
Being small has other advantages as well. Small animals do not fall as hard as large ones – hence insects can fall off high objects safely: they experience different forces than larger animals. For example, flies can walk up vertical surfaces because the adhesive forces are sufficient. Pond Skaters can live on the surface of water bodies because the surface tension is large enough to support their weight. Not all these forces are advantageous: for example, air resistance has a greater effect on small animals than large.
Tagmosis is the grouping of adjacent segments into sections with particular functions. Insects have three – the head for feeding and reception of stimuli, the thorax for locomotion, and the abdomen for reproduction and digestion. The evolution of tagmata allowed for more efficient performance of tasks, for example feeding.
Small animals do not require as many nutrients as large: hence there are many more niches in any environment for small organisms than for larger ones. Consider a single acacia tree (Gullan & Cranston, 1994) that could provide a single meal for a giraffe. The same tree could support a whole ecosystem defined by the life cycles of the dozens of small insect species that live there and their interactions with the tree. Amongst these many species might be, for example, a lycaenid butterfly whose larvae chew the tree’s leaves, a bug that sucks the stem sap, a longicorn beetle that bores into the wood, a midge that galls the flower buds, a bruchid beetle that destroys the seeds, a mealybug that sucks the root sap, and several wasp species that parasitise on these herbivores. Another tree might feed the same giraffe but would have a totally different combination of insect species. Not only does the small size of the insects contribute to this effect but also the evolution of a multitude of insect feeding strategies (Brusca & Brusca, 1990) through the modification of the basic insect body plan.
Modifications of insect feeding parts produced organs able to collect food from a variety of different sources, leading to a diversification of insect species. For example, it is estimated that half of all insects feed on the living tissues of higher plants (phytophagy), but these insects come from only 9 of the 29 insect orders. This suggests that in those insect groups that have successfully breached plant defences, through the evolution of specialised mouth parts, a subsequent radiation occurs, resulting in a much greater diversity of species (Gullan & Cranston, 1994). The Orthoptera (locusts, grasshoppers and crickets) have evolved mouthparts adapted to cutting off and chewing plant matter. These animals consume green plants very efficiently. Other insects have evolved sucking mouthparts (e.g. butterflies for feeding from flowers) that may also be able to pierce the victim’s epidermal tissue (e.g. aphids and mosquitoes). Many flies have a fleshy porous labellum that they use for ‘sponging’ up fluid nutrients (Brusca & Brusca, 1990). These examples demonstrate a little of the diversity that exists from modification of a common basic design. With continued specialisation of different individuals many new species can arise, with no two species occupying the same ecological niche. Many of these specialised niches have minimal levels of available nutrients, and so only small well-adapted animals can survive in them. It is not surprising, therefore, to find that insects are found just about everywhere. Some live in Antarctica, in cracks in the snow, others in the hot springs of Yellowstone at temperatures approaching boiling. Insects can live in horse intestines, in acidity levels comparable to vinegar, or in petroleum in oil fields. Some even live in the sea as lice in the nostrils of sea lions and stay dry as they accompany their hosts of deep dives underwater. These parasitic organisms use their small size to invade their hosts.
As has been shown, the small size of insects has been crucial in their success and has been an important determinant of species richness. Because they are small they can divide up their environments into far smaller liveable pieces than can larger animals, as more opportunities exist for successful establishment. As a result there are more different ecological niches they can fill, leading to a greater diversity of species.
Adaptation, Reproduction and Life cycles.
However there is a catch here. “As organisms get smaller, their control over their individual environment decreases” (Dan Jenzen, Berenbaum, 1995) – therefore insects must adapt to their environment, rather than adapt it to them. 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). Similarly locust swarms can arise within a matter of weeks and may contain up to 10 billion individuals (Berenbaum, 1995). All these factors contribute to give a high level of genetic heterogeneity within an insect species. Because reproduction occurs so quickly and in such great numbers a lot of new individuals are produced, each of which is genetically different because reproduction occurs sexually.
Insects have evolved other adaptations to ensure that reproduction is as successful as possible. The development of internal fertilisation is essential for any fully terrestrial organism. The alternative is indirect sperm transfer, as is practised by most arthropods: for example crustaceans simply discharge sperm into the water near a female, whilst some spiders package their sperm for collection by the female. These methods are all indirect, and, therefore, subject to environmental unpredictability. As has been stated: small organisms are much more at risk from changes in their environment. Direct sperm transfer is more reliable and hence more efficient. It also allows organisms to be fully terrestrial: they have no need to return to water to breed (like amphibians). Another adaptation to terrestrial life is the production of eggs resistant to desiccation, temperature, and so on. For example, the praying mantis equips its eggs with a layer of insulating foam to protect against the cold winter temperatures the eggs must survive before they can hatch in the spring (Berenbaum, 1995).
Considerable diversification has occurred in the way internal fertilisation is accomplished within the insect world. This diversification is necessary because if genitalia are sufficiently complex then only members of the opposite sex of the same species will be able to copulate successfully. Thus diversity of insect genitalia could be another reason for the huge number of different species that exist (Berenbaum, 1995). Often insect genitalia alone are used to identify different species!
Some interesting reproductive strategies have evolved, which, in part, may account for the success of many insect groups. Parthenogenesis (production of individuals from unfertilised eggs) is common. For example, in aphids female diploid individuals can be produced in vast numbers by parthenogenesis, so that when a male arrives he mates with many individuals producing vast numbers of offspring by sexual reproduction. Some species of tiny parasitic wasps go in for polyembrony: when each cell of an embryo (consisting of many cells) produces one individual. Members of the order Hymenoptera (ants, bees and wasps) produce haploid males by parthenogenesis.
Most insect species have two or more life stages, separated by a period of metamorphosis when they undergo a considerable change in body plan. In hemimetabolous development (= incomplete metamorphosis) growth occurs by gradual changes, but in holometabolous development (=complete metamorphosis) growth occurs by a series of dramatic metamorphoses (Brusca & Brusca, 1990). The success of the holometabolous lifestyle is demonstrated by the fact that such species outnumber hemimetabolous insects by a factor of 10 to 1 (Brusca & Brusca, 1990). Holometabolous development is selectively advantageous because it results in the ecological separation of adults from young, thus avoiding intraspecific competition and allowing each stage to develop its own suite of specific survival strategies.
In many insects this bizarre life cycle is adapted so as to make best use of the animal’s environment. For example, under some circumstances, insects can develop continually – for example the larval stage surviving underground in the winter, and then moulting in the summer into the final adult form (e.g. Blister beetles). Many insects survive unsuitable periods as pupae (e.g. the apple blossom weevil lives on apple buds and is inactive for 10-11 months each year). Such periods of inactivity are called a diapause (a physiological state characterised by a cessation of development, a build-up of fat reserves, a reduction in body water and a physiological resistance) and they allow insects to survive unfavourable periods and hence inhabit places that would otherwise be hostile. For example, to withstand cold temperatures many species accumulate large amounts of glycerol, an antifreeze, or produce compounds that protect them from damage caused by ice crystal formation inside cells: so that they can actually freeze up to 90% solid and then thaw out without incident (Berenbaum, 1995).
Behaviour
Organisms must be capable of recognising and responding to environmental differences, to make best use of it. Insects have developed more highly advanced sensory and neuro-motor systems than most other invertebrates, perhaps comparable with those of lower vertebrate animals (Gullan & Cranston, 1994). Ants have been shown to exhibit highly ‘intelligent’ behaviour: some species ‘farm’ aphids by using them to obtain plant sap and protecting them from predators such as ladybirds. Some insects live in a totally different sensory world than us. Many butterflies see patterns on flowers in ultraviolet light, whilst others call to each other in frequencies we cannot hear. The impressive sound of cicadas calling to each other is familiar to many people. Other insects use scent, not song, to attract mates, and they are capable of detecting as little as 100 molecules of pheromone in 1 ml of air (Berenbaum, 1995).
True social behaviour is exhibited by the Hymenoptera (bees, ants and wasps) and Isoptera (termites) and is thought to have evolved at least ten times (Berenbaum, 1995). Social behaviour has enormous advantages for those species that evolve it, including increased defence, specialisation of individual workers (as soldiers, child-minders, hunters etc.), co-operative engineering and co-operative hunting (e.g. bees inform their fellow workers where to find food). This is evolutionarily possible because all the individuals in the colony arise from the same queen, and hence share about 50% of their DNA. The evolution of social behaviour enabled insect species to expand to fill another available ecological niche: by functioning as a collective they are able to achieve much more than they could individually. The intelligence and level of co-ordination shown by these animals is still to be admired and perplex today.
Concluding Factors
Not only are insects the most successful animals, in terms of species diversity, but the phylum, Arthropoda, to which they belong, is also the most successful. There are only about 50,000 vertebrate species in the world, compared with a similar number of crustaceans and about 60,000 arachnids. Many of the factors listed above, such as tagmosis, advanced sensory organs (e.g. compound eyes), small size, high reproductive rate and the exoskeleton are common to all and are the reasons behind the success of the phylum as a whole. Comparison between the insects, crustaceans and arachnids is useful and can be revealing. Three factors stand out: arachnids, in contrast to the most successful insect groups, lack winged flight, lack the complete transformation of the body form during development (metamorphosis) and usually lack dependence on specific food organisms. Mites, the most diverse and abundant of arachnids, are exceptional in having many very specific associations with other living organisms (Gullan & Cranston, 1994).
Flight:
“The evolution of wings and flight undoubtedly contributed to the unparalleled success and diversity of insects” (Kingsolver & Koehl). The fossil evidence shows two radiations of insects. The first coincides with the evolution of the pterygota (winged insects) beginning in the Palaeozoic, although how this occurred is a matter for debate. A comparison of the Pterygota with a sister group, the wingless Apterygotes, reveals that the winged insects are much more speciose (Gullan & Cranston, 1994). The conclusion is unavoidable: the possession of flight correlates with the first radiation of insects.
Flight provides insects with the increased mobility necessary to use patchy food resources and habitats and to evade non-winged predators (Gullan & Cranston, 1994). By flying, insects are able to reach faster speeds than they could on land, for example, horse flies fly at 15mph and have a range of 60 miles (Berenbaum, 1995). Compare the top insect running speed of 10 cm/s (0.23mph) with the flying speed of the Australian Dragonfly at 36mph. Flying insects can find food and mates over much greater distances. Some insects, for example monarch butterflies, migrate to avoid unfavourable conditions. These abilities enhance species survival, but wings also allow insects to reach new habitats by dispersal across a barrier resulting in speciation and hence diversification.
The success of flying animals can also be seen in the vertebrates, where a similar parallel exists. Birds are the most diverse vertebrates (about 8500 species out of 50,000 vertebrates) just as winged insects are the most diverse arthropods.
Angiosperms:
The second radiation of insect species occurred during the Cretaceous (135-65 million years ago). Fossil evidence indicates that the numerical dominance of insects coincides with the angiosperm diversification at this time (Gullan & Cranston, 1994). By the end of the period the fossil insect fauna looks quite modern. As stated above, at least half of all insect species are phytophagic and many more feed on these plant-eating species. Interactions with other organisms, such as plants in the case of herbivorous insects or hosts for parasitic insects, are thought to promote genetic diversification of eater and eaten, called coevolution. The reciprocal nature of these interactions may speed up evolutionary change in one or both partners or sets of partners, perhaps leading to major radiations of these groups (Gullan & Cranston, 1994). Clearly insects and angiosperms (flowering plants) coevolved from the moment the latter arose. Indeed, flowering plants (in their current form) would not have arisen if insects had not already existed: because flowers are primarily designed to attract insects.
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.
References:
Berenbaum, M. R. (1995) Bugs in the System.
Brusca, R. C. and Brusca, G. J. (1990) Invertebrates.
Chapman, R. F. (1982) The Insects: Structure and Function. (3rd edn).
Gullan, P. J. and Cranston, P. S. (1994) The Insects: an Outline of Entomology.
Kingsolver, J. G. and Koehl, M. A. R. (1994) Selective factors in the evolution of insect wings. Ann. Rev. Ent. 39: 425-451.
