Why are insects so Successful? They are the most diverse and numerous animals on the planet, and some species survive virtually unchanged for 35 million years. Their high rates of speciation and persistence through time can be attributed to 6 factors (aka the winning formula):
1. Tough Exoskeleton
Having a tough outer cuticle is akin to having protective bone on the outside of the body (hence ‘exoskeleton’) and in addition to protecting from chemical and physical attacks, this ‘suit-of-armour’ also provides waterproofing and minimizes loss of body fluids (desiccation-proof). The exoskeleton is mostly made out of chitin, a material that allows both the flexibility of rubber and the hardness of metal. The integument is also capable of self-repair, making the exoskeleton a highly advantageous design in the harsher environments where selective pressures may force steps of evolution may take place.
For an insect to both grow in size and possess a rigid bone-like structure on the outside of the body requires replacement of the skin several times in a lifetime. In more primitive insect groups this incomplete metamorphosis occurs 2-5 times in a lifetime so a juvenile nymph may reach adulthood. Complete metamorphosis is required when different life stages of an insect require different forms and function for the habitat they exist in. The massive internal and external reorganisation using previously dormant DNA allows an adult (imago) to emerge from the pupa and fill it’s new ecological role. Through this method, the larval form can be adapted for feeding and growth whereas the adult form can be optimized for dispersal and reproduction. Insects can also use these 2 stages to fill niches occurring at different times of the year that would otherwise be unavailable.
3. Evolution of Flight
Insects are the only invertebrates capable of sustained flight. Flight gives an animal a highly effective means of escape from predators, transport, resource acquisition and dispersal and expansion of a species into new habitats. Natural selection has turned primitive gliding using extra-cuticular membranes (like in the stick insect) into a remarkably efficient means of locomotion, employing principles of aerodynamics that human engineers do not even fully understand. Adept fliers within the order Diptera have developed stabilising gyroscopes (known as halteres) which allow them greater balance and to rotate and switch direction much quicker. Due to efficient energy use during flight, insects such as the desert locust (Schistocerca gregaria), can sustain flight for up to 9 hours during a migration. Dragonflies such as the green darner (Anax junius) can carry up to 15 times their body weight whilst flying, and this is due to specialised muscle fibres and attachments. Flying insects have also been known to hover, and even fly backwards or sideways!
4. Small Size
The small size of insects has obvious advantages, including easier evasion from predators , minimization of resources needed for growth and resources and easy access into tight spaces for nesting or feeding. A smaller size, together with the hardened outer skeleton, also limits the amount of damage that can be sustained whilst falling- smaller objects gain less momentum. A small surface area of the insect body also allows gas exchange to occur all around the body via small holes (spiracles) with a short enough distance between trachea and body cells for oxygen to diffuse into them. One disadvantage of this however is that insects have a maximum body-area to surface ratio; the largest insect is the giant weta (smallest is the fairy fly, a chalcid wasp).
5. High Reproductive Rate
Coupled with the short lengths of each generation, a high reproductive rates allows an insect species to adjust to environmental changes much more rapidly and therefore undergo evolution faster. For example a blow fly (Calliphora sp.) can lay 144 eggs, and (assuming no mortality) within 6 generations one pair of flies can give rise to a progeny of almost 4 billion (luckily predation, disease and climate keep this in check!). Since most insects die before they can reproduce (like the 99.9% mortality rate in emigrating aphids) the effects of the “survival of the fittest” principle are amplified and insect species can evolve new adaptations quickly. This is why aphids can become resistant to pesticides within only a few generations.
6. Efficient Nervous System
In animals a nervous system is required to convert information about the environment into actions that will help that animal survive. Since insect organs are submerged in haemolymph, they can detect slight perturbations in food intake and environmental conditions. The organ tissues can cope quite well with fluctuations in osmotic pressure and ionic concentrations, and a well defined blood-brain barrier make the central nervous system of insects a highly specialised characteristic.
The success of the insects can be largely attributed to these 6 pillars of the winning formula, allowing individuals to: effectively sense and react to their environment, evade predators using armour, flight and a small size, take advantage of a breadth of niches and being able to give rise to new generations in a short time span.