Nature 2022 April

The domain of insects

It is probably hard to believe that over half of all plants, animals, fungi, bacteria and virus species so far identified are insects. Insects represent 56% of all species and a mere 10 trillion in numbers, that’s 1.4 billion insects per human. Over one million insect species have so far been classified and maybe as many as that again remain to be found. The dominance of insects can be further displayed by observing that threequarters of all animals are insects. Birds and mammals, which perhaps are the most visible in our daily lives, account for less than 2% of all species.

Defining an insect can be problematic. Top of the list of criteria are possessing six legs and having a body divided into three sections, during at least part of its life cycle. Wings and external mouthparts, when present, also point towards insects but the latter is not infallible. Adaptation in form though can cause confusion between insects and examples from closely related invertebrates.

We humans could not survive without plants, primarily for producing oxygen by removing carbon dioxide from the atmosphere. However, as many as a quarter of a million plants, including most of those on which we rely on for food, require insects to pollinate their flowers. Furthermore, without insects, vast areas of land in certain regions of the world would very soon be overwhelmed by animal dung and putrefying dead animals. In contrast to those beneficial insects, one large swarm of African locusts can consume more agricultural food in a day than a city the size of London. Mosquitos carrying malaria are normally responsible in a single day for more human deaths than any other single cause.

Ancestors of insects belonging to the animal group Arthropoda were amongst the primordial animal representatives that moved onto land some 540 million years ago. The first primitive insects appeared some 400 million years ago. With tough exoskeletons, insects were able to develop stronger muscles, which could be anchored to a lightweight yet rigid external structure. Firstly, this provided the opportunity to enable greater mobility and speed of movement, both to avoid predation and to be a predator themselves. Secondly, and probably more significantly, these characteristics meant that insects were the first animals to take to the air. The evolution of wings was in the scale of overall time very fast, occurring around 300 million years ago and was a direct consequence of a rapid increase in oxygen by a rapid expansion of green plants. This oxygenrich environment led to an era of the gigantic insects, the largest of which was a dragonfly with 30-inch wingspan!

With this dragonfly in mind there remains today some debate as to how such insects evolved wings. The theory favoured by most leading entomologists involves articulated appendages with muscles which had already evolved whilst insects enjoyed both a terrestrial and amphibious phase in their lifecycle. If you have ever seen the nymphs of dragonflies or beetle larvae in a pond you will have noticed the abdominal flanges that act as gills but also enable rapid movement. Even today, certain insects like water skaters, rather than opting to fly, are seen on ponds orientating their wings like sails using surface breezes to skate across the surface of the water.

Interestingly, 200 million years ago and prior to the Jurassic period and age of the dinosaurs, there would have been no pesky flies to invade a prehistoric larder. Likewise, wasps visiting a picnic in a forest glade would have to wait for another 40 million years!

Almost as soon as some insects developed wings others began to lose them. A typical insect has two pairs of wings but true flies have only one pair. Their hind wings have disappeared save for a tiny stump with a bulb on the tip which plays an important role in providing a counterweight to the lift provided by their wings. Many insects have sacrificed their wings either partially, as in all but the reproductive phase of modern-day ants and termites, or totally like fleas, thrips and lice. With the emergence of large herds and communities of mammals and birds it is not a surprise that fossil records show these animals were accompanied at close quarters by fleas. Who needs wings when you are carried around and can hop from one host to another?

The most important event to transform insects into the largest group of animals was the emergence of flowering plants between 120 to 80 million years ago. The blossoming of flowers enabled the evolution of bees into specialised nectar feeders and pollen collectors and led to the great expansion of butterflies, flies, beetles and other bugs.

The disappearance of dinosaurs 60 million years ago allowed plant-eating reptiles and emerging larger mammals to open the savannah lands, providing new habitats for ants and termites, forsaking their four wings to live underground in very large colonies.

Lepidoptera – butterflies and moths – display the most diversity of form and habitat. Key features are the flamboyant colours and iridescence on wings and body resulting from sophisticated scales and hairs and highly specialised mouthparts in the form of a long, coiled tube or proboscis. The lifecycle of lepidoptera is also highly advanced with camouflaged eggs and caterpillars specialised in quickly building up bulk and nutrients and developing chemical defences against predators. A chrysalis provides a safe environment for an amazing transition from larva to imago (adult butterfly or moth). Female adults developed pheromones to attract males.

Bees and wasps are the most advanced families in the insect kingdom. They are strong fliers, which often, but not always, live in large colonies or social groups of up to 80,000, housed in sophisticated accommodation and exhibiting a wide division of labour and communication between individuals. Caring for offspring is a feature of most bee, wasp and ant species. Some (ants and wasps) are predators and most can sting or bite in predating or defence.

After the establishment of insects as a dominant group of animals it would be a further 55 million years before man’s earliest ancestors began to appear. As our race became civilised, many insects have moved in with us, like cockroaches, silverfish, earwigs, death watch beetles, ants, lice (human, cat, dog, and book), fleas, flies, weevils, bees and wasps, hibernating butterflies and ladybirds, overnighting moths and others.

In the UK there are 22,500 insect species of which just over 50 are native butterflies, 2,500 moths, 40 dragonflies, 35 crickets and grasshoppers, 300 flies, 4,000 beetles, 250 bees, 7,000 wasps and 60 ants – more than enough variety to look out for a few species of each in our villages, woodlands, ponds or fields.

Chris Brown BEM
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