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The first insects

about 400 million years ago Dry land

In short

The oldest fossils thought to be insects are about 400 million years old and come from Rhynie, in Scotland. Insects are related to spiders, crabs and millipedes: they all have jointed legs and a hard outer shell. An insect's body is divided into head, thorax and abdomen, with six legs. When wings appeared, insects could quickly reach new places to live.

Why it matters Insects would become the animal group with the largest number of known species.

Source: Nature, 2004, “New light shed on the oldest insect”

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Illustration: The first insects (Dry land) — Illustration created with AI for Storiascopio

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The story

Small, countless, everywhere

Look around you: a fly on the window, an ant on the ground, a mosquito in the evening. Insects are the most numerous group of animals on the planet. More than a million species have been described, more than half of all the living things we know, and scientists think there are many more still to be discovered. They are found almost everywhere: in deserts, forests, caves, cities. How did they begin?

Arthropods: animals in armour

Insects belong to a big group, the arthropods, which also includes spiders, scorpions, crabs, shrimps and millipedes. They have one thing in common: an outer skeleton, a kind of armour made of a hard substance called chitin, and jointed legs, that is, legs divided into segments. The name comes from this, from the Greek for “jointed feet”. To grow they have to change their armour, in a process called moulting.

A true insect has its body divided into three parts: head, thorax and abdomen. It has six legs attached to the thorax and, usually, two antennae. Many also have wings. According to DNA studies, the closest relatives of insects are the crustaceans: flies and ants are therefore, in their own way, cousins of shrimps.

The Rhynie fossil

The oldest fossil attributed (but disputed) to an insect comes from Rhynie, in Scotland, the same place where the very ancient plants we talked about in the card on the first plants are preserved. In the Rhynie chert, about 407 million years old, the fragment of a tiny animal, a head with two mandibles, was collected in 1919 and named Rhyniognatha hirsti in 1928. In 2004 two entomologists re-examined the mandibles: they had a shape typical of flying insects, and for this reason the fossil was considered the oldest insect in the world.

But the fossil is small and very little of it is left. In 2017 two German researchers, Joachim and Carolin Haug, re-analysed the remains with more modern tools and suggested that it might not be an insect but a myriapod, that is, a relative of centipedes and millipedes. The question is still open. So today, when it is said that insects are about 400 million years old, we have to add that the date is disputed.

Other clues come from DNA. By comparing the genes of insects today and estimating how fast they change, in 2014 a large group of researchers calculated that insects might have originated about 480 million years ago, in the Ordovician. The undisputed hexapods (the group that includes insects) are instead those of Rhynie, about 405 million years old, such as the little springtail Rhyniella; at Gilboa, in New York State, there are arthropod cuticles about 385 million years old that perhaps include ancient insects, but the interpretation is provisional.

The invention of wings

Insects were the first animals to fly, and they did it long before birds, bats and pterosaurs. The first certain winged insects appear in rocks of about 325 to 330 million years ago, in the Carboniferous. How wings arose is one of the big mysteries of biology. One hypothesis says they come from small outgrowths on the sides of the body that served for gliding or for controlling temperature; another says they come from the gills of water-living ancestors. Studies of genes have given arguments for both ideas.

Wings let insects escape predators, look for food in faraway places, and reach plants and flowers. They were a huge success. In the Carboniferous some insects became gigantic: the dragonfly-like insects of the Meganeura group had wings up to about 70 centimetres across, among the biggest ever had by an insect: like those of a small bird of prey. Scientists think that the large amount of oxygen in the air at that time, and the lack of predatory birds, allowed these giants to grow.

Why it happened

Why did insects come out of the water? Plants had by now arrived on dry land, and with plants there was food: leaves, spores, sap, decaying remains. The ancestors of insects, small animals of the water or of wet places, found a rich environment with no competitors.

The outer skeleton was an advantage. The armour, covered in waxy substances, stops the loss of water, which is essential in the air. Insects breathe through small holes on the sides of the body, the spiracles, connected to tiny tubes that carry air to all parts of the body. This system works well in small animals and explains, among other things, why insects do not grow enormous.

Small size was another advantage: an insect can live in a corner of a leaf, eat little and reproduce fast. This favoured the evolution of thousands of different forms.

What changed

Insects changed the ecosystems of the Earth. When, much later, flowers appeared, about 130 million years ago according to fossils, bees, butterflies and beetles became pollinators, that is, they carried pollen from one flower to another. Much of the fruit and vegetables we eat depends on them. Insects are also food for birds, fish, frogs and bats, and they break down leaves and dead animals, returning the substances to the soil.

For human beings insects are both allies and enemies. They produce honey, wax and silk. But some carry serious diseases, such as malaria, carried by certain mosquitoes, and others destroy crops. The struggle against mosquitoes and locusts has gone along with the history of civilisations.

Today many species of insects are in difficulty because of pesticides, loss of habitats and climate change. Some studies in Europe have shown a sharp fall in the total weight of flying insects in certain areas. Protecting them means protecting the food we live on.

Key people and places

People

Michael Engel and David Grimaldi: American entomologists who in 2004 re-examined Rhyniognatha (described in 1928) and considered it the oldest known fossil insect.
Joachim and Carolin Haug: German palaeontologists who in 2017 suggested that Rhyniognatha might be a myriapod and not an insect.
William Mackie: Scottish geologist who in 1912 discovered the Rhynie chert, where plants and animals 407 million years old are preserved.

Places

Rhynie, Aberdeenshire (Scotland): A village near Aberdeen where the chert preserves a whole ecosystem 407 million years old.
Gilboa, New York (United States): A place with the remains of a forest 385 million years old, where fragments of arthropods are found, perhaps including very ancient insects (a provisional idea).
Commentry, France: A Carboniferous deposit where the fossils of the big Meganeura dragonfly-like insects were found in the 19th century.

Good to know

Meganeura, with a wingspan of up to about 70 centimetres, was not a true dragonfly but a close relative: it belonged to a group of flying insects that is now completely extinct.
According to a widely used estimate, about ten quintillion insects live on Earth: roughly 1.4 billion for every human being.
Insects breathe without lungs: air enters through small holes in the body and reaches the cells through tiny tubes.
Ants have lived in large, organised societies, with queens and workers, for more than 100 million years.
The word “insect” comes from the Latin insectum, “cut into”, because the body of these animals looks divided into segments.