Insects, arachnids and myriapods

On a pale sheet, colourful insects painted side by side: a cicada with transparent wings, several beetles including a longhorn beetle, and a spider. — Studies of insects painted in watercolour by Johannes Bronckhorst: a cicada, several beetles, a ladybird, a spider and a stick insect. Johannes Bronckhorst, CC0, source

If an encyclopaedia followed the number of species, most of its pages would go to six-legged animals.

Books do not do this. Neither does science.

This page exists to correct that.

The real proportions

Our page on classification (5.1) establishes a fact documented by the biologist Robert May: the worldwide effort to describe species is divided roughly equally among three large groups — vertebrates, plants and invertebrates.¹

Yet:

Plants number about ten times more known species than vertebrates.
Invertebrates, about a hundred times more.

A third of the research budget and time for each group, even though one of them is a hundred times larger than the others.

Why this bias exists

This is not an interpretation: it is the researchers’ explicit finding. Mammals and birds are better studied because their features resemble our own.²

We have eyes, they have eyes. We have a mouth in roughly the same place. They show attitudes that we recognise: fear, hunger, attention. A spider or a millipede has none of this in a form familiar to us.

We study first what we recognise.

This is not a moral failing. It is a cognitive and institutional bias: a leaning that steers research without researchers always being aware of it.

“An inordinate fondness for beetles”

A famous retort is attributed to the British biologist J.B.S. Haldane. Asked by theologians what the study of nature had taught him about the Creator, he is said to have replied that the Creator must have “an inordinate fondness for beetles”.

Here is what the history of science can establish about it:

The substance is sound: Haldane expressed this idea on several occasions; the palaeontologist Stephen Jay Gould traced it back in 1993.
The exact wording varies: The word-for-word quotations circulating in popular literature derive from later, secondary transcriptions.
The setting is probably apocryphal: The scene with theologians comes from a paper by the ecologist G. Evelyn Hutchinson, who himself presents it as possibly apocryphal.³

The fact behind the quip

At the time Haldane was speaking, about one in four described animal species was a beetle.

Today, the orders of magnitude remain striking. Insects are the most diverse group of animals on the planet: about one million named species — compared with some 6,500 species of mammals.⁴

On their own, they account for 85% of arthropod species, two thirds of animal species, and nearly half of all species known on Earth.⁵

Beetles, the subject of the quip, account for around 40% of described arthropod species — insects, spiders and millipedes included.⁶

What the counts hide

A large proportion of catalogued insect species are known from very few specimens, sometimes from a single specimen kept in a museum. Most of the inventory rests on observations that have not been repeated.⁷

What we have counted depends on where we have looked.

And how many are there really?

Estimates have shifted a great deal, and in a direction worth noting. A few decades ago, thirty million insect species or more were put forward, based on reasoning about the number of insects specific to each plant species.⁸

Recent work considers these figures highly unlikely. By combining several independent methods, a 2018 synthesis converges on about 5.5 million insect species — including one and a half million beetles — and seven million terrestrial arthropods in total.⁹

With one million named species, this means that 80% of insects remain to be discovered.¹⁰

The direction of the revision is worth remembering: here, the estimate was divided by five. A figure that moves is not necessarily a figure that rises.

Three groups named in the title

This page has three names in its title. It would be paradoxical to deplore the invisibility of these animals without ever saying what they are.

All three belong to the same large group, the arthropods: animals with an external, jointed skeleton that has to be shed in order to grow. None of them grows gradually as we do. They moult — they climb out of a shell that has become too small, and the new one hardens in the air.¹¹

	Legs	Body	Antennae	Wings
Insects	6	3 parts	1 pair	often
Arachnids	8	2 parts	none	never
Myriapods	many	numerous rings	1 pair	never

Sources for the table¹²

Counting the legs is enough to tell them apart in almost every case. A spider is not an insect, and neither is a millipede.

A millipede never has a thousand legs. Depending on the species, they number from a few dozen to just over seven hundred. The name is an old exaggeration that has stayed in the language — like the catfish, which is not a cat.

Two lives in one

This is what makes insects so hard to compare with the rest of the animal world.

In many of them, the young animal and the adult look nothing alike. A caterpillar and a butterfly are the same individual. So are a white grub in the soil and a beetle.

Between the two, a profound reorganisation of the body: this is metamorphosis.¹³

This is not a mere curiosity. It changes the way the species occupies the world:

The caterpillar eats leaves; the butterfly drinks nectar.
The larva lives in water; the adult flies.
The two forms do not compete for the same food or the same space.

A single animal exploits two environments in the course of its life, without ever competing with itself. This is one of the explanations put forward for their extraordinary diversity.

Not all species work this way: in grasshoppers or true bugs, the young already resembles a smaller version of the adult and changes through successive moults, without upheaval.¹⁴

They flew first

One last fact, which puts things back in proportion as surely as the counts at the beginning of this page.

Insects conquered the air perhaps ninety million years before the first flying vertebrate. When the flying reptiles appeared, the air had been occupied for a very long time.¹⁵

So these are not small animals added at the margins of a world of vertebrates. They were there before, in greater numbers, and in more places.

Why this matters

What we learn about the living world shapes the idea we form of it. If popular imagery shows only furred or feathered animals, we build the illusion of a world populated by large animals, in which insects are mere background scenery.

That world does not exist.

This bias has concrete consequences for conservation choices, research funding and the perception of collapses in biodiversity. We protect only what we see.¹⁶

Correcting a proportion is not an exercise in bookkeeping: it is realigning our picture with reality.

Note 1

Part of the problem is that taxonomic effort is approximately divided 1∶1∶1 among vertebrates, plants, and invertebrates, whereas plant species are roughly 10 times, and invertebrates 100 times, more numerous than vertebrates […]

May, Robert M., “Why Worry about How Many Species and Their Loss?”, PLoS Biology, vol. 9, no. 8, e1001130, 2011