Search for “insects” and the questions underneath are always the same: what exactly counts as one, how many are there, how do scientists sort them, and do they do anything for us. Here are the short answers, with where each one comes from, and with what our own Field Guide data adds.
What makes an animal an insect
Every adult insect has the same body plan:
- Six legs, all attached to the middle section of the body.
- Three body parts: head, thorax and abdomen.
- One pair of antennae on the head.
- A hard outer skeleton (exoskeleton), which it has to shed in order to grow.
- Usually wings: most insects have two pairs, flies have one pair, and a few, such as the silverfish, never have any.
Most insects also have compound eyes made of many small units, and they breathe through tiny holes along the sides of the body rather than through a mouth or lungs. The Amateur Entomologists’ Society and the Royal Entomological Society both describe insects in these terms.
The leg count is the quickest test. Eight legs means a spider, a tick, a mite or a scorpion. Many more than that means a centipede, a millipede or a woodlouse. None of them are insects, and our guide are spiders insects? has the three-second version of the test.
In our own Field Guide, more than 150 of the 181 cards are insects. The others are the animals people still call bugs: spiders and their relatives, centipedes, millipedes, woodlice, snails, slugs and an earthworm.
How many species of insects there are
About a million insect species have been named, which is more than every other group of animals put together. The Smithsonian Institution puts the number of known living insects at around 900,000, and other sources round it up to about a million as more are described each year.
The real number is much higher, and nobody knows it exactly. The Smithsonian quotes estimates running from about two million to as many as 30 million. The figure most often quoted now comes from the entomologist Nigel Stork, whose 2015 study in PNAS and 2018 review in the Annual Review of Entomology put the total at about 5.5 million insect species. If that is right, roughly four in five insect species on Earth have never been named.
Beetles are the largest group by far. Roughly 400,000 beetle species have been described, which is about a quarter of all the animal species known to science.
How insects are classified
Insects sit in the animal kingdom, in the phylum Arthropoda: animals with jointed legs and an exoskeleton. That phylum also holds spiders, crustaceans, centipedes and millipedes. Insects are the class Insecta within it.
The level most people actually use is the order. There are a few dozen insect orders, and five of them hold most of the species:
| Order | Common name | Example you can find |
|---|---|---|
| Coleoptera | Beetles | Seven-spot ladybird |
| Diptera | Flies | Marmalade hoverfly |
| Lepidoptera | Butterflies and moths | Peacock butterfly |
| Hymenoptera | Bees, wasps and ants | Buff-tailed bumblebee |
| Hemiptera | True bugs | Green stink bug |
Below the order come families, genera and species. Our types of insects guide goes through the main orders one by one, with how to recognise each.
The other way to sort insects is by how they grow up. Some go through complete metamorphosis: egg, larva, pupa, adult, like a caterpillar turning into a butterfly. Others go through incomplete metamorphosis, where the young look like small wingless adults and grow by moulting. Of the cards in our Field Guide with life-cycle data, 101 go through complete metamorphosis and 43 through incomplete. The insect life cycles guide explains both.
What insects do for us
Pollination. The UN’s Food and Agriculture Organization says that three out of four crops producing fruits or seeds for human food depend, at least in part, on pollinators, and most of those pollinators are insects: bees above all, but also hoverflies, butterflies, moths and beetles. Apples, pears, avocados and pumpkins are among the crops FAO names.
Pest control. Ladybirds and lacewings are predators of aphids. The University of California’s integrated pest management programme lists both as natural enemies worth protecting in gardens and on farms, because their larvae eat aphids, mites and other soft-bodied pests.
Clean-up. Dung beetles, burying beetles, flies and many others break down dung, dead animals and fallen plants and return them to the soil. Without them the litter of a forest floor would pile up.
Food for other animals. Insects are a main food for many birds, freshwater fish, bats, frogs and lizards. A garden with more insects usually has more birds.
Food and materials for people. Honey and beeswax from bees, silk from the silkworm moth’s cocoon. FAO’s work on edible insects estimates that insects are part of the traditional diet of at least two billion people.
Science. The common fruit fly has been one of the main laboratory animals in genetics for more than a century.
Front desk note: when someone asks what insects are for, the honest answer is that most of what we grow, and most of what eats what we grow, runs through them. They were here first and we are guests in their kitchen.
When to see the most insects
Our seasonal bars come from more than nine million iNaturalist research-grade observations of the species in the Field Guide. Taken together, July has the most records and January the fewest, and July is also the single most common peak month across our species. Most of those records come from the northern hemisphere, so in the south the pattern runs about six months later. Each species page shows its own month-by-month pattern.
If you have found one and want to know which order, family and species it belongs to, a photo in the identifier will give you a name, a confidence level and a safety read, and the A–Z of insect names lists every card in the Field Guide.
Sources
- Smithsonian Institution: Numbers of insects (species and individuals)
- Stork, N. E. (2018). How many species of insects and other terrestrial arthropods are there on Earth? Annual Review of Entomology 63
- Stork, N. E. et al. (2015). New approaches narrow global species estimates for beetles, insects, and terrestrial arthropods. PNAS 112
- Food and Agriculture Organization of the United Nations: Why we need bees and Edible insects
- University of California IPM: Beneficial predators (Quick Tips)
- Royal Entomological Society: Understanding insects; Amateur Entomologists’ Society: Insect fact files
- Seasonal and life-cycle figures: Peekabug Field Guide, built from iNaturalist research-grade observations
Questions
What are the characteristics of insects?
An adult insect has six legs, a body in three parts (head, thorax and abdomen), one pair of antennae and a hard outer skeleton. Most also have compound eyes and two pairs of wings, though flies have one pair and some insects, such as silverfish, never have wings. Anything with eight legs, such as a spider or a tick, is not an insect.
How many species of insects are there in the world?
About a million insect species have been named and described, which is more than all other animals put together. The real total is much larger: a widely cited 2018 review by the entomologist Nigel Stork put it at about 5.5 million, with most still unnamed. Beetles alone make up roughly a quarter of all described animal species.
What is the classification of insects?
Insects are animals in the phylum Arthropoda, which they share with spiders, crustaceans, centipedes and millipedes, and they form the class Insecta. Within the class they are sorted into orders, such as beetles (Coleoptera), flies (Diptera), butterflies and moths (Lepidoptera), bees, wasps and ants (Hymenoptera) and true bugs (Hemiptera). Each order is split into families, genera and species.
What are the benefits of insects to humans?
They pollinate a large share of our food: the UN's Food and Agriculture Organization says three out of four crops that produce fruits or seeds for food depend at least in part on pollinators. Insects also eat crop pests, break down dead plants, dung and carcasses, feed birds, fish and other animals, and give us honey, silk and wax. Many people around the world eat insects as part of their traditional diet.
