Spider vs Insect: Legs, Body Parts and Key Differences

Key differences at a glance

Spiders belong to the class Arachnida, together with scorpions, harvestmen, ticks and mites. Insects form the class Insecta, the most diverse group of animals on Earth, with around a million described species against roughly 50,000 known spiders. A familiar pair to compare is the European garden spider and the western honey bee.

Eight legs versus six

All adult spiders have eight walking legs, attached to the front body part. Insects have six, attached to the middle section, the thorax. Near the spider’s mouth sits a pair of shorter leg-like limbs, the pedipalps, which work as feelers and food handlers, and in males as organs for transferring sperm. This can make a spider seem to have ten legs.

Spider legs have seven segments, while insect legs have five main ones. Spiders also stretch some leg joints by raising their internal fluid pressure rather than with muscles, which is why a dead spider’s legs curl up beneath it.

Two body parts versus three

A spider’s body has two parts joined by a narrow waist called the pedicel. The front part, the cephalothorax or prosoma, carries the eyes, the fangs, the pedipalps and all eight legs. The rear part, the abdomen, holds the heart, most of the gut, the reproductive organs, the breathing organs and the silk glands.

An insect’s body has three parts. The head carries the antennae, eyes and mouthparts, the thorax carries the legs and wings, and the abdomen holds digestion and reproduction. In many insects, such as bees, wasps and ants, the waist between thorax and abdomen is narrow too, but the three-part plan is still clear.

Antennae, wings and eyes

Spiders have no antennae. They sense touch, sound-like vibrations and air currents with thousands of fine hairs on their legs, and they detect tension in their skeleton with tiny slit-shaped organs. Insects use their antennae to smell, taste and feel.

No spider has wings, while most adult insects have two pairs, or one pair in flies. Insects were the first animals to fly, more than 300 million years ago. Spiders get airborne only by ballooning on silk threads carried by the wind.

The eyes differ too. Most spiders have eight simple eyes, though some have six, fewer or none. Hunting spiders such as jumping spiders have sharp vision, while web builders see poorly and rely on vibrations. Insects have two compound eyes made of many tiny units, often several thousand in a dragonfly, plus up to three small simple eyes.

Mouthparts, venom and silk

Spiders eat with chelicerae, jaws that end in hollow fangs. Almost all spiders have venom glands, and they digest prey outside the body and suck up the liquid. Insects have far more varied mouthparts: biting mandibles in beetles and grasshoppers, a sucking tube in butterflies, and piercing beaks in bugs and mosquitoes.

Venom in insects is the exception, not the rule. Bees, wasps and many ants sting, using a modified egg-laying tube, so only females can sting.

Silk shows another clear contrast. Every spider makes silk, at every age, from spinnerets at the tip of the abdomen, and orb weavers can make up to seven types, as described in how spiders make silk. Among insects, silk is produced by only some groups and mostly by larvae, such as silkworm caterpillars and caddisfly larvae, usually from glands near the mouth. Webspinners are unusual insects that spin silk from glands in their front feet.

Growing up: molting versus metamorphosis

Spiders hatch from eggs as tiny versions of their parents. They grow by molting their exoskeleton several times, but their body plan does not change. Insects mostly go through metamorphosis. Grasshoppers and true bugs hatch as nymphs that look roughly like small adults and gain wings at the final molt. Beetles, flies, butterflies and bees go through complete metamorphosis, changing from egg to larva to pupa to a very different adult.

Diets, numbers and size

Spiders are almost all predators, and insects are their main food. A 2017 estimate put the prey killed by the world’s spiders at 400–800 million tonnes a year, most of it insects. Insects, by contrast, eat almost everything: leaves, nectar, wood, dung, blood, fungi and other animals. Many, like the honey bee, are vital pollinators.

Insects win easily on diversity. About a million insect species have been described, and scientists estimate that several million more remain unnamed. Spiders, at roughly 50,000 described species, are still one of the largest groups of predators on land, and new species are named every year.

Neither group gets very large, because an exoskeleton and breathing through tubes limit body size on land. The biggest spiders, such as the goliath birdeater and the giant huntsman spider, have leg spans around 30 cm (12 in), described in largest spiders in the world. The biggest insects reach similar wingspans or lengths, but most species in both groups are only a few millimeters long.

How are spiders and insects related?

Spiders and insects are both arthropods: invertebrates with a hard exoskeleton, a segmented body and jointed legs. Their lineages split well over 500 million years ago. Spiders belong to the chelicerates, a group that includes scorpions, mites, horseshoe crabs and sea spiders; the horseshoe crab connection is explained in is a horseshoe crab a crab?. Insects, surprisingly, are more closely related to crustaceans such as shrimps, crabs and woodlice than they are to spiders.

Look-alikes that are neither

Several animals are often mistaken for spiders or insects. Harvestmen are arachnids, but not spiders: their body looks like a single oval, and they have no silk or venom. Ticks and mites are eight-legged arachnids too. Woodlice and pill bugs look like insects but are crustaceans, as explained in are pill bugs insects?. Counting legs and body parts and looking for antennae is still the simplest way to sort them out.