Crab Spider (Thomisidae)

The name crab spider is loosely used for up to 2100 species of similar-looking spiders in the family Thomisidae.

The crab spider has many common names, such as flower spiders, flower crab spiders, and shield crab spider.

How To Identify a Crab Spider

Crab spiders are known for their crab-like appearance and how they run sideways in a crab-like way. They also have large front eyes, and their bodies have flat abdomens and long legs which resemble that of a crab.

Considering there are thousands of species of crab spider, it’s hard to state the actual size. However, when measuring abdomens, most range from 4-10mm. Their bodies appear to be made of two long sections connected by a narrow waist, which gives it the “crab” look when walking.

In terms of colour and markings, many crab spiders, such as the flower crab spider (misumena vatia) can change colour and blend into their surroundings. After a couple of days of waiting patiently on a plant, a flower crab spider can become the same colour or adopt similar markings as its background, hence the name.

Some other key identifying characteristics:

  • The females have a larger body size than the male crab spiders.
  • They have very long front legs, which are longer and stronger than their back legs.
  • There are no real visible differences between males and females except for their size and eye placement.
  • The crab spider has 6 eyes: 2 large ones in front and 2 small ones on each side.
A flower crab spider (Misumena vatia) on thistle
A flower crab spider (Misumena vatia) on a thistle

Hunting Behavior and Diet

As crab spiders don’t spin webs to trap insects as other spiders do, they are considered ambush predators, which lay in wait, pouncing on their prey.

Crab spider venom is not very potent as they do not capture their prey with their bite; instead, they inject a toxin that causes paralysis and death. They catch their prey by grabbing it with their superfast and strong front legs.

In general, the Thomisidae family of spiders are good predators, known for catching many insects such as mosquitoes, moths, sawflies and butterflies. They feed mostly on ants, small caterpillars, moths and flies but will also eat other insects such as houseflies, crickets, grasshoppers and aphids.

Crab spiders have excellent eyesight. They can detect movement, even when it is done slowly, which helps them hunt at night. Although they are primarily daytime feeders, their acute eyesight allows them to feed when little light is present, such as during the evening or morning sunrise/sunset.

Many spiders within the Thomisidae family lay waiting on flowers in their camouflaged state. They can often be mistaken for bird droppings, which allows them to lay waiting in open surroundings for their next meal. Giant crab spiders (found in the USA and many parts of Central America) have the ability to jump, and therefore have a wider variety of diet.

Yellow crab spider caught the bee on the sunflower
A yellow crab spider catches a bee on a sunflower

Where To Find a Crab Spider

The crab spider can be found in many areas around the world and prefers warmer climates, mostly in meadows, flower gardens, fields, orchards and trees. In the UK, they are almost exclusively present in the gardens of southern regions, where temperatures are typically higher than up north.

A white crab spider
A white crab spider

Are Crab Spiders Dangerous?

Crab spiders are not harmful to humans and only bite if they are disturbed. The bite itself is not very painful, but there have been some reports of it causing a stinging sensation lasting up to 5 minutes after the bite happens.

The fangs are barely strong enough to pierce the human skin; if any, giant crab spiders can. However, the venom produced by any type of crab spider is not very strong and has little to no effect on humans.

The crab spider does not always inject venom when biting, so you will usually feel a slight pinch instead of pain upon being bitten by one. The very few recorded bites are known to cause mild pain similar to a bee sting.

A Green Crab Spider (Diaea dorsata)
A green crab spider (Diaea dorsata)

Mating Habits

It’s not uncommon for females to eat males before and after mating. Therefore, the smaller males often creep up on females from behind, mate, and sneak off before being eaten.

After mating, the female crab spider can lay up to 30 eggs at one time, which she will hide under bark or logs somewhere outside of her nest. She also weaves silk-like webbing around them and attaches them to the bark until they hatch, which takes about 6 – 10 weeks, depending on the temperature. The webbing will continue to be removed by the spider until the eggs hatch.

Females have been known to carry their babies in the spider nest to protect them. This is done by covering their eggs with silk-like webbing so they are hidden from predators.

2 thoughts on “Crab Spider (Thomisidae)”

  1. Martyn Chillmaid

    Hi Sam,
    I received your article from Google because I have been researching crab spiders. I was photographing a lizard orchid and discovered a small crab spider on it. It was golden in colour with a triangular abdomen ( similar to the yellow crab spider). Its really standout feature is its two front pairs of legs which are dark to black and folded back at the main leg joint. Any ideas? Many thanks.

    Martyn

    1. Hi Martyn,
      Thanks for your query. It definitely sounds like a species of crab spider, but not one we’re familiar with i’m afraid.

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