Spectacular Crab Spiders of the Family Thomisidae: Are They Dangerous to Humans?
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The crab spider lurking in your garden looks stranger than most other spiders. These small members of the family Thomisidae live throughout the country, from suburban backyards to remote bushland, and their unusual appearance can startle people who notice them for the first time, and given Australia’s reputation for dangerous wildlife, that moment of surprise can come with concern along with the question: “Are Crab Spiders Dangerous?”
What is a Crab Spider?
The common name “crab spider” comes from the way these spiders hold their bodies and move. Their two front pairs of legs are much longer and more robust than their back two legs, and they extend outward in a crab-like fashion. This body shape allows the crab spider to grab prey with lightning speed. When threatened or moving across a surface, they can scuttle sideways or run backwards, just like a true crab on the beach.
Crab spiders have flattened bodies that help them press close to petals and leaves. Their body length varies but most Australian flower crab spiders range from 3 to 11 millimetres. Males are usually smaller than females, which can reach up to 10 millimetres in length, while the male measures around 6 millimetres.
Hunting Without a Web: The Ambush Strategy
Unlike many spiders that build webs to trap their prey, crab spiders are ambush predators that rely on stealth and patience. The spider finds a suitable hunting spot, often on a flower where insects will visit, and then it waits. It can remain motionless for hours or even days, with its powerful front legs spread wide, ready to seize any insect that comes within reach. It grabs the prey with its strong spines on the front legs and delivers a venomous bite that quickly immobilises the victim. The venom of a crab spider is potent enough to subdue insects much larger than the spider itself.
One of the most remarkable features of certain crab spider species is their ability to change colour to match the flower they are hunting on. The spectacular crab spider can shift from white to yellow over between 10 and 25 days, while the reverse change is closer to six days.
Most crab spiders complete their entire life cycle within a single year. Adults mature in late summer or autumn, mate, and the females lay egg sacs that hatch in spring and grow through the warmer months until they reach maturity, and the cycle begins again.
Are Crab Spiders Dangerous to Humans?
The short answer is no. Crab spiders are not dangerous to humans, and they pose no significant health risk. While these spiders are venomous, their venom is designed to work on small insects, not on large mammals like us. Crab spiders are not aggressive toward people and will not bite unless they feel directly threatened.
In humans this venom rarely causes anything more than minor, localised irritation if a bite occurs. The fangs of most crab spiders are small and may not even break human skin. When a bite does occur, it feels similar to a mild bee sting or a mosquito bite. Even then, serious reactions are rare and are usually associated with individual allergies rather than the venom itself.
What To Do If You're Bitten by A Crab Spider
While a crab spider bite is nothing to worry about, it helps to know basic first aid for spider bites in general. The same principles apply whether you have been bitten by a harmless species or you are unsure what bit you.
Clean the bite area with soap and water to reduce the risk of infection. Apply a cold compress or ice pack wrapped in a cloth to the bite site for 10 to 15 minutes to help with pain and swelling. You can take over-the-counter pain relief if needed, and an antihistamine cream may help with itching. Watch the bite over the next day or two to ensure symptoms are improving.
Learn First Aid for Spider Bites
Encounters with wildlife are part of life in Australia, and while crab spiders are harmless, other creatures require a more cautious response. Knowing how to provide first aid for bites and stings helps you protect yourself, your family, and your community when it matters most. We offer nationally accredited courses that teach practical skills for managing emergencies, from spider bites and snake encounters to everyday injuries. A first aid course gives you the knowledge to stay calm and act correctly when someone needs help.
FAQs
Can You Find Crab Spiders Inside?
Crab spiders prefer to stay outdoors where their prey lives, and they have no reason to enter buildings.
What Is the Deadliest Spider in Australia?
The Sydney funnel-web spider is considered the most dangerous spider in Australia due to its highly toxic venom and willingness to bite when threatened. No deaths have occurred since antivenom became available in 1981, but bites still require immediate emergency treatment.
How Do I Know if a Spider Bite Is Serious?
Seek medical attention if symptoms spread beyond the bite site, if you develop nausea, vomiting, abdominal pain, or profuse sweating, or if the bite area shows signs of infection such as increasing redness, warmth, or pus.
