Crab Spider
Araneae
What Are They?
This little ambusher is a crab spider most likely from the genus Thomisus, the "flower crab spiders." Rather than spinning a web, it does something far sneakier: it sits perfectly still on a flower and waits for an unsuspecting fly, bee, or other pollinator to land right next to it. By the time the insect notices, it's too late.
Why Does It Walk Sideways?
Look closely at the legs in the photo the front two pairs are much longer and heftier than the back two, and they're held out to the sides rather than pointing forward. That's what gives crab spiders their name: they scuttle sideways and diagonally, crab-style, instead of walking straight ahead like most spiders.
The Disappearing Trick
Here's the really clever part. Many Thomisus crab spiders can actually change colour to match the flower they're sitting on shifting between white, yellow, or pink over the course of about two days. Scientists have found this isn't just a passive trick of the light: it's controlled by hormones, the very same one spiders use to trigger moulting. A spider placed on a yellow flower will gradually turn yellow; move it to white, and it fades back though turning yellow happens about twice as fast as fading back to white. Some related species take an even stranger approach: instead of hiding, they reflect ultraviolet light that's invisible to us but creates a glowing beacon to a bee's eyes, essentially advertising "flower here" to lure prey in rather than hiding from it.
How Does It Actually Catch Prey?
A crab spider on a flower detects an approaching insect from about 2 centimetres away and strikes when it's within about half a centimetre. Remarkably, it can overpower prey up to three times its own size a single bite behind the head delivers a venom potent enough to subdue something that much bigger, after which digestive juices dissolve the victim's insides so the spider can drink it, leaving only an empty husk behind.
What Do the Young Ones Eat When There's No Prey Around?
Here's a fact that surprises most people: spiderlings that can't find insect prey for a while don't necessarily starve. Research has shown they can survive by feeding directly on pollen and nectar from the flowers they live on a backup food source that's unusual for a predator.
Good Guys in the Garden
Despite lurking on flowers to ambush visitors, crab spiders are considered beneficial. They prey on aphids, mites, and thrips pests that damage crops making them a quiet form of natural pest control.
Where Do They Live?
Crab spiders (the wider family, Thomisidae, of which Thomisus is one genus) are found across almost every habitat in Africa grasslands, shrubs, flowers, and trees and even use silk threads to "balloon" on the wind to disperse to new areas. In Kenya specifically, several Thomisus species have been recorded from Nairobi down to the coast and up to the Rift Valley, and this individual was photographed at Lolldaiga House in the Lolldaiga Conservancy, Laikipia a region where closely related crab spiders are also well documented.
Family Life
Females lay their eggs inside a curled-up leaf and then guard the egg sac fiercely until they die without eating anything during that entire watch. It's a one-shot, all-in investment in the next generation.
Worldwide Diversity
Crab spiders are a big family worldwide there are roughly 2,100 known species across some 170 genera, and Thomisus alone makes up more than 130 of them. So while we can't say with certainty exactly which Thomisus species this is just from a photo (that would need a close look under a microscope), we can say with confidence it's a flower crab spider caught doing exactly what its family does best: hiding in plain sight and waiting for lunch to walk by.