Flower Crab (Misumena)

Physical description and identification

Misumena is a genus of flower-hunting crab spiders in the family Thomisidae, erected by Pierre André Latreille in 1804. The name needs handling with care: long lists of forty-odd Misumena species circulate online, taken from older literature, but most of those names have since been moved to other genera or sunk as synonyms. Current catalogues accept only a few, and one of them — Misumena vatia, the goldenrod crab spider — accounts for essentially everything the public sees of the genus.

The build is pure crab spider: a broad, flattened body, short rear legs, and two long heavy front leg pairs held permanently open like a trap. These spiders walk sideways and backwards as readily as forwards, which is how the family got its name.

Adults

• Size: females are 6–10 mm in body length, males 3–5 mm and much lighter. A female may be several times her mate’s mass.
• Colour: adult females are white, pale greenish or bright yellow, smooth and waxy rather than hairy, many with a pair of red streaks down the sides of the abdomen. Males are marked quite differently: dark reddish-brown carapace and front legs, pale yellow hind legs, small whitish abdomen. Only adult females change colour.
• Eyes: eight small eyes in two strongly curved rows, the lateral pairs raised on low bumps and none enlarged — enough to rule out jumping spiders at a glance.
• Distinctive feature: a plain face. Misumena has no ridge across the front of the head, which is the quickest way to separate it from Misumenoides, whose species all carry a raised white carina below the eyes.

Eggs and egg sacs

A mated female stops feeding, binds a broad leaf into a folded chamber with silk and lays a flattened papery sac inside it, holding anything from a few dozen eggs to well over a hundred. She seals herself in beside it, guards it without eating for about three weeks, and dies within days of the spiderlings emerging.

Spiderlings

The young are pale translucent miniatures. They receive no food from the mother — the claim that spiderlings are fed by her and help her build nests is simply wrong, and she is usually dead before they disperse. They scatter on silk lines or by ballooning, hunt thrips and aphids in low vegetation, overwinter as juveniles and mature the following spring; see our guide to the spider life cycle.

Hunting instead of a web

No Misumena builds a capture web. Silk is spun for drag-lines, moulting anchors, courtship and the egg chamber, but never as a snare — see our guide to spider webs. The spider settles head-down in a flower head, holds the long front pairs apart and waits, sometimes for hours. When a bee or butterfly lands within reach the legs close and the fangs go in behind the head, and the venom acts fast enough that heavier prey cannot pull free.

The colour change is the genus’s signature and it is slow. A yellow ommochrome pigment is secreted into the outer layer of the cuticle to turn the female yellow, then withdrawn and excreted to return her to white. Going yellow takes roughly 10 to 25 days; going back to white takes about five or six. Nothing about it resembles the instant switching of a chameleon.

Where flower crab spiders live

This is where popular accounts go furthest wrong. Misumena is not a genus of South America and Africa. It is Holarctic, and M. vatia occurs across most of North America from southern Canada southward, throughout Europe including the United Kingdom, and eastward across temperate northern Asia. Tropical records almost always refer to other thomisid genera.

The habitat requirement never changes: open, sunny ground with flowers that insects visit — meadows, old fields, prairie, roadside verges, hedge banks, clearings and garden borders. The favoured platforms are broad, flat-topped blooms that give a stable footing and draw plenty of traffic, such as daisies, asters, goldenrod and umbels. Closed forest is avoided, because the strategy depends on pollinators arriving.

Are flower crab spiders venomous?

Yes. Like almost all spiders, Misumena species use venom to subdue prey, and they depend on it more than most: with no web to hold an insect and a body lighter than many of their targets, they must immobilise a struggling bee within a second or two or lose both the meal and their grip on the flower.

Effective against insects is not the same as dangerous to people. No Misumena venom is medically significant to humans or pets. The genus is unrelated to the recluse spiders and the widow spiders, lacking both the tissue-damaging enzymes of the first group and the neurotoxins of the second, and the fangs are short.

Can flower crab spiders bite?

They can, but it is a rare event. A disturbed flower crab spider freezes, then drops off the bloom on a silk line; biting is reserved for being squeezed, which in practice means a flower picked and gripped without looking.

The reported effect is a brief pinprick with slight redness that fades within hours. No systemic symptoms and no tissue damage have been documented from the genus. Wash the area with soap and water; a cold pack is enough if it is sore. Spreading swelling, hives or breathing difficulty would indicate an allergic reaction rather than the venom, and warrants medical attention as after any insect sting.

Ecological importance and behavior of flower crab spiders

Misumena spiders are sit-and-wait predators of exceptional patience. A female can hold the ambush posture for hours and may keep the same flower head for days, with long gaps between meals broken by occasional very large catches. She selects blooms that are already attracting insects, so the plant does the advertising for her.

Diet: honey bees, bumblebees, solitary bees, hoverflies and other flies, small butterflies and skippers, beetles and the occasional wasp. Prey twice the spider’s mass is routine.

Natural predators: insectivorous birds, lizards, shrews, ants, larger spiders, and spider-hunting and mud-dauber wasps that paralyse crab spiders to stock their nest cells. The pigment change lowers the spider’s visibility both to the ultraviolet-sensitive eyes of bees and to the colour vision of birds, so the same camouflage works against predator and prey at once.

Prey-predator dynamics: hunting at the flower puts these spiders in the pollination pathway. A resident female removes pollinators and can reduce that bloom’s seed set, but she also takes flower-feeding pests, and visitors learn to avoid flowers where a spider is visible.

Seasonal behaviour: adults appear from late spring, peak through midsummer and are largely gone by early autumn. Males mature earlier, wander between flower heads and rarely survive long after mating. Females feed hard, mate, then commit everything to a single egg sac. The full cycle takes about a year, sometimes two where the season is short.

Relationship with humans: entirely harmless. Unsprayed borders, native perennials and an unmown corner do more for these spiders than anything else, and no control is ever warranted.

Species in the genus and similar spiders

Quick facts

Did you know?

• The genus was erected in 1804 by Latreille, a founder of modern arthropod classification, though its type species had already been named by Carl Clerck in 1757.
• Species lists for Misumena are a trap: the forty-odd names quoted in older sources are mostly historical combinations that no longer belong to the genus.
• The colour change is asymmetric — up to three and a half weeks to go yellow, under one to come back to white — because laying the pigment down is slower than clearing it.
• They are a textbook case of camouflage working on two unrelated visual systems at once: the ultraviolet-sensitive eyes of bees and the four-cone colour vision of birds.
• A female regularly kills bumblebees heavier than herself, something no comparable spider manages without a web to hold the prey still.

In summary, Misumena is a small northern-hemisphere genus with an outsized reputation built almost entirely on one species. Its spiders spin no capture web, repaint themselves over days rather than seconds, and are completely harmless to the person holding the flower.