The Life-cycle of a Corfu Crab Spider (Thomisus onustus)

Introduction

Lots of people are going to be put off by the mere mention of the word ‘SPIDER’ in the title of this blog but, if you can keep down the repulsion and desire to run off screaming, you’ll come out the other end with a, hopefully, different view of what spiders are all about.

Crab Spiders are harmless (to humans), small, often brightly coloured spiders, named for their remarkable crab-like appearance and their habit of scuttling sideways. They belong to the family ‘Thomisidae’ which includes approximately 170 genera with over 2,100 species. Their common name ‘crab spider’ is mostly linked to species in this family, where they are also known as ‘flower spiders’ or ‘flower crab spiders’. This article about the life-cycle of a Corfu crab spider has been a long time in the making, I hope you enjoy the photos and information in this blog.

My interest in crab spiders was sparked by a small study I did of the life on an Oregano bush in my Corfu garden during the month it was flowering in 2024. The study is worth reading as an introduction to this story about one of the crab spider species I witnessed last year. The Blog is available on my website (naturakerkyra.com) or you can read it directly, here: https://www.naturakerkyra.com/oregano-life 

A female Crab Spider (Thomisus onustus)

The Crab Spider Life Cycle

About

The Crab Spider (Thomisus onustus) is a small crab spider commonly found across Europe and parts of western Asia. It is particularly associated with flowers in dry, open habitats such as meadows, scrublands, and gardens. Its life history is closely tied to the blooming season of flowering plants from April to June in Corfu. Adults display remarkable colour changes, shifting over days between white, yellow, or pink to match their chosen flower heads. It is an adaptation that allows them to become nearly invisible to both prey and predators, sometimes they get it wrong and becoming obvious when you look at the flowers, bright yellow on a pink flower is a bit of a giveaway.

Early Life

The life-cycle begins in late spring or early summer when spiderlings (newly emerged baby spiders) emerge from silken egg sacs concealed beneath leaves or tucked away between petals. These sacs were created weeks earlier by females who mated shortly after reaching adulthood.

The mother crab spider with newly hatched spiderlings.

Growth and Hunting

Spiderlings undergo several moults as they grow and, in the early stages, they prey on small insects and avoid larger threats by mimicking flower parts or go into hiding. Unlike web-building spiders, such as orb-weavers, crab spiders are ambush predators hiding on or within flowers, leaves or fruit where the ability to change colour to match their surroundings allows it to blend in with the background. Once settled on a flower head, they patiently wait for insect visitors. Their powerful front legs act like traps which quickly snap closed and are strong enough to overpower an amazing range of insects such bees, wasps, moths, butterflies and pretty much anything that comes near them. Once they have captured their victim, their venom subdues the prey, and the spider then sits and feeds on it for hours.

Bees are a major resource for the crab spiders.

From bees to butterflies, crab spiders will catch anything.

Even drone flies and bush-crickets aren't too big for a crab spider.

The Golden Digger Wasp (Sphex funerarius) would seem like an unlikely victim.

The Common Red Soldier Beetle mystery:

On the face of it, pretty much anything that visits a plant that is the home of a crab spider is likely to become a victim of their insatiable appetite. However, while studying the insect behaviour on an oregano bush I was puzzled to find that a beetle seemed to lead a charmed life around crab spiders. 

A crab spider oblivious to Common Red Soldier Beetles (Rhagonycha fulva).

The oregano bushes were covered in a swarm of Common Red Soldier Beetles (Rhagonycha fulva) which were having a huge sexual orgy wherever you looked on the bush. With so many of the beetles all over the flowers, you would think that the crab spiders would be feasting heavily on them, but this was not the case. The crab spiders completely ignored them to the extent that beetles were even climbing over the spiders but I never saw a crab spider with its fangs in a red soldier beetle. 

Upon investigation I found that a distinctive feature of some soldier beetles is their ability to produce a toxic compound called 'cantharidin' which can cause skin and mucous membrane irritation. It is used by the beetles as a defensive mechanism against predators. Not all soldier beetles produce this toxin but I guess these particular red soldier beetles do, and it protects them from this crab spider species. 

Mating and Reproduction

A male and female crab spider mating.

It typically takes one growing season for a spider to mature, although this may vary depending on temperature and climate. Reproduction begins in spring or early summer when the new generation has reached adulthood. Adult 'Thomisus onustus' are sexually dimorphic with females up to 10mm larger than the males, which are typically only about a quarter of the size of a female and reach maturity earlier.

A male, lucky to escape with only two legs missing.

Although smaller, males are more mobile than females, spending less time hunting and more time searching for a mate, being guided by pheromone trails of the females. After locating a female, the male mates by cautiously approaching and transferring sperm using their 'pedipalps' (pedipalps are prominent appendages that often resemble a small pair of legs). Courtship is a very cautious affair, the male must approach the female without triggering her predatory strike, which often means sneaking up on the female when she is distracted by a meal she has just captured. Once the male has initiated mating, it is completed quickly and the male leaves swiftly to avoid becoming a second meal, although often a male will lose a couple of legs on its left side because they come within range of the females fangs.

The mother guarding her egg sac while spiderlings emerge onto a lavender flower.

The fertilised female lays her eggs in a carefully constructed silk sac, typically hidden on or near a flower head or leaf. When the egg sac is secure, the female crab spider remains with it until the spiderlings emerge, refusing to feed and fending off any threats to her nest. 

Once the baby spiders have emerged after about 2-4 weeks, depending on the temperature, the female will stay for a couple of days but then leave to die soon after the young spiders disperse, having completed their single breeding cycle. The tiny spiderlings leave soon after hatching, some walk to nearby plants while others release fine strands of silk which get caught by the wind and carry them away to begin a new life cycle.  The method of riding air currents on silk threads is called ‘ballooning’.

Tiny spiderlings balooning themselves away from the nest to begin a new life-cycle.

From a tiny wind-borne spiderling to a ferocious camouflaged flower assassin, the life of the Crab Spider (Thomisus onustus) is a brief but intricate thread in the tapestry of a Corfu summer.