Chameleons and octopuses may be famed for their extraordinary camouflage skills, but this ability is not unique and many animals, big and small, can match their surroundings. Perhaps one of the smallest is the tiny crab spider, Thomisus onustus, which measure between 1 and 10 mm. These tiny arachnids match the vibrant yellows, pinks and whites of their flowery hosts, where they lie in wait for their pollinator prey. Little is known about the reason behind the crab spiders' camouflage (pollinators visit flowers regardless of a spider's colour and these spiders have few predators themselves) or even what controls the reversible colour changes. During her PhD, Ana L. Llandres studied the benefits of camouflage; however, when it came to her post-doc at the IRIB, France, she wanted to turn her attention to the regulation of camouflage: ‘What are the factors that makes spiders change colour?’ With the help of her post-doctoral advisor,

Jérôme Casas, and lab members Florent Figon, Jean-Philippe Christides and Nicole Mandon, she set out to investigate (p. 3886).

In early April 2012, Llandres travelled to the Extremadura region of Spain, and over the course of 4 days captured 160 female spiders hiding out in bright yellow corn marigolds. Llandres then allowed her spiders to adapt to their new white holding tanks over the course of 23 days, watching them gradually turn a whitish shade. She then wanted to find out whether simply changing the background colour was enough to get the spiders to change colour. So she placed 32 spiders in yellow containers and 18 remained in white containers in a sunny outdoor garden. Measuring light reflectance to precisely distinguish the spiders' colour, she found that, after 15 days, the initially whitish spiders matched their coloured abodes, becoming even whiter in the white containers and yellow in the yellow containers.

But what was controlling this colour change? During the spiders' first 23 days in the lab, Llandres recalls: ‘I saw that spiders that moulted tended to change to a white colour at a slower speed compared to spiders that did not moult. We know from previous studies that when spiders moult they show a peak of ecdysone [hormone] just prior to the moulting event, which made us think that ecdysone hormone could also be linked to the process of yellowing in this species’. To test this, the team decided to inject the spiders with the synthetic version of the hormone, hydroxyecdysone, and keep them in a yellow container in the lab. Sure enough, in the 3 days following the injection the spiders had adopted a yellower colour. However, by day 6, they had reverted to their white colour again. Llandres explains that when hormones are injected, spiders quickly break them down. Without the hormone, and in spite of the yellow backdrop, these spiders just couldn't maintain their yellow colour. In contrast to the earlier experiment, Llandres points out: ‘Natural illumination is much stronger than the illumination present in the laboratory and this may be an important factor that may affect a spider's colour change.’

In conclusion, the study has highlighted that both environmental and hormonal factors control the colour switching. What's more, the results suggest that spiders can actively choose to camouflage themselves based on their background, and won't always just choose a flower that matches their colour. However, the question still remains why are they camouflaging themselves in the first place? By eventually developing a way to actively manipulate the direction of colour change, the group hope they will be in a unique position to answer this century-old question.

A. L.
Environmental and hormonal factors controlling reversible colour change in crab spiders
J. Exp. Biol.