Spider Egg-Sac Colouration: A Predator’s Eye View

Animal colouration has a long tradition in generating and testing theories central to evolutionary biology since colouration can influence animal fitness by serving as visual signals in sexual selection or by providing protection through cryptic, mimetic and aposematic colouration. Until recently most studies have not investigated animal colouration from an animal’s perspective.Eggs are so important for gene transmission, that providing some protection is often taken granted. Egg colouration is expected to be adaptive. However, previous studies of egg colouration have primarily concentrated on the adaptive significance of bird egg colouration, little attention has been paid to egg colouration of invertebrates, including spiders. In spiders, eggs are wrapped by silk, which called egg-sac, and egg-sac exhibits a diversity of colours. Why spider egg-sacs have different colours is unclear. In the present study, I investigated the adaptive significance of spider egg-sac colouration from a predator’s eye view through modeling visual systems of known avian (blue tits) and hymenopteran predators. In Chapter 2, I collected egg-sacs of 23 spider species and the related backgrounds, then measured and compared their spectral reflectance. My results revealed there were significant differences in the spectral reflectance among egg-sacs of different spider species. To evaluate whether predators could detect egg-sacs from their backgrounds, I calculated chromatic and achromatic contrasts of egg-sacs against spiders or background vegetations in the visual system of bird and hymenopteran predators. My results showed that not all the spider egg-sac colours are perfect from a predator’s eyes, at least, not all being cryptic. Green and brown egg-sacs may offer a selective advantage against hymenopteran and avian predators as green and brown spider egg-sacs matched background, thus making egg-sacs inconspicuous to predators. In addition, there exists colour polymorphism in egg-sac colouration in the same species. Thus, in Chapter 3, I focused Philoponella prominens, a uloborid spider with polymorphism of egg-sac colouration and its body colouration. I assessed the colour-coordination between the adult females and their egg-sacs by comparing spectral reflectance and analyzing colour contrasts among different colour morphs from a predator’ eye view. The results showed that black and brown females and their dull egg-sacs matched well in their colouration, and were less conspicuous to predators, which also were supported by field statistic data. In Chapter 4,I compared the reflectance of inter-population variation in egg-sac colouration of the same species which were collected from different geographies, a theridiid spider Achaearanea tepidariorum and a wolf spider Pardosa sumatrana, to assess the influence of possible ecological factors on spider egg-sac colouration.In summary, this was the first time to investigate adaptiveness of spider egg-sac colouration from a predator’s eye view. We believed that this study would widen our horizon about animal cryptic and mimetic protective colouration. Also, it provided us more supports for the study of the evolution of animal visual systems.