The evolution of a visual signal will be affected by signaller and receiver behaviour, and by the physical properties of the environment where the signal is displayed. Crab spiders are typical sit-and-wait predators found in diverse ambush sites, such as tree bark, foliage and flowers. Some of the flower-dweller species present a UV+-white visual lure that makes them conspicuous and attractive to their prey. We hypothesised that UV+-white colouration was associated with the evolution of a flower-dwelling habit. In addition, following up on results from a previous study we tested whether the UV+-white colouration evolved predominantly in flower-dwelling species occurring in Australia. We measured the reflectance of 1149 specimens from 66 species collected in Australia and Europe, reconstructed a crab spider phylogeny, and applied phylogenetic comparative methods to test our hypotheses. We found that the flower-dwelling habit evolved independently multiple times, and that this trait was correlated with the evolution of the UV+-white colouration. However, outside Australia non-flower-dwelling crab spiders also express a UV+-white colouration. Therefore, UV+-white reflectance is probably a recurring adaptation of some flower-dwellers for attracting pollinators, although it may have other functions in non-flower-dwellers, such as camouflage.
Sampled trees from the Bayesian analysis
Randomly sampled phylogenetic trees from the posterior distribution returned from the Bayesian phylogenetic inference ran in MrBayes 3.2.2. Sampled 100 trees from each one of four independent searches in MrBayes, in a total of 400 trees. Trees have been ultrametricised and pruned.
Bayesian phylogenetic analysis
Results from the Bayesian phylogenetic analysis ran in MrBayes 3.2.2. Four independent searches with three heated chains and one cold chain each. Default MrBayes’ priors. Searches initiated with random trees. Markov chain Monte Carlo simulations (MCMC) were run for 10 million iterations and trees sampled every 10,000 iterations.
Specimen reflectance data. The first column shows wavelength (nm) values. Each one of the following columns represents the average reflectance value (%) of five measurements of one specimen. Reflectance values have been smoothed (R function 'smooth.spline' with argument 'spar'=0.7), and negative values converted to zero. Column names indicate taxon names. More detail can be found in 'specimen data.csv' file.
Specimen data. Columns: (1) SEQ = sequential row numbers; (2) ID = researcher’s specimen identification number; (3) Clade = taxon name; (4) family = spider taxonomic family; (5) country = country of specimen collection; (6) spectrum_file_name = reflectance data file name, same as in 'spec.files.csv' file.
Consensus tree from the Bayesian analysis
Majority rule consensus tree returned from the Bayesian analysis ran in MrBayes 3.2.2.
Phylogenetic tree data
Phylogenetic tree data. Columns: (1) taxon = phylogenetic tree tip labels; (2) flw.categ = flower dwelling habit, either a flower-dweller (FLW) or a non-flower dweller (NOFLW); (3) biog_reg = region of collection, either collected in Australia or in Europe plus Malaysia; (4) sample.size = reflectance data sample sizes.
Phylogenetic tree. Majority rule consensus tree from the Bayesian analysis ultrametricised and pruned. See file 'tree FULL.nex' for the original tree.
Flower-dwelling habit of Thomisidae
Number and percentage of individuals per taxon collected directly on flowers (flower) and number individuals collected on other substratum or using a sweepnet (non-flower).
ESM Table 2.xlsx