The spectral reflectance curves of freshly freeze-killed bees were measured using a spectrophotometer (AvaSpec-2048, Avantes, Eerbeek, this website the Netherlands) in the UV and visible range, and a calibrated light source (DH2000, Ocean Optics, Dunedin, FL, USA); the setup is described in Chittka & Kevan (2005). In addition, we inserted a special attenuator (Inline Fibre Optic Attenuator, 0–100%; 200–2000 nm;
Avantes) into the light path from the light source to the probe to allow spectral reflectance measurements of small target areas (∅2 mm). To calculate colour receptor signals, we used the blue tit Cyanistes caeruleus as a representative avian insectivore. Blue tit colour vision has input from four photoreceptor types (single cones), whose sensitivity is determined by the opsin visual pigment as well as oil droplets and ocular media that filter incoming light; the receptors are maximally sensitive in the UV ([ultraviolet sensitive (UVS); λmax=374 nm], the blue [short wavelength sensitive (SWS); λmax=455 nm], the green [medium wavelength sensitive
(MWS); λmax=539 nm] and the red [long wavelength sensitive (LWS); λmax=607 nm: Hart, 2001; Hart & Vorobyev, 2005). The so-called double cones consist of a large principal cone, filtered by an oil droplet whose absorptance varies somewhat between dorsal and ventral eye regions, and a smaller accessory cone which, in blue tits, does not have an oil droplet (Hart Selleckchem AZD1208 et al., 2000). Double cones are thought not to contribute to colour vision, but to be important in motion and shape vision (Hart & Hunt, 2007); hence, we also calculated their responses to bumblebee colour patterns. As the extent to which principal and accessory members are optically and electrically coupled is not fully known, we calculated separate quantum catches for the two types; we also calculated principal cone quantum catches for the two subtly different types of oil droplets present in the dorsal and ventral eye regions. The spectral sensitivity
curves for single and double cones in conjunction with their respective oil droplets and filtering by ocular media have been kindly provided by Nathan Hart (University of Western Australia). All passeriform birds Tobramycin studied so far possess a tetrachromatic set of single cones, with limited interspecific variation in the tuning of photopigments (Bowmaker et al., 1997; Hart, 2001). Among the 12 different passerines studied, for example, the wavelengths of maximum absorbance ranged from 355 to 380 nm for the UV pigment, 440 to 454 nm for the short-wave pigment, 497 to 504 nm for the medium-wave pigment and 557 to 567 nm for the long-wave pigment (Hart, 2001). The blue tit thus serves as a typical example for a passerine bird. The relative amount of light absorbed by each spectral photoreceptor type (i) is For UV photography (Fig. 1b), we used a Nikon D70 digital SLR camera (Tokyo, Japan) fitted with a Nikon UV Nikkor f4.