Head size of H. scabiosae bees sampled in 2008 (white bars) and 2009 (grey bars). (a) Foundresses, first brood females and second brood females. (b) First brood males and second brood males. Solid lines indicate the median for each category, boxes the interquartile range, and whiskers the most extreme values within 1.5 times the interquartile range. Sample sizes for each category (number of individuals/number of nests) are indicated above the x-axis. Different letters indicate significant differences between groups (Tukey's tests). Females and males were analysed separately.