Figure 4.

Chromosome numbers of species with varying numbers of worker castes. Worker caste number is defined as the following: 0 = no morphological difference between worker and reproductive castes, 1 = morphologically different worker and reproductive castes, but no within-worker caste polymorphism, 2 = two separate morphologically distinct worker castes. 3 = three or more morphologically distinct workers castes. The boxplot shows the interquartile range that contains values between 25th and 75th percentile. The line inside the box shows the median. The two ‘whiskers’ show the largest/smallest observation that is less than or equal to the upper quartile plus/minus 1.5 the length of the interquartile range.