Size Dimorphism in Bumblebees: a Result of Caste Specific Differences in Fat Body Metabolism?

Most highly social insects exhibit a distinct size dimorphism between the female castes: queens being larger than workers. Nutritional factors are known to influence caste specific development to a great extent, but little is known about this in bumblebees. To test whether size differentiation in Bombus terrestris results from differences in food intake and fat body metabolism, as was suggested in earlier research, we compared the weight gain of the midgut and the fat body, as well as its general composition for last instar queen and worker larvae. The present results show that the relative growth rates and the general composition of the fat body in queen and worker larvae are similar, indicating that caste specific size dimorphism is not owing to differences in fat body metabolism. The timing of the onset of defecation appeared to have a pronounced effect on the determination of midgut weight and body mass, and it is discussed that this might have complicated the correct interpretation of earlier results.