We describe an inexpensive device for rapidly measuring volumes of small biological structures, and its application to brain-body scaling in insects. Invertebrates, with bodies a fraction as big as a small rodent's brain, often rival the behavioral sophistication of mammals. Does the size (volume or mass) of the nervous system impose fundamental constraints on behavioral complexity, or have small animals evolved specializations that squeeze extra computational power from tiny brains? A basic step toward addressing such questions is to evaluate how whole brains and nervous systems scale with body size. This has been extensively studied among vertebrates, but neglected in invertebrates despite their much greater diversity. The paucity of invertebrate data may in part reflect technical problems. Some brain-body scaling data have been provided from histological studies, but reconstructing volumes from sections is time-consuming, and the role of shrinkage artifacts is uncertain. Our method avoids these drawbacks, using the principle of fluid displacement to directly measure intact volumes. This is straightforward for large volumes; smaller volumes are measured by carefully controlling fluid levels with a micrometer syringe, matching objects to an appropriate container size, accounting for evaporation, and exploiting fluid surface tension and optical properties. Calibrations using a series of spherical test ball sizes yield standard errors of /- 5% down to 0.26 microliters; planned refinements may extend comparable accuracy to smaller volumes. Preliminary data from insects will be presented. The device is easily portable to any field station with a dissecting microscope, stable platform, walls and a roof, and the method is applicable to studies of a variety of biologically interesting volumes.
