A More Standardized Method for Imaging and Measuring Drosophila Thorax Length Using ImageJ

Background

Thorax measurements in Drosophila melanogaster are an important and widely used metric in studies of life history, plasticity, and heritability (e.g., Imasheva et al., 1999; Bergland et al., 2008; Lafuente et al., 2018). This is due to the well-established relationship between thorax size and metrics of fitness, such as fecundity and lifespan (Hasson et al., 1992). Conventional thorax measurements in many studies choose to measure from the tip of the scutellum to the edge of the thorax. However, unlike for other sets of morphometric measurements (e.g., those made on the wings, in which the veins and vein junctions make clear anchor points and landmarks), the thorax provides fewer obvious fixed and identifiable edge points. This can make thorax measurements more challenging and potentially unstandardized across different studies. For example, the angle of measurements may be arbitrary, which can introduce high variation between replicated measurements. Here, we outline a protocol for making consistent thorax length measurements based upon a fixed bristle point situated at the anterior end of the thorax when viewed laterally. This fixed point allows for a repeatable angle to be drawn from the scutellum, thereby significantly improving the consistency across replicated measurements when compared to a non-landmark-guided approach.