Hydrofoil-like legs help stream mayfly larvae to stay on the ground

Abstract

Adaptations to flow have already been in the focus of early stream research, but till today morphological adaptations of stream insects are hardly understood. While most previous stream research focused on drag, the effects of lift on ground-living stream insects have been often overlooked. Stream mayfly larvae Ecdyonurus sp. graze on algae on top of the stones and therefore inhabit current exposed places in streams. They have a dorso-ventrally flattened body shape, which is known to reduce drag. However, this body shape enhances lift too, increasing the danger for the animal of getting detached from the substrate. Using microscopic techniques, 3D-printing, and drag and lift measurements in a wind tunnel, our experiments show that the widened femora of Ecdyonurus sp. can generate negative lift, contributing to counterbalance the (positive) lift of the overall body shape. The larvae can actively regulate the amount of lift by adjusting the femur's tilt or optimizing the distance to the ground. This shows that morphological adaptations of benthic stream insects can be very elaborate and can reach far beyond adaptations of the overall body shape. In the presented case, Ecdyonurus sp. takes advantage of the flow to overcome the flow's challenges.

Keywords: Drag; Ecdyonurus sp.; Femur; Lift; Torrenticol.

Fig. 1

a Ecdyonurus larva in flow tank. Frame taken from a video. The black arrow shows the direction of flow. The length of the larva is approximately 9 mm. b Light microscopy image of a typical cross section of the middle region of the first femur of Ecdyonurus sp. embedded in epoxy resin

Fig. 2

Schematic top view of the experimental setup in the wind tunnel. The artificial femur profile was mounted between two endplates on the two component force balance. The distance between the leading edge of the ground board and the leading edge of the femur profile was 100 mm. The distance to the ground (h) varied from 4–60 mm by moving the motorized translation stage mounted to a ground board

Fig. 3

Lift coefficients (C_L) of all femur models of Ecdyonurus sp. at different Reynolds numbers (Re) and angles of attack (AOAs). All values are means with standard errors (n = 10). At all Re, C_L increased linearly (0.03 C_L/AOA (°)) between AOAs of − 26° and 0°. The standard errors of C_L were very small at Re ≥ 4500 but considerable at Re 1700

Fig. 4

Drag coefficient (C_D), lift coefficient (C_L), and C_L/C_D coefficients versus AOA for the femur model of Ecdyonorus sp. M1 at a Reynolds number of 1700. Values are means with standard errors (n = 10)

Fig. 5

The lift (C_L) and drag (C_D) coefficient of the femur model (M1, Re = 1700) with increasing true ground clearance h/c (black symbols) and in unbounded air (grey symbols) for different angles of attack (AOA: − 10° and − 20°). All values are means with standard error (n = 10). The ground effect is visible for true ground clearance of h/c ≤ 2.0 showing decreasing C_L and C_D for lower h/c

Fig. 6

The lift-to-drag ratio (C_L/C_D) of the femur model (M1, Re = 1700, n = 10) with an increasing true ground clearance h/c (black symbols) and in unbounded air (grey symbols) for different angles of attack (AOA: − 10° and − 20°). A more negative C_L/C_D ratio means a higher fluid-induced force in the direction to the ground. Most extreme values were attained at h/c 1–1.5 for both AOAs.

Fig. 7

Schematic view of the boundary layer thickness (ẟ) and position of the femur model (c = 100 mm) at different ground clearances (h/c) during wind tunnel tests at Re = 1700. The calculated boundary layer thickness was 3.8 mm (equals h/c 0.38) at the position (x) of the femur model. Interaction of the model and boundary layer would only take place at h/c < 0.38. The thickness of a laminar boundary layer on a flat plate and several velocity profiles within it are estimated according to Vogel (1994)