Many ways to land upright: novel righting strategies allow spotted lanternfly nymphs to land on diverse substrates

Abstract

Unlike large animals, insects and other very small animals are so unsusceptible to impact-related injuries that they can use falling for dispersal and predator evasion. Reorienting to land upright can mitigate lost access to resources and predation risk. Such behaviours are critical for the spotted lanternfly (SLF) (Lycorma delicatula), an invasive, destructive insect pest spreading rapidly in the USA. High-speed video of SLF nymphs released under different conditions showed that these insects self-right using both active midair righting motions previously reported for other insects and novel post-impact mechanisms that take advantage of their ability to experience near-total energy loss on impact. Unlike during terrestrial self-righting, in which an animal initially at rest on its back uses appendage motions to flip over, SLF nymphs impacted the surface at varying angles and then self-righted during the rebound using coordinated body rotations, foot-substrate adhesion and active leg motions. These previously unreported strategies were found to promote disproportionately upright, secure landings on both hard, flat surfaces and tilted, compliant host plant leaves. Our results highlight the importance of examining biomechanical phenomena in ecologically relevant contexts, and show that, for small animals, the post-impact bounce period can be critical for achieving an upright landing.

Keywords: anti-predator; body rotation; dropping; invasive; manoeuvrability; rebound.

Figure 1.

(a) Fourth instar SLF nymphs on a trunk. (b) Close-up of a fourth instar SLF nymph showing our definition of body length (grey arrow) (black line = 10 mm scale bar). Photographs of the SLF's preferred native host tree, A. altissima, showing (c) the release distance (white bar = 200 mm) used in most experiments in this study and (d) a view from the ground looking upward into the canopy, showing how the densely overlapping leaflets offer many landing opportunities for falling nymphs.

Figure 2.

(a) Schematic of the dropping experiment, showing how the five different phases of motion used to analyse the outcomes were defined. (b) Illustration of the five orientations used to describe releases, impacts and final landings. (Red arrows indicate the dorsoventral axis.) (c) Geometry used to define the cranial–caudal body axis angle, θ, relative to horizontal.

Figure 3.

Distribution of orientation at impact and final landing for live SLF fourth instar nymphs dropped on a hard surface from each of five release orientations. (Red arrow points towards the dorsal surface when viewed in lateral aspect.)

Figure 4.

(a) Superimposed sequence of video frames recorded every 15 ms showing an SLF nymph falling 200 mm. (Scale bar, 10 mm.) (b) Stereotypical falling posture assumed by SLF nymphs after dropping. (c) Measured (open circles) and fitted (red line) vertical position, y, of the specimen shown in (b) plotted versus time. (d) Fitted terminal falling speed distributions for live and dead specimens artificially dropped from 20 cm and live specimens voluntarily releasing from 35 cm (black circles: mean; error bars: 95% CI; grey circles: all data, jittered for visibility).

Figure 5.

Rate of SLF nymph body rotation in the image plane upon release, at an approximate midpoint during the fall, and immediately before impact. (a) Rotation rate magnitudes show different trends during the fall period for live nymphs (circles) relate to dead specimens with legs spread (triangles) or tucked (squares). (b) Plots of the change in rotation rate magnitude between the midpoint and at impact relative to release (equivalent to scaling the initial rotation rate at release to zero) for different release methods and specimen preparations. (Error bars show 95% CI, which were similar to instrumental measurement uncertainties. Horizontal distances between data points are proportional to time; data also are jittered for visibility.)

Figure 6.

Distribution of impact and final landing orientations for SLF nymphs (a) dropped artificially from tweezers and (b) releasing voluntarily onto a hard paper surface. The distributions from (b) recorded at 200 mm below the release point corresponded to the same falling distance as those recorded for impact in (a). From left to right, top to bottom in each plot: the dotted lines represent model predictions for upright landings (29.3%) and upside-down landings (29.3%) at impact, and the expectation for upright versus upside-down final landing orientation (50%), if landing orientation were random (red arrows, dorsoventral axis).

Figure 7.

(a) Typical image sequence for SLF nymphs falling onto A. altissima leaflets. (b) Orientation distributions at the first impact and landing for specimens that successfully landed on leaflets. Because we only characterized successful landings on leaves, these results cannot be compared to the data and models shown in figure 6. (c) Image sequence showing bouncing from a leaflet (impact, 0 ms; red arrows, dorsoventral axis).

Figure 8.

(a) Image sequence from a video of fourth instar SLF nymph landing on its back on the hard substrate, bouncing and finally landing upright (impact, 0 ms; red arrows, dorsoventral axis). Distributions for (b) impact speed, v_imp, and (c) bounce height for live and dead specimens artificially dropped from 20 cm onto the hard substrate and leaves and live specimens voluntarily releasing from 35 cm (black circles: mean; error bars: 95% CI; grey circles: all data, jittered for visibility). (d) Bounce height versus $v_{\mathrm{imp}}^2$ (∝ kinetic energy before impact).