Recently, swaying behavior has been interpreted as serving a balancing function (Kelty-Stephen, 2018; Cuthill et al., 2019), based on reanalysis by Kelty-Stephen (2018) using the data presented in Bian et al. (2016). Bian et al. (2016) examined the movement of the phasmid Extatosoma tiartum in response to wind cues. While the insects would sway in response to wind stimulus, the frequency of swaying declined over time. The number of sways was higher in variable wind conditions compared with constant wind conditions, suggesting that the insects react to environmental cues such as wind stimulus and modify their swaying behavior in response. In the presence of plants in the background, insect swaying was consistent with the movement of the wind-blown plants. Reanalyzing part of the same dataset, Kelty-Stephen (2018) proposed that the swaying behavior allows the insects to achieve stability in response to wind-like stimulation. Kelty-Stephen (2018) further proposes that these data provide evidence for multifractal complexity in postural stabilization under wind-like stimulation and point to similarities between phasmid and human postural sway. Kelty-Stephen (2018) proposed that the reduction in sway exhibited by phasmids can be explained by non-linear interactions across time consistent with tensegrity principles and dismissed Bian et al.’s (2016) suggestion of anti-predator behavior on the part of the insects. Such a balancing function could be more relevant to perching than hanging insects, with the latter relying on gravity to remain stable.

We do not seek to question Kelty-Stephen’s (2018) analysis. However, we must point out that the author has foremost, incorrectly characterized the phasmids described in Bian et al. (2016) to ‘perch upon a branch’ (Kelty-Stephen, 2018, p. 8), when they were, in fact, hanging from a branch (Figures 1A,B). It is unclear why Kelty-Stephen (2018) took this perspective on phasmid swaying behavior and whether this inaccuracy influenced the author’s interpretations. While phasmids can walk when perching, it is uncommon. E. tiaratum (Phasmatidae) typically hangs from perches, rarely spending time ‘upright’. We found a similar pattern for four species of phasmids that we had collected from the field, maintained in the laboratory and observed over a three-day period (Figures 1C–F; Table 1). Hanging from the top or side of the enclosure, host plant or even another phasmid was the overwhelming position observed, while perching was relatively rare (60 out of 1,343 observations, 4%). Swaying behavior was not ubiquitous across these four species: swaying was common in Lonchodes brevipes (Diapheromeridae), occasionally observed in Calvisia flavopennis (Lonchodidae) and Marmessoidea rosea (Lonchodidae), and rarely observed in Haaniella echinata (Heteropterygidae). Indeed, our study (Pohl et al., 2022) indicate that even for the same species (L. brevipes), individuals at different life stages appear to sway to different extents. More nuanced studies in the future are crucial to understand the factors affecting swaying behavior, but the appropriate biological context to consider stabilizing mechanisms in such systems is of a hanging organism.

We find it surprising that the author misrepresented the work in this way, particularly as the hanging phasmids considered in Bian et al. (2016) contrasts with upright focal organisms in references on postural sway (e.g., Straube et al., 1987; Clayton et al., 2003; Hutchinson et al., 2007; Munafo et al., 2016; Dewolf et al., 2021). Demonstrating tensegrity principles in a hanging organism seems novel and worthy of further attention; but evaluation of the significance and broader implications of these findings are relevant only when the behavior is placed in the correct context. Kelty-Stephen (2018) was, perhaps, too quick to dismiss the potential for sway to represent an adaptive behavior to avoid predators. We do not claim here that the data provides definitive evidence that it does, nor did Bian et al. (2016). However, we argue that Kelty-Stephen (2018) dismisses the possibility based on a limited consideration of the broader investigation, and we do not wish for this premature and inaccurate characterization to propagate (e.g., Cuthill et al., 2019).

In characterizing Bian et al.’s (2016) study, Kelty-Stephen (2018) correctly describes how phasmids decreased swaying over time and that these data were from trials in which no plants were present. It is from this account alone that Kelty-Stephen (2018) rejects the adaptation explanation, suggesting that decreasing sway could be regarded as “a morbid wish to stand out to predators” (p. 16). This is a flawed proposition because there were no plants present in the trial so continuing to sway would also offer no camouflage benefit. What Kelty-Stephen (2018) did not mention is that the aim of the initial experiment by Bian et al. (2016) was to determine whether wind initiates swaying in hanging insects; which was clearly demonstrated. A second experiment subsequently showed that insects swayed more under variable wind, which is a more natural wind stimulus, compared with constant wind as used in the first experiment. Together, these two experiments by Bian et al. (2016) showed that wind initiates swaying and that insects can control swaying. The presence of both wind and insect control of swaying are necessary for a putative motion camouflage explanation; however, neither were undertaken in the presence of plants and make no attempt to relate swaying behavior with plant movement.

Similarities between phasmid and plant movement was investigated in a third experiment. Here insects and plants were filmed in natural conditions and in many circumstance the movement of both matched in the frequency domain. There were some exceptions that serve to highlight the potential for non-moving objects to standout from moving ones, but also to prompt consideration of the circumstance that do and do not lead to swaying in natural environments. These are outlined in Bian et al. (2016) and highlight the complex factors that contribute to animal behavior, a point which seems to have alluded Kelty-Stephen (2018). Thus, contrary to Kelty-Stephen’s (2018) assertion that Bian et al. (2016) “failed to find evidence that phasmids exploited the wind in the way they predicted” (p. 16), the complete dataset in Bian et al. (2016) showed that there most certainly is the potential for insects to benefit from swaying in wind and enough evidence was provided to take the next step, which is to confirm such behavior confers a survival advantage for the insects. Such investigations necessarily require an understanding of predator motion vision systems and behavior.