Stick insects or leaf insects (order Phasmatodea) are renowned for their remarkable camouflage, resembling twigs, bark or leaves. However, beyond their motionless appearance, the wings of these insects conceal an evolutionary story far less static than one might assume. Of the approximately 3,500 described species, around 40% possess fully formed, functional wings (macropterous), while the remaining 60% exhibit drastically reduced wings (micropterous or brachypterous) or lack them altogether (apterous) (Fig. 1).

How can this morphological diversity be explained? And, more intriguingly, is it possible that some clades have lost and regained wings over the course of their evolution? Below, we will explore how fossil discoveries, genetic data and phylogenetic discussions have shaped—and at times challenged—our understanding of wing evolution in phasmids.

1 - A glimpse at the origins and fossil record in the late 20th century

To understand how this topic evolved from being a marginal concern into a small ‘revolution’ in evolutionary entomology—with phasmids at its centre—we must go back to the late 20th century and briefly review what was known at that time.

The evolutionary history of phasmids is closely tied to the evolution of wings. To understand it, one must go back to a much larger group: the Pterygota, which includes the vast majority of present-day insects, both winged and those descended from winged forms. The term comes from the Greek pterygōtos (πτερυγωτός), which literally means 'winged'.

On the other hand, it is worth considering a fundamental evolutionary principle: Dollo’s Law. Formulated in the late 19th century by the palaeontologist Louis Dollo, this law holds that a complex structure lost during evolution cannot be regained in its original form. From this viewpoint, insect wings are regarded as highly complex anatomical structures. They are not simple appendages: their formation and function require the coordination of numerous structural components (such as specialised venation, thoracic musculature, joints and sensory organs), as well as a complex genetic network involving multiple developmental and regulatory pathways (Chapman, 2012). Therefore, according to this law, the re-evolution of wings once completely lost would be extremely unlikely.

At that time, the phasmid fossil inventory relied almost exclusively on a few Mesozoic sites in Asia. The ‘stars’ were the winged members of the now-classic superfamily †Susumanioidea Gorochov, 1988 (Fig. 2). These specimens display two pairs of complete wings with intricate venation, and for decades they constituted virtually the only unreservedly accepted fossil phasmids. Beyond that core, the picture was much more blurred: an isolated wing described as †Permophasma kovalevi Gorochov, 1994 (Late Permian of Mongolia), some Triassic fragments of uncertain affinity, and two Cenozoic inclusions that not all authors considered unequivocal phasmids.

Under all these premises, it was assumed that phasmids had arisen from ancestors with fully developed wings, and that throughout their evolution, various clades had independently lost them with no possibility of recovery. This hypothesis would explain the wide range of wing development observed today in the group: from completely apterous species to those with functional wings for flight, including multiple intermediate forms.

And it is precisely within this framework of knowledge that, at the beginning of the 21st century, a proposal emerged that would radically challenge traditional ideas: the hypothesis that wings could have “re-evolved” in phasmids, put forward by Whiting, Bradler & Maxwell (2003).

2 - The great controversy: re-evolution of wings?

One of the most intense debates on phasmid evolution erupted with the article by Whiting, Bradler & Maxwell (2003). Based on an unprecedented molecular sampling in stick insect studies (genes 18S, 28S and H3) and a series of phylogenetic reconstructions, the authors put forward a proposal as bold as it was provocative: the ancestral lineage of stick insects had lost its wings very early on and, subsequently, some lineages had independently regained them on at least four occasions (Fig. 3). If confirmed, this would represent the most convincing case to date—and, if I am not mistaken, the first supported by comprehensive molecular analysis—of wing re-evolution in insects, challenging the deeply ingrained view that wings arose only once and that losses are definitive.

The key support for this hypothesis lay in the reconstruction of the ancestral state. Under their parsimony and maximum likelihood models, the common ancestor of phasmids emerged with over 95% probability of being apterous. Moreover, the winged taxa did not form a separate clade: they were nested among wingless groups, which could suggest successive episodes of functional ‘recovery’. Whiting et al. clarified that this did not imply that wings appeared de novo: many apterous phasmids retain thoracic musculature, neural circuits and, crucially, the basic genetic machinery shared with legs; it would be sufficient to reactivate that latent developmental plan to bring wings back.

The shock was immense and, barely a year later, the reply from Trueman et al. (2004) arrived. Their critique did not focus on the molecular data, but rather on the methodology. They argued that Whiting et al. had only tested the ‘apterous ancestor’ option and had not compared, under the same criteria, the alternative scenario of an alate ancestor with multiple losses. When they readjusted the gain/loss rates (without reaching extreme values), they found that the ancestor could be reconstructed as winged with the same—or greater—parsimony; it was enough to assume that wing loss is a couple of times more probable than wing gain, a very reasonable assumption given that loss has been documented thousands of times and regeneration never.

In a brief but forceful reply, Whiting and Whiting (2004) argued that their phylogenetic analysis, based on maximum parsimony and maximum likelihood methods, consistently placed six apterous lineages at the base of the phasmid evolutionary tree. In their view, this pattern is difficult to explain if one assumes an alate ancestor, thereby reinforcing the apterous ancestor hypothesis. They also challenged the validity of the parsimony argument employed by Trueman et al., pointing out that the 6:1 cost ratio assumed to favour the alate ancestor scenario was, in their opinion, excessively high and unrealistic for completely ruling out the possibility of reversals. Finally, they emphasised that their critics did not present an alternative phylogenetic topology based on new data that would invalidate theirs, meaning that the hypothesis of wing re-evolution, although contentious, ought to remain a viable possibility.

In summary, the work of Whiting et al. (2003) raised the possibility that in certain stick insect clades, the genetic and neuromuscular circuits associated with flight had not been completely extinguished but had remained latent and, under appropriate evolutionary pressure, could be ‘reactivated’ to produce functional wings again; Trueman et al. reminded us that more parsimonious explanations based solely on repeated losses exist; and Whiting’s 2004 reply insisted that with the evidence available, re-evolution could not be ruled out.

Two decades on, the debate continues, fuelled by new fossils and macro-analyses, but that exchange marked the beginning of a minor revolution in the way we think about evolutionary irreversibility.

3 - New fossils and new analyses: a more complex picture

From 2019 onwards, the debate on the evolutionary history of phasmids has been profoundly enriched by the contribution of new fossils, macroevolutionary studies and experimental developmental work. Far from closing the discussion, these investigations have opened new questions and nuanced previous interpretations.

New Cretaceous fossils

The fossil record has increased substantially this century, although perhaps the most interesting discovery is that of Yang et al. (2019). These authors described three adult males preserved in Kachin amber from Myanmar, mid-Cretaceous (≈ 99 Ma), and erected for them the new family †Pterophasmatidae Yang, Shih, Ren & Gao, 2019, with three monotypic genera for the following species: †Pterophasma erromera Yang, Shih, Ren & Gao, 2019, †Leptophasma physematosa Yang, Shih, Ren & Gao, 2019, and †Meniscophasma sinulatum Yang, Shih, Ren & Gao, 2019.

The particularity of these new specimens is that they exhibit complete wings, yet their wing morphology shows intermediate traits between the most primitive fossils and modern phasmids. This led the authors to propose that †Pterophasmatidae occupies a possible transitional stage between †Susumanioidea and present-day Euphasmatodea.

Another important curiosity is that other fossils associated with phasmids have been found at the same site over the past two decades, and these display a wide morphological diversity. Consequently, this could indicate that the mid-Cretaceous represented a key period of radiation and evolution for stick insects, possibly linked to the extensive diversification of angiosperms that occurred at the same time (Sauquet et al. 2017).

Despite the relevance of these findings, which provide new pieces to the complex puzzle of phasmid evolution, they do not constitute conclusive proof either for or against the hypothesis of wing loss and subsequent recovery throughout their evolutionary history.

Ecomorphology, macroevolutionary models and extensive phylogenies

However, since 2020 the debate on the evolutionary history of wings in stick insects has been reignited, with two particularly interesting lines of research: one ecomorphological and the other phylogenetic-macroevolutionary.

The first is characterised by the work of Yu Zeng and colleagues (2020, 2023). Their aim was not to determine whether lost wings can ‘reappear’, but to explain why so many modern phasmids exhibit highly reduced wings or have lost them entirely, and how this reduction relates to ecology, biomechanics and, decisively, sexual dimorphism (Fig. 4). To this end, they assembled an unprecedented inventory—over 750 species—and measured, separately for females and males, body lengths, wing areas, wing loads and dimorphism indices (SSD for body size; SWD for wing size).

Their data show that the general rule in phasmids is to find larger-bodied females with very short or absent wings, and smaller males whose pairs of wings—when present—are proportionally longer than those of females. According to the authors, this decoupling reflects different pressures: females allocate resources to producing more eggs and can afford to forgo flight; males, in contrast, retain better dispersal capacity for locating mates. When both sexes abandon flight, the dimorphism tends to collapse.

At the order-wide scale, Zeng and colleagues observed that phasmids capable of flight converge on a narrow range of wing load, whereas non-flying species occupy an enormous range: aerodynamic selection matters (literally) only while the wings serve for flight. Moreover, they demonstrated that wing reduction is not merely ‘trimming tissue’. It involves reconfiguring the thoracic musculature, redistributing mass among legs, abdomen and thorax, and simplifying venation to produce wings that are more rigid and cheaper to manufacture.

Meanwhile, other research groups chose to tackle the ‘million-dollar question’ from a phylogenetic and macroevolutionary perspective. Thus, Forni et al. (2022) took the debate a step further by incorporating, for the first time, SSE (State-Speciation-Extinction) macroevolutionary models to explicitly assess how gains and losses of wings influence phasmid diversification. Unlike previous approaches, which were limited to ancestral-state reconstructions or morphological comparisons, this team—of which I was fortunate enough to be a part—compiled a large-scale phylogeny with more than 300 species, based on seven molecular markers (18S, 28S, H3, COI, COII, 12S and 16S), and fitted a suite of over 40 alternative evolutionary models.

The key finding was that models allowing both wing loss and recovery fit the data significantly better. In other words, out of more than forty combinations of models evaluated, those permitting bidirectional transitions—wings lost and then regained—fit the observed ‘winged’ distribution pattern on the phylogenetic tree much more closely. In other words: if we assume that wings can re-evolve, the evolutionary history of phasmids makes more statistical sense.

Moreover, by employing advanced HiSSE models, researchers discovered that species groups capable of "reversing" wing loss exhibited higher net diversification rates—that is, they generated more new species and experienced fewer extinctions—compared to groups in which wing loss was irreversible. This finding suggests that retaining the potential to recover a lost trait may confer a significant long-term evolutionary advantage.

However, the authors themselves warned that this advantage could artificially inflate the apparent frequency of reversals. To control for this bias, two statistical tools were combined: BiSSE (which measures whether a binary character, such as ‘with or without wings’, influences diversification rates) and HiSSE (which allows unobserved factors—‘hidden states’—to modulate those rates without being confounded with the primary character).

Ultimately, Forni et al. (2022) did not claim to have proven beyond doubt that wings have re-evolved, but they did show that, even with stringent robustness tests—such as phylogenetic resampling or extreme penalisation of gain probabilities—the data fit better in scenarios where wings were not lost forever, but could reappear at multiple points in phasmid evolutionary history.

A few months later, Bank & Bradler (2022) published a study that in turn introduced several novel elements: an even more extensive phylogeny (based on 513 species and five genes: 18S, 28S, H3, COI and COII) and a particularly interesting approach focused on the presence or absence of ocelli—photoreceptive organs that contribute to horizon detection and three-dimensional spatial orientation during flight, among other functions (Fig. 5). Ocelli in insects have not been intensively studied in a phylogenetic context.

Starting from a robust phylogenetic reconstruction, they reconstructed the ancestral state of both the wing apparatus and the ocelli, and evaluated how these two characters are distributed throughout the tree. Their aim was twofold: on the one hand, to identify how many times wings and ocelli had been lost; on the other, to assess whether there was evidence of their reappearance.

One of the most striking results was that the majority of extant winged species lack ocelli, while a few species do possess them. At first glance, one might assume this represents simple ancestral retention, although the current pattern does not clearly support that option. However, if we assume—as their results indicate—that the last common ancestor of phasmids was apterous and lacked ocelli, then the present distribution requires multiple re-gains of both characters. The authors acknowledge that this hypothesis depends on that initial reconstruction (a wingless, ocelli-less ancestor), but they demonstrate that their results hold under multiple analytical conditions.

Most interestingly, for both wings and ocelli, models allowing reversals—even though they do not quantify diversification rates—better explain the observed distribution of these characters on the tree than those that impose irreversible loss.

4 - Function, mechanics and evolutionary costs of wings

So far we have discussed when and how wings could have been lost or reappeared. But what functions do they actually fulfil when present? What costs and benefits do they entail?

The evolution of wings in stick insects cannot be understood solely as a matter of flight ‘yes or no’. In reality, wings in this group have acquired a functional diversity that goes far beyond aerial locomotion.

In some species, especially males, the wings still allow active flight or gliding, which likely facilitates dispersal, mate searching, access to a greater diversity of food sources, predator evasion, and so on. But in many other cases, wings have been transformed into tools with alternative functions. A clear example is found in deimatic defence mechanisms: certain species, such as Metriophasma diocles (Westwood, 1859), suddenly unfurl their coloured wings when disturbed, creating a surprise effect that deters predators (Bedford, 1978) (Fig. 7). In other cases, such as Pterinoxylus crassus Kirby, 1889, the hindwings bear structural modifications that enable sound production by stridulation (Bedford, 1978; Hennemann et al., 2020), adding an acoustic dimension to their defensive repertoire.

On the other hand, maintaining functional wings carries a cost. Developing them requires investment in tissues, venation, specialised thoracic musculature, neuromotor control and maintenance during the adult stage (Roff & Fairbairn, 1991). In stable environments, where individuals seldom need to travel long distances, and where camouflage or crypsis predominates, these costs may not pay off. In such contexts, natural selection can progressively favour brachypterous or apterous forms, where resources previously allocated to flight are redirected, for example, towards higher fecundity (Maginnis, 2006), since females can invest more in egg production. Indeed, many phasmid species exhibit males with functional wings for flight, while females are larger, brachypterous or even completely apterous.

Furthermore, the wing apparatus does not evolve in isolation. Its development competes with other physiological demands, as Maginnis (2006) demonstrated in Sipyloidea sipylus (Westwood, 1859)—currently considered a synonym of Sipyloidea chlorotica (Audinet-Serville, 1838)—where leg regeneration during the nymphal stage negatively affected wing growth: regenerated individuals exhibited smaller wings and reduced flight capability (Fig. 6). This type of resource competition suggests that wings, even when present, may not always be guaranteed full functionality.

Overall, wing evolution in phasmids cannot be summarised as a mere structural loss or gain. It likely reflects, instead, a complex adaptive reorganisation of the body, intertwining factors such as habitat ecology, behaviour, life history and reproductive priorities. Therefore, talking about wings in stick insects is talking about flight but also camouflage, defence, cost–benefit trade-offs and evolutionary plasticity.

With all these elements on the table—fossils, phylogenies, functions and costs—it is time to ask what we have truly learned and where the debate is headed.

5 - Perspectives and conclusions

Two decades after Whiting et al. (2003) shook things up with the wing re-evolution hypothesis, the question remains open. But it is no longer solely a theoretical duel between parsimony and likelihood, nor a discussion confined to an eccentric insect clade. Wing evolution in phasmids has become a paradigmatic case study for exploring how evolutionary irreversibility works—and to what extent it can be broken.

The current scenario is much richer and more nuanced. New fossils have expanded the palaeontological record; SSE models have made it possible to evaluate the role of wings in diversification with more refined statistical tools; and recent works have introduced unexpected variables—such as ocelli distribution or the role of sexual dimorphism—that provide new clues for interpreting the puzzle.

Beyond the technicalities, what emerges is a broader lesson about evolution: it rarely follows straight paths. The history of phasmids reminds us that even structures as complex as wings, whose loss was assumed to be definitive, may re-emerge under certain conditions if the underlying genetic and physiological circuits have not been completely lost. There are no guarantees, but neither are there absolute red lines.

Have wings truly re-evolved in some phasmids? The definitive answer remains pending, although all the evidence we have so far indicates that it is most likely the case. Perhaps, just as these insects appear to be twigs or leaves but are not, so their evolutionary history appeared linear… until we discovered that it is not.

Referencias

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