Habitat fragmentation caused by human activities in natural environments, altering ecosystem characteristics such as area reduction, loss of vegetation, changes in microclimatic conditions and the isolation of remaining vegetation fragments (1, 2). These changes reduce the size of habitat patches, supporting smaller populations and increasing the risk of stochastic extinction (3). Furthermore, greater inter-patch distances increase dispersal challenges, potentially intensifying the extinction vortex (3, 4). These effects are further exacerbated by deforestation and global temperature increases, significantly modifying the ecology and behaviour of many organisms (5). Some studies on insects involved in disease transmission show increased prevalence in deforested areas (6–8). However, other studies suggest that forest conservation could boost vector abundance, potentially enhancing their invasion of human environments (9, 10). These contrasting conclusions may arise from differences in the ecological dynamics, life history, and phenotypic traits of insect vectors, which exhibit distinct dispersal behaviours, habitats, and interactions with their environments.

Dispersal refers to the movement from birth to breeding sites and involves three stages: emigration, transience, and immigration (11–14). Its evolution balances the costs and benefits at each stage, which are shaped by individual, social, and environmental factors (13, 15–17). Morphological phenotypic traits underlying dispersal are classified as enabling traits (necessary for dispersal), enhancing traits (which reduce costs or improve efficiency), and matching traits (which facilitate non-random movements) (3). In fragmented landscapes, dispersal is further challenged by increasing distances between habitat patches, requiring traits that improve efficiency and reduce costs (18). Adverse developmental environments can amplify genetic variation or reduce developmental stability, increasing phenotypic plasticity and enabling individuals to adapt to fragmented habitats. This plasticity may lead to genetic changes through genetic assimilation or accommodation (19–22).

Head and wing size and shape are key determinants of dispersal in insects and influence flight performance, orientation, and energy efficiency (18, 23–25). Morphological variation in these traits can mediate responses to selective pressures imposed by habitat fragmentation, promoting the evolution of forms that optimize flight in open or structurally complex environments. For instance, narrower wings may reduce energetic costs during long-distance flight, while changes in head morphology may relate to enhanced sensory processing or aerodynamics (23, 24, 26). These associations between form and function suggest that morphological traits may reflect adaptations to environmental structure and provide insight into dispersal dynamics in changing landscapes (3).

Symmetry in biological structures, a fundamental feature controlled by the genome, reflects developmental stability. In contrast, deviations from symmetry, such as fluctuating asymmetry (FA), indicate an organism’s capacity to buffer environmental variability and maintain consistent phenotypic expression (27, 28). Together, these mechanisms highlight how phenotypic traits and their plasticity mediate dispersal processes in fragmented habitats, providing insights into the evolutionary dynamics of species. Specifically, developmental stability buffers environmental variation and ensures consistent phenotypic expression within individuals possessing a particular genotype and environment (29). Thus, research on developmental stability along an anthropization gradient represents a valuable approach to understanding how environmental modification influences the dispersive characteristics of an insect species. To our knowledge, this is the first study using this approach in Triatominae species (review in 30).

This study focuses on Triatoma garciabesi Carcavallo, Cichero, Martinez, Prosen & Ronderos, 1967 and T. guasayana Wygodzinsky & Abalos, 1949 (Hemiptera: Reduviidae, Triatominae), secondary vectors of Trypanosoma cruzi Chagas, 1909 (Kinetoplastida, Trypanosomatidae), the etiological agent of Chagas disease. These species sustain the parasite transmission cycles in sylvatic environments and may connect wild and domestic cycles (31, 32). Both are spread across Argentina, Bolivia, and Paraguay (33, 34). Triatoma garciabesi, an arboreal species linked to birds, inhabits the loose bark of Prosopis sp. and maintains high population densities year-round (35). In contrast, T. guasayana, a terrestrial species associated with mammals, resides in dry cacti, fallen logs, and bromeliads, with population densities declining in winter (36, 37). Both species are known to invade rural houses during summer in the western Chaco Seco ecoregion of Argentina (35, 38–42).

Habitat degradation caused by human activities affects triatomine populations, promoting their dispersal into artificial ecotopes (43). Host populations decline and habitat loss impose selective pressures on flight-related traits, potentially enhancing mobility in dynamic landscapes (3, 44). For these two species, a recent study demonstrates that flight-dispersal characteristics changed in response to varying degrees of anthropization (45). These findings may be linked to the direct impact of fragmentation on the phenotypic specialization hypothesis, which proposes that landscape fragmentation exerts selective pressure on populations. Combined with greater inter-patch distances, this pressure may drive the evolution of alternative dispersal strategies (3).

In this research, we aim to understand how landscape fragmentation acts as a selective pressure on the phenotypic plasticity of flight-related traits in two species of triatomines. Particularly, our goal was to determine the effects of landscape metrics on changes in the shape and size of the head and wings in both species of triatomines. We hypothesize that habitat fragmentation promotes phenotypic shifts in flight-related traits in T. garciabesi and T. guasayana, favouring morphologies associated with enhanced dispersal capacity. Moreover, these phenotypic changes are shaped according to the life-history characteristics of each species, which could lead to different responses depending on their reproductive strategies, life cycles, and ecological adaptations. Specifically, we expect individuals from more fragmented landscapes to exhibit larger head and wings and particular shape modifications that improve aerodynamic efficiency. Moreover, we predict that morphological symmetry will be higher in these populations, suggesting directional selection towards optimal flight performance rather than increased developmental stress.

Our study further supports the hypothesis that the fragmentation of the landscape acts as a selective pressure on developmental stability in structures related to flight dispersion (3). Previous research by Fiad et al. (45) demonstrated that anthropization influences T. garciabesi and T. guasayana, leading to phenotypic changes across an anthropization gradient. However, the relationship between fragmentation and developmental stability regarding flight dispersal remained unclear. Here, we examine the direct impact of fragmentation on developmental stability related to flight dispersion in two Triatominae species. The primary focus of this study is that habitat fragmentation due to anthropization gradients influences the variation in flight-related traits differently between the two triatomine species studied. These differences can likely be attributed to their life history characteristics and their varying sensitivity to changes in their surrounding landscape. The effects of fragmentation caused by anthropization on morphological traits such as head and wing structure may be more pronounced in T. garciabesi than in T. guasayana. For T. garciabesi, the greater degree of asymmetry in the eyes and head and the greater size of the head indicate a stronger response to landscape fragmentation, particularly in environments with numerous habitat patches and higher landscape complexity. These findings may reflect phenotypic specialization in response to fragmented habitats, supporting the hypothesis that landscape fragmentation influences dispersal decisions by increasing costs. These drive phenotypic specializations that enhance the ability to cross the matrix and travel longer distances (3). In contrast, T. guasayana exhibited less pronounced morphological changes, possibly indicating lower sensitivity to habitat fragmentation or a more rigid response to landscape modifications. This variation between the species suggests that the degree of landscape alteration can shape species-specific adaptations, with T. garciabesi demonstrating greater sensitivity to environmental pressures. Overall, these results underline the importance of considering the interplay between landscape characteristics when studying morphological adaptations in response to habitat anthropization. This study is the first to examine how landscape fragmentation shapes morphological changes in two triatomine species.

Our findings reveal that head asymmetry differs significantly between intermediate and low anthropization levels, suggesting that specific environmental pressures may influence developmental stability in particular habitats. Previous studies have detected shape variations between these levels (45), and our results indicate that these differences may be driven by FA rather than symmetrical modifications. This aligns with findings in other Triatominae species, where increased FA has been associated with environmental stress during insect development (review in 30).

Several stress factors may contribute to this pattern, including extreme temperatures, changes in vegetation cover, the quality of food sources from hosts, and exposure to environmental chemicals such as insecticides (45, 64–68). Beasley et al. (69) postulated that the relationship between FA and stress is stronger when the source of stress is anthropogenic. Furthermore, it has been proposed that this species is sensitive to vegetation changes and landscape features (40, 45, 68). In this context, environmental stressors such as changes in the availability of suitable refuges and/or reduced food sources for this species due to landscape anthropization could be key factors. Increasing anthropization in the Arid Chaco has led to the reduction of large extensions of mature forest and a decline in avian biodiversity (70–73). This process directly affects T. garciabesi by reducing the availability of both shelters and bird hosts, which serve as a crucial blood source for this species. The resulting habitat degradation could further exacerbate the stress experienced by individuals in more fragmented environments. These stress factors, associated with habitat loss and reduced food availability, may directly impact the developmental stability of specific morphological structures. Our results indicate that habitat fragmentation increases FA in T. garciabesi, particularly in the anteocular region, where asymmetry was greatest in highly fragmented landscapes. In contrast, compound eyes tended to remain more symmetrical in those environments. FA-driven changes in head morphology involved compression of the anteocular region, resulting in more compact heads with greater ocular convexity. These characteristics resembled those described for macropterous individuals of T. guasayana, which tend to have shorter anteocular distances and smaller head sizes than micropterous specimens (23, 24, 74). These changes could suggest potential flight-related adaptations. Alternatively, such asymmetrical modifications may reflect developmental instability rather than true adaptation. Since FA reflects perturbations during development, it may disrupt the integration between eye shape and head structure, potentially affecting visual performance and limiting effective dispersal in fragmented landscapes. This altered head morphology may partially explain the observed reduction in population density from low to intermediate and highly anthropized areas (45).

Beyond the head, we also found significant differences in wing shape symmetry between intermediate and low anthropization levels. This suggests that habitat fragmentation may influence this trait through selective pressures that disrupt the canalization process (22). Additionally, the relationship between wing symmetry and patch number suggests an adaptation process to fragmentation. In highly fragmented landscapes with intermediate levels of anthropization, an adaptive response may arise, favouring individuals with traits that enhance long-distance flight. In contrast, phenotypic characteristics may be better suited for short-range dispersal in less fragmented environments, corresponding to low or high anthropization levels. Despite these differences in wing shape, wing asymmetry remained stable across the fragmentation gradient. This persistence suggests that wing asymmetry may be relatively conserved and potentially useful as an indicator of local environmental adaptation (30). Its maintenance could also be driven by developmental constraints or functional factors such as flight, which may restrict morphological variability and regulate developmental stability (3, 75).

Although the centroid size of the head and wings in T. garciabesi was associated with landscape metrics, the weak relationship coefficients suggest these effects are not biologically significant. Only sex emerged as a relevant factor, indicating sexual dimorphism in those traits and suggesting that females were larger than males. The existence of sexual dimorphism is well-known in Triatominae (76, 77). According to Cox et al. (78) and Fairbairn et al. (79), sexual dimorphism may be the result of ecological and reproductive pressures. In triatomines, it could be related to feeding habits, population density, habitat, or climatic changes (76, 80, 81). Some authors have proposed that insects with larger heads and wings have greater dispersal capacity (3, 82, 83). In the context of the sexual dimorphism observed in this study, this could enhance flight performance in mated females.

Head shape symmetry in T. guasayana differed significantly between low and intermediate levels of anthropization, as well as between low and high levels. Additionally, variations in both the symmetric and FA components of head shape drove differences between low and intermediate anthropization levels. In contrast, differences between low and high anthropization levels were solely attributed to changes in the FA component. The FA component of wing shape also differed significantly between low and intermediate levels of anthropization. Previous studies have reported head and wing shape differences between these levels of anthropization, suggesting that such changes may reflect morphological adaptations to enhance flight dispersal in more fragmented landscapes (45). However, the weak correlation between landscape metrics and both the symmetric and FA shape components suggests that landscape fragmentation might indirectly influence phenotypic plasticity.

Fragmentation may reduce the variation of local conditions within patches while increasing variation among patches (4). These local conditions create patch-specific selective pressures that act on the phenotypic plasticity of the populations inhabiting them, potentially altering dispersal propensities (3). The less pronounced morphological changes observed in T. guasayana compared to T. garciabesi align with previous findings. Our results, along with those of Fiad et al. (84, 45), suggest that, in this species, seasonality may play a more significant role than anthropization metrics in shaping flight-related traits.

In a study conducted in the arid Chaco of La Rioja province, T. guasayana was more frequently found in areas with low vegetation cover near houses (≤100 meters) and intermediate vegetation cover within a 1000 meters radius. Our findings indicate that both the symmetric and asymmetric components of shape begin to associate with landscape metrics beyond 2400 meters from houses. This suggests that these shape components influence dispersal beyond this distance, to fragmentation metrics such as PLAND, number of patches, aggregation index, and landscape shape index. These associations may reflect a direct impact of fragmentation on developmental stability related to flight dispersal. Wing asymmetry mechanically compromises flight performance and function in insects (85–87).

Triatoma guasayana exhibits a dispersal strategy predominantly adapted for flight, enabling a greater number of individuals to traverse the landscape and more frequently invade rural dwellings (36). Thus, individuals with lower asymmetry in flight-related traits may have an advantage for dispersal. Individuals of T. guasayana that develop during winter and emerge as adults in early summer, within a 2400 meters radius around houses, are the most likely to fly and invade households in the Arid Chaco. In these cases, domestic animals serve as a key food resource. The availability of such resources, combined with the surrounding semi-wild environments, may favour the persistence of wild foci that invade households during warmer seasons. The relatively lower evidence of developmental instability within the 2400 meters radius could enable these insects to explore different environments until suitable ones are identified for establishment and colony growth. This reflects greater resilience to environmental pressures associated with anthropization. This pattern may also explain why, in this area, captures of T. guasayana were three times higher than those of T. garciabesi, and why their density index remained similar across anthropization levels (45).

In summary, T. guasayana appears to exhibit greater morphological stability and adaptability to fragmented environments compared to T. garciabesi. Its flight-oriented dispersal strategy and lower developmental instability near human dwellings may enhance its ability to exploit semi-wild and domestic ecotones. This resilience may partially explain its higher prevalence across the anthropization gradient and its potential role as a persistent vector in rural areas of the Arid Chaco. These findings have important epidemiological implications, as they suggest that T. guasayana may maintain high dispersal potential and population densities even in fragmented landscapes. This, in turn, could facilitate its role as a bridge vector between sylvatic and domestic environments, increasing the risk of Chagas disease transmission in human-occupied areas.

Our results show that habitat fragmentation imposes differential selective pressures on Triatoma garciabesi and T. guasayana, leading to divergent dispersal behaviour. T. garciabesi exhibits higher developmental instability and marked morphological shifts in fragmented landscapes, suggesting a more plastic and potentially more energetically costly dispersal syndrome. In contrast, T. guasayana maintains more stable morphological traits across the fragmentation gradient, which may reflect a more conservative or less environmentally responsive dispersal strategy. These species-specific responses highlight the role of habitat structure in shaping dispersal-related traits and point out the importance of incorporating landscape configuration into our understanding of the adaptive and movement dynamics of these vector species.