Vision is likely the main sense involved in predator–prey interactions. While predators' visual systems are typically tuned to detect and precisely follow moving targets, prey's visual systems present adaptations that allow them to rapidly detect and evade approaching chasers (Hughes, 1977). Compared to other sensory modalities, vision offers several advantages, mainly because it allows the detection of visual targets at long distances and simultaneously provides a precise representation of the surrounding space with high temporal resolution. In such inter-specific interactions, predators and prey define their approaching or evasive responses based on a continuous evaluation of how they see the other part moving (Domenici et al., 2011a, Domenici et al., 2011b). These interactions are hard to study in the animals' natural environment mainly due to the difficulties of properly registering the behavior of the animals and the impossibility of manipulating the variables involved in the chase. Laboratory studies, instead, are more limited from an ecological point of view but partially solve some of those inconveniences, permitting the study of the visuomotor transformations underlying predator–prey interactions.

Reduviidae (Hemiptera: Heteroptera) is a large family of ca. 7000 species of true bugs that includes mostly predator species, commonly known as assassin bugs (Ambrose and Kumar, 2016). In addition, the family includes a subfamily of ca. 140 species called Triatominae, commonly named kissing bugs, whose principal diagnostic character is their obliged hematophagy (Schofield, 1988). Regardless of their different feeding habits, most reduviids possess relatively large globose compound eyes laterally protruding from their head capsule (Weirauch, 2008). These are apposition compound eyes in which the ommatidia possess an open rhabdom composed of a ring of six rhabdomeres (R1-6) surrounding a central pair of rhabdomeres (R7-8) (Fischer et al., 2000, Reisenman et al., 2002). While most predatory assassin bugs are diurnal, kissing bugs are markedly crepuscular and nocturnal. In triatomines, it has been shown that screening pigments form a dynamic ‘pupil’, whose diameter changes with both the time of the day and the light intensity (Reisenman et al., 2002). This light filter enables triatomines to adjust their eyes for varying light conditions. In low light, the pupil opens to enhance sensitivity, while in bright light, the pupil constricts and improves spatial resolution (Nilsson and Ro, 1994).

Studies on the visual capabilities of reduviids are rather scant. Concerning assassin bugs, the role of vision in their predatory behavior has been studied in the field and in laboratory-controlled conditions by presenting prey dummies (Parker, 1969, Haridass and Ananthakrishnan, 1980, Claver and Ambrose, 2001). These studies uncovered how different visual parameters of the prey (e.g. size, speed, direction of motion) trigger different phases of the predatory behaviors displayed by these insects (e.g. freezing, onset of chase, slow approach, running approach). However, we found no studies investigating the role that vision could play in defensive behaviors or other biological contexts of assassin bugs.

Studies on kissing bugs' visual capabilities are also scant. This is curious since triatomines are epidemiologically relevant insects. They are vectors of the parasite Trypanosoma cruzi, the agent responsible for Chagas Disease in the Americas (WHO, 2025). Besides, since the pioneering work of Wigglesworth and Gillett (1934), their physiology and behavior have been extensively studied (Lazzari et al., 2013, Barrozo et al., 2017). Triatomines feed on sleeping warm-blooded hosts almost exclusively during the night. The principal sources of information that have been involved in guiding their feeding behavior are olfactory and thermal cues (Guerenstein and Lazzari, 2009). Although the role of vision in their feeding behavior has been classically neglected, other visually guided behaviors of triatomines have been described. Two of the most epidemiologically relevant species, Rhodnius prolixus and Triatoma infestans, avoid highly illuminated spaces guided by a marked negative phototaxis (Reisenman et al., 1998, Reisenman and Lazzari, 2006). In contrast, some triatomine species show positive phototaxis to dim punctual light sources, evidenced by an attraction to the source of light in both natural environments and laboratory experiments (Noireau and Dujardin, 2001, Minoli and Lazzari, 2006, de la Fuente et al., 2007, Minoli et al., 2018, Ortiz et al., 2023). While these phototactic behaviors require rather simple visual organs and neural processing, visual behaviors that involve object detection require better spatial resolution and more complex visual processing. This more complex form of spatial vision, usually referred to as image vision, supports behaviors such as pursuit, escape, communication, among others (reviewed in Nilsson, 2022). The only antecedent on such higher visual performance in triatomines was published by Lazzari and Varjú (1990). They found that T. infestans can fixate a moving target in the posterior-lateral region of their visual field; and they suggested that this capability would allow them to visually track an eventual predator while still being able to escape in a direction away from it.

In the current study, we explore the role of vision in mediating defensive behaviors in reduviids, particularly in triatomines. In nature, triatomines are predated by birds, rodents, spiders, entomophagous insects, and even by their hosts if they awaken during the insect's feeding event. Under laboratory-controlled conditions, R. prolixus was presented with looming stimuli that simulate the approach of a predator. We observed that the animals consistently responded by freezing or escaping from the visual stimulus. Moreover, we found that they can rapidly concatenate these behaviors according to the ongoing information provided by the stimulus. By confronting animals with looming stimuli with different dynamics, we found that the escape response was principally evoked by stimuli that simulate the approach of an object with a sustained velocity. When we modified the salience of the stimulus by changing its contrast with the background, we found that the animals displayed freezing responses no matter the magnitude of the contrast but evoked escape responses only to high-contrast stimuli. In addition, we studied the plasticity of the defensive behaviors by presenting a massed stimulation protocol; however, we found no habituation of the defensive responses. The results obtained in the current work indicate a clear role for the well-developed visual system of kissing bugs in supporting defensive behaviors. We found that these defensive behaviors are elicited in a fast and adaptive manner according to the risk that the visual stimuli entail.