Characterizing long-range search behavior in Diptera using complex 3D virtual environments

The exemplary search capabilities of flying insects have established them as one of the most diverse taxa on Earth. However, we still lack the fundamental ability to quantify, represent, and predict trajectories under natural contexts to understand search and its applications. For example, flying insects have evolved in complex multimodal 3D environments, but we do not yet understand which features of the natural world are used to locate distant objects. Here, we independently and dynamically manipulate 3D objects, airflow fields, and odor plumes in virtual reality over large spatial and temporal scales. We demonstrate that that flies make use of features such as foreground segmentation, perspective, motion parallax, and integration of multiple modalities to navigate to objects in a complex 3D landscape while in flight. We first show that tethered flying insects of multiple species navigate to virtual 3D objects. Using the apple fly, Rhagoletis pomonella, we then measure their reactive distance to objects and show that these flies use perspective and local parallax cues to distinguish and navigate to virtual objects of different sizes and distances. We also show that apple flies can orient in the absence of optic flow by using only directional airflow cues, and require simultaneous odor and directional airflow input for plume following to a host volatile blend. The elucidation of these features unlocks the opportunity to quantify parameters underlying insect behavior such as reactive space, optimal foraging, and dispersal, as well as develop strategies for pest management, pollination, robotics and search algorithms.

Methods
We assessed critical parameters of long range search including motion parallax, perspective, reactive distance, anemotaxis, and plume following using a multimodal virtual reality arena (MultiMoVR). To this end, we provided photorealistic scenes and perspective-accurate stimuli of 3D tree models along with grass and sky textures in a 1025 m x 1025 m landscape, including directional airflow and odor. This landscape was presented in a periodic boundary condition such that as the animal approaches the end of the virtual landscape, it is seamlessly teleported to the opposite side, so the animal can essentially translate infinitely in any direction. Using this arena, we show that R. pomonella can approach and discriminate virtual objects of varying sizes and distances in a complex 3D environment, respond to directional airflow based on velocity, and orient to directional odor flux in VR. We also show that multiple Dipteran species, including a North American pest (R. pomonella), a tropical vector (Aedes aegypti), an Asian species (Pselliophora laeta) and a cosmopolitan pollinator (Eristalis tenax) can navigate towards virtual objects at distances found in nature using this system.