The study of sensory systems in insects has a long-spanning history of almost an entire century. Olfaction, vision, and gustation are thoroughly researched in several robust insect models and new discoveries are made every day on the more elusive thermo- and mechano-sensory systems. Few specialized senses such as hygro- and magneto-reception are also identified in some insects. In light of recent advancements in the scientific investigation of insect behavior, it is not only important to study sensory modalities individually, but also as a combination of multimodal inputs. This is of particular significance, as a combinatorial approach to study sensory behaviors mimics the real-time environment of an insect with a wide spectrum of information available to it. As a fascinating field that is recently gaining new insight, multimodal integration in insects serves as a fundamental basis to understand complex insect behaviors including, but not limited to navigation, foraging, learning, and memory. In this review, we have summarized various studies that investigated sensory integration across modalities, with emphasis on three insect models (honeybees, ants and flies), their behaviors, and the corresponding neuronal underpinnings.

Insects rely heavily on sampling information from their environment for survival and fitness. Different sensory channels with dedicated downstream neural substrates are programmed to fine tune these signals and translate them into usable instructions to mediate complex behaviors. Sensory information can also be learned and stored as memories that influence decisions in the future. The learning of unimodal sensory signals, especially olfaction, has been studied extensively in different conditioning paradigms in different insects. Using the Drosophila melanogaster model in our work, we first investigated differences in the efficiencies of aversive associative visual and olfactory learning using sensory cues that are innately very attractive, such as blue or green light as well as food odors found in fruits or yeast. We then combined the presentation of visual and olfactory sensory stimuli during training to study the effect of bimodal integration on learning performance. When trained unimodally, flies did not easily learn visual stimuli, but when trained bimodally, they developed a significant short-term visual memory after a single learning trial. The bimodal training also suppressed the phototaxis response of the flies to near abolishment. However, a similar training did not enhance the short-term olfactory memory and in some cases, even caused reduction in strength. An enhancement after bimodal training was only seen with a weak long-term olfactory memory retrieved after 24h. Our study demonstrates that bimodal sensory integration is not always synergistic, but is conditional upon the training paradigm and inherent learning abilities of the sensory modalities involved.