Naturalistic Behavior
Prior to understanding the sensorimotor control behind the behavior, it is imperative to obtain a detailed description of the elementary motor patterns and their relationship with the sensory environment. Whereas the mechanisms underlying the encoding of odorant stimuli have only been investigated in gaseous phase, nearly nothing is known about the use of olfaction in natural conditions. To address this problem, we devised a new behavioral paradigm to study the role of larval olfaction in guiding motor behaviors in a microfluidic dig-and-dive chamber. When introduced onto a slab of low-percentage agarose, larvae commence digging. This exploratory behavior represents food search while also helping avoid parasites and predators. We observe that perhaps because of the lack of food cues, digging is often followed by a dive into the agarose. During a diving episode, larvae are unable to breathe due to the submersion of their spiracles in the semi-fluid agarose. As a consequence of the critical necessity of respiration, dives tend to be short and punctuated by long rests at the surface. Our custom-designed microfluidics chamber allows us to not only monitor the digging and diving behaviors but also to control the release of odorant cues at the bottom of the chamber. We have developed a machine-vision algorithm to track larval paths during the course of the assay. This software permits us to reconstruct the temporal evolution of the larval posture at a high resolution. By quantifying the behavior of wild type larvae and individuals with impairments in their olfactory function, we find that the presence of food-related odors significantly increases the frequency and depth of digging. This finding highlights the importance of olfaction to the sensory ecology of the Drosophila larva.
