Superhydrophobic diving flies (Ephydra hians) and the hypersaline waters of Mono Lake

The remarkable alkali fly, Ephydra hians, deliberately crawls into the alkaline waters of Mono Lake to feed and lay eggs. These diving flies are protected by an air bubble that forms around their superhydrophobic cuticle upon entering the lake. To study the physical mechanisms underlying this process we measured the work required for flies to enter and leave various aqueous solutions. Our measurements show that it is more difficult for the flies to escape from Mono Lake water than from fresh water, due to the high concentration of Na_2CO_3 which causes water to penetrate and thus wet their setose cuticle. Other less kosmotropic salts do not have this effect, suggesting that the phenomenon is governed by Hofmeister effects as well as specific interactions between ion pairs. These effects likely create a small negative charge at the air–water interface, generating an electric double layer that facilitates wetting. Compared with six other species of flies, alkali flies are better able to resist wetting in a 0.5 M Na_2CO_3 solution. This trait arises from a combination of factors including a denser layer of setae on their cuticle and the prevalence of smaller cuticular hydrocarbons compared with other species. Although superbly adapted to resisting wetting, alkali flies are vulnerable to getting stuck in natural and artificial oils, including dimethicone, a common ingredient in sunscreen and other cosmetics. Mono Lake’s alkali flies are a compelling example of how the evolution of picoscale physical and chemical changes can allow an animal to occupy an entirely new ecological niche

Plume-Tracking Behavior of Flying Drosophila Emerges from a Set of Distinct Sensory-Motor Reflexes

Background: For a fruit fly, locating fermenting fruit where it can feed, find mates, and lay eggs is an essential and difficult task requiring the integration of olfactory and visual cues. Here, we develop an approach to correlate flies’ free-flight behavior with their olfactory experience under different wind and visual conditions, yielding new insight into plume tracking based on over 70 hr of data. Results: To localize an odor source, flies exhibit three iterative, independent, reflex-driven behaviors, which remain constant through repeated encounters of the same stimulus: (1) 190 ± 75 ms after encountering a plume, flies increase their flight speed and turn upwind, using visual cues to determine wind direction. Due to this substantial response delay, flies pass through the plume shortly after entering it. (2) 450 ± 165 ms after losing the plume, flies initiate a series of vertical and horizontal casts, using visual cues to maintain a crosswind heading. (3) After sensing an attractive odor, flies exhibit an enhanced attraction to small visual features, which increases their probability of finding the plume’s source. Conclusions: Due to plume structure and sensory-motor delays, a fly’s olfactory experience during foraging flights consists of short bursts of odor stimulation. As a consequence, delays in the onset of crosswind casting and the increased attraction to visual features are necessary behavioral components for efficiently locating an odor source. Our results provide a quantitative behavioral background for elucidating the neural basis of plume tracking using genetic and physiological approaches

Empirical Individual State Observability

A dynamical system is observable if there is a one-to-one mapping from the system's measured outputs and inputs to all of the system's states. Analytical and empirical tools exist for quantifying the (full state) observability of linear and nonlinear systems; however, empirical tools for evaluating the observability of individual state variables are lacking. Here, a new empirical approach termed Empirical Individual State Observability (E-ISO) is developed to quantify the level of observability of individual state variables. E-ISO first builds an empirical observability matrix via simulation, then applies convex optimization to efficiently determine the subset of its rows required to estimate each state variable individually. Finally, (un)observability measures for these subsets are calculated to provide independent estimates of the observability of each state variable. Multiple example applications of E-ISO on linear and nonlinear systems are shown to be consistent with analytical results. Broadly, E-ISO will be an invaluable tool both for designing active sensing control laws or optimizing sensor placement to increase the observability of individual state variables for engineered systems, and analyzing the trajectory decisions made by organisms.Comment: 10 pages, 3 figure

Distinct activity-gated pathways mediate attraction and aversion to CO₂ in Drosophila

Carbon dioxide is produced by many organic processes and is a convenient volatile cue for insects that are searching for blood hosts, flowers, communal nests, fruit and wildfires. Although Drosophila melanogaster feed on yeast that produce CO₂ and ethanol during fermentation, laboratory experiments suggest that walking flies avoid CO₂. Here we resolve this paradox by showing that both flying and walking Drosophila find CO₂ attractive, but only when they are in an active state associated with foraging. Their aversion to CO₂ at low-activity levels may be an adaptation to avoid parasites that seek CO₂, or to avoid succumbing to respiratory acidosis in the presence of high concentrations of CO_2 that exist in nature. In contrast to CO₂, flies are attracted to ethanol in all behavioural states, and invest twice the time searching near ethanol compared to CO₂. These behavioural differences reflect the fact that ethanol is a unique signature of yeast fermentation, whereas CO₂ is generated by many natural processes. Using genetic tools, we determined that the evolutionarily conserved ionotropic co-receptor IR25a is required for CO₂ attraction, and that the receptors necessary for CO₂ avoidance are not involved in this attraction. Our study lays the foundation for future research to determine the neural circuits that underlie both state- and odorant-dependent decision-making in Drosophila

Octopaminergic modulation of the visual flight speed regulator of Drosophila

Recent evidence suggests that flies' sensitivity to large-field optic flow is increased by the release of octopamine during flight. This increase in gain presumably enhances visually mediated behaviors such as the active regulation of forward speed, a process that involves the comparison of a vision-based estimate of velocity with an internal set point. To determine where in the neural circuit this comparison is made, we selectively silenced the octopamine neurons in the fruit fly Drosophila, and examined the effect on vision-based velocity regulation in free-flying flies. We found that flies with inactivated octopamine neurons accelerated more slowly in response to visual motion than control flies, but maintained nearly the same baseline flight speed. Our results are parsimonious with a circuit architecture in which the internal control signal is injected into the visual motion pathway upstream of the interneuron network that estimates groundspeed

Mosquitoes Use Vision to Associate Odor Plumes with Thermal Targets

All moving animals, including flies [1, 2 and 3], sharks [4], and humans [5], experience a dynamic sensory landscape that is a function of both their trajectory through space and the distribution of stimuli in the environment. This is particularly apparent for mosquitoes, which use a combination of olfactory, visual, and thermal cues to locate hosts [6, 7, 8, 9 and 10]. Mosquitoes are thought to detect suitable hosts by the presence of a sparse CO2 plume, which they track by surging upwind and casting crosswind [11]. Upon approach, local cues such as heat and skin volatiles help them identify a landing site [12, 13, 14 and 15]. Recent evidence suggests that thermal attraction is gated by the presence of CO2 [6], although this conclusion was based experiments in which the actual flight trajectories of the animals were unknown and visual cues were not studied. Using a three-dimensional tracking system, we show that rather than gating heat sensing, the detection of CO2 actually activates a strong attraction to visual features. This visual reflex guides the mosquitoes to potential hosts where they are close enough to detect thermal cues. By experimentally decoupling the olfactory, visual, and thermal cues, we show that the motor reactions to these stimuli are independently controlled. Given that humans become visible to mosquitoes at a distance of 5–15 m [16], visual cues play a critical intermediate role in host localization by coupling long-range plume tracking to behaviors that require short-range cues. Rather than direct neural coupling, the separate sensory-motor reflexes are linked as a result of the interaction between the animal’s reactions and the spatial structure of the stimuli in the environment