Despite their small brains, insects can navigate over long distances by orienting using visual landmarks [1], skylight polarization [2-9], and sun position [3, 4, 6, 10]. Although Drosophila are not generally renowned for their navigational abilities, mark-and-recapture experiments in Death Valley revealed that they can fly nearly 15 km in a single evening [11]. To accomplish such feats on available energy reserves [12], flies would have to maintain relatively straight headings, relying on celestial cues [13]. Cues such as sun position and polarized light are likely integrated throughout the sensory-motor pathway [14], including the highly conserved central complex [4, 15, 16]. Recently, a group of Drosophila central complex cells (E-PG neurons) have been shown to function as an internal compass [17-19], similar to mammalian head-direction cells [20]. Using an array of genetic tools, we set out to test whether flies can navigate using the sun and to identify the role of E-PG cells in this behavior. Using a flight simulator, we found that Drosophila adopt arbitrary headings with respect to a simulated sun, thus performing menotaxis, and individuals remember their heading preference between successive flights-even over several hours. Imaging experiments performed on flying animals revealed that the E-PG cells track sun stimulus motion. When these neurons are silenced, flies no longer adopt and maintain arbitrary headings relative to the sun stimulus but instead exhibit frontal phototaxis. Thus, without the compass system, flies lose the ability to execute menotaxis and revert to a simpler, reflexive behavior.

To follow a straight course, animals must maintain a constant heading relative to a fixed, distant landmark, a strategy termed menotaxis. In experiments using a flight simulator, we found that Drosophila adopt arbitrary headings with respect to a simulated sun, and individuals remember their heading preference between successive flights—even over gaps lasting several hours. Imaging experiments revealed that a class of neurons within the central complex, which have been previously shown to act as an internal compass, track the azimuthal motion of a sun stimulus. When these neurons are silenced, flies no longer adopt and maintain arbitrary headings, but instead exhibit frontal phototaxis. Thus, without the compass system, flies lose the ability to execute menotaxis and revert to a simpler, reflexive behavior.One sentence summarySilencing the compass neurons in the central complex of Drosophila eliminates sun navigation but leaves phototaxis intact.

Despite the ecological importance of long-distance dispersal in insects, its mechanistic basis is poorly understood in genetic model species, in which advanced molecular tools are readily available. One critical question is how insects interact with the wind to detect attractive odor plumes and increase their travel distance as they disperse. To gain insight into dispersal, we conducted release-and-recapture experiments in the Mojave Desert using the fruit fly, Drosophila melanogaster. We deployed chemically baited traps in a 1 km radius ring around the release site, equipped with cameras that captured the arrival times of flies as they landed. In each experiment, we released between 30,000 and 200,000 flies. By repeating the experiments under a variety of conditions, we were able to quantify the influence of wind on flies’ dispersal behavior. Our results confirm that even tiny fruit flies could disperse ∼12 km in a single flight in still air and might travel many times that distance in a moderate wind. The dispersal behavior of the flies is well explained by an agent-based model in which animals maintain a fixed body orientation relative to celestial cues, actively regulate groundspeed along their body axis, and allow the wind to advect them sideways. The model accounts for the observation that flies actively fan out in all directions in still air but are increasingly advected downwind as winds intensify. Our results suggest that dispersing insects may strike a balance between the need to cover large distances while still maintaining the chance of intercepting odor plumes from upwind sources.

25Despite the ecological importance of long-distance dispersal in insects, its underlying mechanistic 26 basis is poorly understood. One critical question is how insects interact with the wind to increase 27 their travel distance as they disperse. To gain insight into dispersal using a species amenable to 28 further investigation using genetic tools, we conducted release-and-recapture experiments in the 29Mojave Desert using the fruit fly, Drosophila melanogaster. We deployed chemically-baited traps 30 in a 1 km-radius ring around the release site, equipped with machine vision systems that captured 31 the arrival times of flies as they landed. In each experiment, we released between 30,000 and 32200,000 flies. By repeating the experiments under a variety of conditions, we were able to quantify 33 the influence of wind on flies' dispersal behavior. Our results confirm that even tiny fruit flies could 34 disperse ~15 km in a single flight in still air, and might travel many times that distance in a 35 moderate wind. The dispersal behavior of the flies is well explained by a model in which animals 36 maintain a fixed body orientation relative to celestial cues, actively regulate groundspeed along 37 their body axis, and allow the wind to advect them sideways. The model accounts for the 38 observation that flies actively fan out in all directions in still air, but are increasingly advected 39 downwind as winds intensify. In contrast, our field data do not support a Lévy flight model of 40 dispersal, despite the fact that our experimental conditions almost perfectly match the core 41 assumptions of that theory. 42 Significance Statement 43Flying insects play a vital role in terrestrial ecosystems, and their decline over the past few 44 decades has been implicated in a collapse of many species that depend upon them for food. By 45 dispersing over large distances, insects transport biomass from one region to another and thus 46 their flight behavior influences ecology on a global scale. Our experiments provide key insight into 47 the dispersal behavior of insects, and suggest that these animals employ a single algorithm that 48 is functionally robust in both still air and under windy conditions. Our results will make it easier to 49 study the ecologically important phenomenon of long-distance dispersal in a genetic model 50 organism, facilitating the identification of cellular and genetic mechanisms. 51 52

In male spiders, genitalia, sexual behaviour and secondary sex morphology tend to diverge rapidly across species, presumably as a result of sexual selection. In the three Leucauge species for which pre-and copulatory courtship behaviour is known, females clamp the male chelicerae prior to and during copulation. This brings the basal segment of the male's chelicerae into contact with the anterior surface of the female's chelicerae. The basal segment of male's chelicerae has also morphological features such as sturdy, abundant setae which are thought to have evolved to stimulate females, as well as other morphological features whose specific function is yet unknown. We show here that in a fourth species, Leucauge sp., the female does not clamp the male's chelicerae; as expected, this absence is associated with a lack of secondary sexual differences in the male chelicerae.