A `bright zone' in male hoverfly (Eristalis tenax) eyes and associated faster motion detection and increased contrast sensitivity

Eyes of the hoverfly Eristalis tenax are sexually dimorphic such that males have a fronto-dorsal region of large facets. In contrast to other large flies in which large facets are associated with a decreased interommatidial angle to form a dorsal `acute zone' of increased spatial resolution, we show that a dorsal region of large facets in males appears to form a `bright zone' of increased light capture without substantially increased spatial resolution. Theoretically, more light allows for increased performance in tasks such as motion detection. To determine the effect of the bright zone on motion detection, local properties of wide field motion detecting neurons were investigated using localized sinusoidal gratings. The pattern of local preferred directions of one class of these cells, the HS cells, in Eristalis is similar to that reported for the blowfly Calliphora. The bright zone seems to contribute to local contrast sensitivity; high contrast sensitivity exists in portions of the receptive field served by large diameter facet lenses of males and is not observed in females. Finally, temporal frequency tuning is also significantly faster in this frontal portion of the world, particularly in males, where it overcompensates for the higher spatial-frequency tuning and shifts the predicted local velocity optimum to higher speeds. These results indicate that increased retinal illuminance due to the bright zone of males is used to enhance contrast sensitivity and speed motion detector responses. Additionally, local neural properties vary across the visual world in a way not expected if HS cells serve purely as matched filters to measure yaw-induced visual motion

Calcium Sets the Physiological Value of the Dominant Time Constant of Saturated Mouse Rod Photoresponse Recovery

Background: The rate-limiting step that determines the dominant time constant (tD) of mammalian rod photoresponse recovery is the deactivation of the active phosphodiesterase (PDE6). Physiologically relevant Ca 2+-dependent mechanisms that would affect the PDE inactivation have not been identified. However, recently it has been shown that tD is modulated by background light in mouse rods. Methodology/Principal Findings: We used ex vivo ERG technique to record pharmacologically isolated photoreceptor responses (fast PIII component). We show a novel static effect of calcium on mouse rod phototransduction: Ca 2+ shortens the dominant time constant (tD) of saturated photoresponse recovery, i.e., when extracellular free Ca 2+ is decreased from 1mMto,25 nM, the tD is reversibly increased,1.5–2-fold. Conclusions: We conclude that the increase in tD during low Ca 2+ treatment is not due to increased [cGMP], increased [Na +] or decreased [ATP] in rod outer segment (ROS). Also it cannot be due to protein translocation mechanisms. We suggest that aCa 2+-dependent mechanism controls the life time of active PDE

Animal navigation: A noisy magnetic compass?

Diverse organisms use Earth’s magnetic field as a cue inorientation and navigation. Nevertheless, eliciting magneticorientation responses reliably, either in laboratory or naturalsettings, is often difficult. Many species appear to preferentiallyexploit non-magnetic cues if they are available, suggesting that themagnetic sense often serves as a redundant or ‘backup’ source ofinformation. This raises an interesting paradox: Earth’s magnetic fieldappears to be more pervasive and reliable than almost any othernavigational cue. Why then do animals not rely almost exclusively onthe geomagnetic field, while ignoring or downplaying other cues?Here, we explore a possible explanation: that the magnetic sense ofanimals is ‘noisy’, in that the magnetic signal is small relative tothermal and receptor noise. Magnetic receptors are thus unable toinstantaneously acquire magnetic information that is highly precise oraccurate. We speculate that extensive time-averaging and/or otherhigher-order neural processing of magnetic information is required,rendering the magnetic sense inefficient relative to alternative cuesthat can be detected faster and with less effort. This interpretation isconsistent with experimental results suggesting a long time course formagnetic compass and map responses in some animals. Despitepossible limitations, magnetoreception may be maintained by naturalselection because the geomagnetic field is sometimes the onlysource of directional and/or positional information available

Heading variations resolve the heading-direction ambiguity in vertical-beam radar observations of insect migration

Vertical-beam entomological radars provide precise measurements of the body alignment of individual overflying insects but are unable to distinguish which of the two axial directions the insect is heading towards. Insects migrating at altitude typically show common alignment, although with a broad spread. We show here that when observations from multiple individual insects are available, and the insects have airspeeds of about 2 ms–1 or greater, the spread in heading directions allows the heading ambiguity to be resolved (though at the sample rather than the individual level). A vector analysis of radar-measured track direction, track speed, and heading will provide consistent results for all the insects in a sample only when the heading direction is chosen correctly. With the heading then resolved, the analysis can continue to estimation of representative values for the airspeed of the insects in the sample and for the speed and direction of the wind they were flying in. This general approach can be implemented in two different ways, which we term the ‘cluster’ and ‘projection’ methods. When applied to an intense migration of large insects, probably moths, these methods produced highly consistent results from hour to hour and from one 150 m height interval to the next. Simulations show that the methods are not liable to directional bias and reveal when they are rendered ineffective by small sample sizes or low insect airspeeds; they also indicate that the cluster method handles small sample sizes better than the projection method, and its use is therefore recommended. A comparison with two previously proposed methods that use meteorological data to resolve the ambiguity shows that the new methods are more reliable. Use of this objective means of resolving the heading ambiguity will increase confidence in radar-based studies of the orientation behaviour of insect migrants and their responses to cues like sky illumination patterns, the Earth’s magnetic field, and wind

Photoreceptor Processing Speed and Input Resistance Changes during Light Adaptation Correlate with Spectral Class in the Bumblebee, Bombus impatiens

Colour vision depends on comparison of signals from photoreceptors with different spectral sensitivities. However, response properties of photoreceptor cells may differ in ways other than spectral tuning. In insects, for example, broadband photoreceptors, with a major sensitivity peak in the green region of the spectrum (>500 nm), drive fast visual processes, which are largely blind to chromatic signals from more narrowly-tuned photoreceptors with peak sensitivities in the blue and UV regions of the spectrum. In addition, electrophysiological properties of the photoreceptor membrane may result in differences in response dynamics of photoreceptors of similar spectral class between species, and different spectral classes within a species. We used intracellular electrophysiological techniques to investigate response dynamics of the three spectral classes of photoreceptor underlying trichromatic colour vision in the bumblebee, Bombus impatiens, and we compare these with previously published data from a related species, Bombus terrestris. In both species, we found significantly faster responses in green, compared with blue- or UV-sensitive photoreceptors, although all 3 photoreceptor types are slower in B. impatiens than in B. terrestris. Integration times for light-adapted B. impatiens photoreceptors (estimated from impulse response half-width) were 11.3±1.6 ms for green photoreceptors compared with 18.6±4.4 ms and 15.6±4.4 for blue and UV, respectively. We also measured photoreceptor input resistance in dark- and light-adapted conditions. All photoreceptors showed a decrease in input resistance during light adaptation, but this decrease was considerably larger (declining to about 22% of the dark value) in green photoreceptors, compared to blue and UV (41% and 49%, respectively). Our results suggest that the conductances associated with light adaptation are largest in green photoreceptors, contributing to their greater temporal processing speed. We suggest that the faster temporal processing of green photoreceptors is related to their role in driving fast achromatic visual processes

Relating Information, Encoding and Adaptation: Decoding the Population Firing Rate in Visual Areas 17/18 in Response to a Stimulus Transition

Neurons in the primary visual cortex typically reach their highest firing rate after an abrupt image transition. Since the mutual information between the firing rate and the currently presented image is largest during this early firing period it is tempting to conclude this early firing encodes the current image. This view is, however, made more complicated by the fact that the response to the current image is dependent on the preceding image. Therefore we hypothesize that neurons encode a combination of current and previous images, and that the strength of the current image relative to the previous image changes over time. The temporal encoding is interesting, first, because neurons are, at different time points, sensitive to different features such as luminance, edges and textures; second, because the temporal evolution provides temporal constraints for deciphering the instantaneous population activity. To study the temporal evolution of the encoding we presented a sequence of 250 ms stimulus patterns during multiunit recordings in areas 17 and 18 of the anaesthetized ferret. Using a novel method we decoded the pattern given the instantaneous population-firing rate. Following a stimulus transition from stimulus A to B the decoded stimulus during the first 90ms was more correlated with the difference between A and B (B-A) than with B alone. After 90ms the decoded stimulus was more correlated with stimulus B than with B-A. Finally we related our results to information measures of previous (B) and current stimulus (A). Despite that the initial transient conveys the majority of the stimulus-related information; we show that it actually encodes a difference image which can be independent of the stimulus. Only later on, spikes gradually encode the stimulus more exclusively

A Guide for Using Flight Simulators to Study the Sensory Basis of Long-Distance Migration in Insects

Studying the routes flown by long-distance migratory insects comes with the obvious challenge that the animal’s body size and weight is comparably low. This makes it difficult to attach relatively heavy transmitters to these insects in order to monitor their migratory routes (as has been done for instance in several species of migratory birds. However, the rather delicate anatomy of insects can be advantageous for testing their capacity to orient with respect to putative compass cues during indoor experiments under controlled conditions. Almost 20 years ago, Barrie Frost and Henrik Mouritsen developed a flight simulator which enabled them to monitor the heading directions of tethered migratory Monarch butterflies, both indoors and outdoors. The design described in the original paper has been used in many follow-up studies to describe the orientation capacities of mainly diurnal lepidopteran species. Here we present a modification of this flight simulator design that enables studies of nocturnal long-distance migration in moths while allowing controlled magnetic, visual and mechanosensory stimulation. This modified flight simulator has so far been successfully used to study the sensory basis of migration in two European and one Australian migratory noctuid species

Australian Bogong moths Agrotis infusa (Lepidoptera : Noctuidae), 1951–2020: decline and crash

The Bogong moth Agrotis infusa is well known for its remarkable long-distance migration – a return journey from the plains of southeast Australia to the Australian Alps – as well as for its cultural significance for Indigenous Australians. Each Spring, as many as four billion moths are estimated to arrive in the Australian Alps to aestivate in cool mountain caves and in boulder fields, bringing with them a massive annual influx of energy and nutrients critical for the health of the alpine ecosystem. However, a massive decline in moths present at their aestivation sites has occurred over the past 3 years, with only a few individuals present where hundreds of thousands could earlier be found. In order to understand the possible sources of decline, we analysed historical records of Bogong moth numbers at aestivation sites in the Australian Alps, including observations on Mt. Gingera (NSW) in the early 1950s, observations from 1980 onwards in the Snowy Mountains (NSW) and an almost-unbroken series of observations each summer over the past 53 years in three caves at different elevations on Mt. Buffalo (Victoria). This analysis shows that moth numbers were probably steady from 1951 until about 1980, fluctuated and slowly fell from then until 2016 and dramatically crashed in 2017. In the Murray–Darling Basin, the main winter breeding ground of Bogong moths, changes in farming practices, such as increasing land clearing for crops (which has removed around a quarter of a billion moths annually from the mountains compared to pre-European levels), has probably driven some of the decline in Bogong moth numbers observed from 1980 to 2016. The impact of insecticide remains unclear and is in urgent need of further study. Even though we found little evidence that increasing global temperatures per se are responsible for the Bogong moth decline, the Australian climate has nonetheless become drier and warmer over past decades, possibly hampering the survival of immature stages in the breeding areas and confining adult aestivation to gradually higher elevations. The crash in moth numbers from 2017 is most likely due to the recent severe drought in the moth's breeding grounds

The giant eyes of giant squid are indeed unexpectedly large, but not if used for spotting sperm whales

© The Author(s), 2013. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in BMC Evolutionary Biology 13 (2013): 187, doi:10.1186/1471-2148-13-187.We recently reported (Curr Biol 22:683–688, 2012) that the eyes of giant and colossal squid can grow to three times the diameter of the eyes of any other animal, including large fishes and whales. As an explanation to this extreme absolute eye size, we developed a theory for visual performance in aquatic habitats, leading to the conclusion that the huge eyes of giant and colossal squid are uniquely suited for detection of sperm whales, which are important squid-predators in the depths where these squid live. A paper in this journal by Schmitz et al. (BMC Evol Biol 13:45, 2013) refutes our conclusions on the basis of two claims: (1) using allometric data they argue that the eyes of giant and colossal squid are not unexpectedly large for the size of the squid, and (2) a revision of the values used for modelling indicates that large eyes are not better for detection of approaching sperm whales than they are for any other task. We agree with Schmitz et al. that their revised values for intensity and abundance of planktonic bioluminescence may be more realistic, or at least more appropriately conservative, but argue that their conclusions are incorrect because they have not considered some of the main arguments put forward in our paper. We also present new modelling to demonstrate that our conclusions remain robust, even with the revised input values suggested by Schmitz et al

Dung beetles use their dung ball as a mobile thermal refuge

At midday, surface temperatures in the desert often exceed 60°C. To be active at this time, animals need extraordinary behavioural or physiological adaptations. Desert ants, for instance, spend up to 75% of their foraging time cooling down on elevated thermal refuges such as grass stalks [1]. Ball-rolling dung beetles work under similar thermal conditions in South African savannahs. After landing at a fresh dung pile, a beetle quickly forms a dung ball and rolls it away in a straight line, head down, walking backwards [2]. Earlier studies have shown that some dung beetles maintain an elevated body temperature to gain a competitive advantage [3], [4] and [5], and that heat shunting may prevent overheating during flight [6] and [7]. However, we know little about the behavioural strategies beetles might employ to mitigate heat stress while rolling their dung balls. Using infrared thermography and behavioural experiments, we show here that dung beetles use their dung ball as a mobile thermal refuge onto which they climb to cool down while rolling across hot soil. We further demonstrate that the moist ball functions not only as a portable platform, but also as a heat sink, which effectively cools the beetle as it rolls or climbs onto it