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.

Evidence for a southward autumn migration of nocturnal noctuid moths in central Europe.

Insect migrations are spectacular natural events and resemble a remarkable relocation of biomass between two locations in space. Unlike the well-known migrations of daytime flying butterflies, such as the painted lady (Vanessa cardui) or the monarch butterfly (Danaus plexippus), much less widely known are the migrations of nocturnal moths. These migrations - typically involving billions of moths from different taxa - have recently attracted considerable scientific attention. Nocturnal moth migrations have traditionally been investigated by light trapping and by observations in the wild, but in recent times a considerable improvement in our understanding of this phenomenon has come from studying insect orientation behaviour, using vertical-looking radar. In order to establish a new model organism to study compass mechanisms in migratory moths, we tethered each of two species of central European Noctuid moths in a flight simulator to study their flight bearings: the red underwing (Catocala nupta) and the large yellow underwing (Noctua pronuba). Both species had significantly oriented flight bearings under an unobscured view of the clear night sky and in the Earth's natural magnetic field. Red underwings oriented south-southeast, while large yellow underwings oriented southwest, both suggesting a southerly autumn migration towards the Mediterranean. Interestingly, large yellow underwings became disoriented on humid (foggy) nights while red underwings remained oriented. We found no evidence in either species for a time-independent sky compass mechanism as previously suggested for the large yellow underwing.

The Earth's Magnetic Field and Visual Landmarks Steer Migratory Flight Behavior in the Nocturnal Australian Bogong Moth.

Like many birds [1], numerous species of nocturnal moths undertake spectacular long-distance migrations at night [2]. Each spring, billions of Bogong moths (Agrotis infusa) escape hot conditions in different regions of southeast Australia by making a highly directed migration of over 1,000 km to a limited number of cool caves in the Australian Alps, historically used for aestivating over the summer [3, 4]. How moths determine the direction of inherited migratory trajectories at night and locate their destination (i.e., navigate) is currently unknown [5-7]. Here we show that Bogong moths can sense the Earth's magnetic field and use it in conjunction with visual landmarks to steer migratory flight behavior. By tethering migrating moths in an outdoor flight simulator [8], we found that their flight direction turned predictably when dominant visual landmarks and a natural Earth-strength magnetic field were turned together, but that the moths became disoriented within a few minutes when these cues were set in conflict. We thus conclude that Bogong moths, like nocturnally migrating birds [9], can use a magnetic sense. Our results represent the first reliable demonstration of the use of the Earth's magnetic field to steer flight behavior in a nocturnal migratory insect.

Corrigendum: The Australian Bogong Moth Agrotis infusa: A Long-Distance Nocturnal Navigator.

[This corrects the article on p. 77 in vol. 10, PMID: 27147998.].

The Australian Bogong Moth Agrotis infusa: A Long-Distance Nocturnal Navigator.

The nocturnal Bogong moth (Agrotis infusa) is an iconic and well-known Australian insect that is also a remarkable nocturnal navigator. Like the Monarch butterflies of North America, Bogong moths make a yearly migration over enormous distances, from southern Queensland, western and northwestern New South Wales (NSW) and western Victoria, to the alpine regions of NSW and Victoria. After emerging from their pupae in early spring, adult Bogong moths embark on a long nocturnal journey towards the Australian Alps, a journey that can take many days or even weeks and cover over 1000 km. Once in the Alps (from the end of September), Bogong moths seek out the shelter of selected and isolated high ridge-top caves and rock crevices (typically at elevations above 1800 m). In hundreds of thousands, moths line the interior walls of these cool alpine caves where they "hibernate" over the summer months (referred to as "estivation"). Towards the end of the summer (February and March), the same individuals that arrived months earlier leave the caves and begin their long return trip to their breeding grounds. Once there, moths mate, lay eggs and die. The moths that hatch in the following spring then repeat the migratory cycle afresh. Despite having had no previous experience of the migratory route, these moths find their way to the Alps and locate their estivation caves that are dotted along the high alpine ridges of southeastern Australia. How naïve moths manage this remarkable migratory feat still remains a mystery, although there are many potential sensory cues along the migratory route that moths might rely on during their journey, including visual, olfactory, mechanical and magnetic cues. Here we review our current knowledge of the Bogong moth, including its natural history, its ecology, its cultural importance to the Australian Aborigines and what we understand about the sensory basis of its long-distance nocturnal migration. From this analysis it becomes clear that the Bogong moth represents a new and very promising model organism for understanding the sensory basis of nocturnal migration in insects.

An experimental displacement and over 50 years of tag-recoveries show that monarch butterflies are not true navigators.

Monarch butterflies (Danaus plexippus) breeding in eastern North America are famous for their annual fall migration to their overwintering grounds in Mexico. However, the mechanisms they use to successfully reach these sites remain poorly understood. Here, we test whether monarchs are true navigators who can determine their location relative to their final destination using both a "compass" and a "map". Using flight simulators, we recorded the orientation of wild-caught monarchs in southwestern Ontario and found that individuals generally flew in a southwest direction toward the wintering grounds. When displaced 2,500 km to the west, the same individuals continued to fly in a general southwest direction, suggesting that monarchs use a simple vector-navigation strategy (i.e., use a specific compass bearing without compensating for displacement). Using over 5 decades of field data, we also show that the directional concentration and the angular SD of recoveries from tagged monarchs largely conformed to two mathematical models describing the directional distribution of migrants expected under a vector-navigation strategy. A third analysis of tagged recoveries shows that the increasing directionality of migration from north to south is largely because of the presence of geographic barriers that guide individuals toward overwintering sites. Our work suggests that monarchs breeding in eastern North America likely combine simple orientation mechanisms with geographic features that funnel them toward Mexican overwintering sites, a remarkable achievement considering that these butterflies weigh less than a gram and travel thousands of kilometers to a site they have never seen.

Motion parallax processing in pigeon (Columba livia) pretectal neurons.

In the visual system of invertebrates and vertebrates there are specialised groups of motion-sensitive neurons, with large receptive fields, which are optimally tuned to respond to optic flow produced by the animals' movement through the 3-D world. From their response characteristics, shared frame of reference with the vestibular or inertial system, and anatomical connections, these neurons have been implicated in the stabilisation of retinal images, the control of posture and balance, and the animal's motion trajectories through the world. Using standard electrophysiological techniques and computer-generated stimuli, we show that some of these flow-field neurons in the pretectal nucleus lentiformis mesencephali in pigeons appear to be processing motion parallax. Two large overlapping planes of random dots moving independently were used to simulate motion parallax, in which one with larger dots was moved fast and the other with smaller dots was moved slowly in the opposite direction. Their neural responses to these two superimposed planes were facilitated above those produced by a single plane of moving dots and those produced by two layers moving in the same direction. Furthermore, some of these neurons preferred backward motion in the visual field and others preferred forward motion, suggesting that they may separately code visual objects 'nearer' and 'farther' than the stabilised ('on') plane during forward translational motion. A simple system is proposed whereby the relative activity in 'near', 'far' and 'on' populations could code depth through motion parallax in a metameric manner similar to that employed to code color vision and stereopsis.

A taxonomy of different forms of visual motion detection and their underlying neural mechanisms.

A simple taxonomy of different forms of visual motion is presented to show that there may be a hierarchical system of processing of visual motion in the brain, and that this is first split into self-produced motion and object motion, and then further into various forms of animate and inanimate motion patterns. Further refinement results in specific mechanisms which stem from specific demands of an animal's life-style and ecological niche. Examples are presented of the underlying neural mechanisms for some of these different classes of visual motion processing, such as simple object motion, looming and time to collision, and stereopsis from the object motion processing subsystem. In contrast, other examples of the neural mechanisms from the self-produced motion system include simple canonical flow field analysis, translation and rotation for guiding action in 3D space, and motion parallax for depth perception. The taxonomy thus provides a framework that may guide future research on how the brain detects and processes other dynamic visual patterns.

Looming responses of telencephalic neurons in the pigeon are modulated by optic flow.

The movement of animals through space filled with various objects requires the interaction between neuronal mechanisms specialized for processing local object motion and those specialized for processing optic flow generated by self-motion of the animal. In the avian brain, visual nuclei in the tectofugal pathway are primarily involved in the detection of object motion. By contrast, the nucleus of the basal optic root (nBOR) and the pretectal nucleus lentiformis mesencephali (nLM) are dedicated to the analysis of optic flow. But little is known about how these two systems interact. Using single-unit recording in the entopallium of the tectofugal pathway, we show that some neurons appeared to be integrating visual information of looming objects and whole-field motion simulating optic flow. They specifically responded to looming objects, but their looming responses were modulated by optic flow. Optic flow in the nasotemporal direction, typically produced by the forward movement of the bird, only mildly inhibited the looming responses. Furthermore, these neurons started firing later than when the looming object was presented against a stationary background. However, optic flow in other directions, especially the temporonasal direction, strongly inhibited their looming responses. Previous studies have implicated looming-sensitive neurons in predator avoidance behavior and these results suggest that a bird in motion may need less time to initiate an avoidance response to an approaching object.

The neural mechanisms of long distance animal navigation.

Animal navigation is a complex process involving the integration of many sources of specialized sensory information for navigation in near and far space. Our understanding of the neurobiological underpinnings of near-space navigation is well-developed, whereas the neural mechanisms of long-distance navigation are just beginning to be unraveled. One crucial question for future research is whether the near space concepts of place cells, head direction cells, and maps in the entorhinal cortex scale up to animals navigating over very long distances and whether they are related to the map and compass concepts of long-distance navigation.

Do monarch butterflies use polarized skylight for migratory orientation?

To test if migratory monarch butterflies use polarized light patterns as part of their time-compensated sun compass, we recorded their virtual flight paths in a flight simulator while the butterflies were exposed to patches of naturally polarized blue sky, artificial polarizers or a sunny sky. In addition, we tested butterflies with and without the polarized light detectors of their compound eye being occluded. The monarchs' orientation responses suggested that the butterflies did not use the polarized light patterns as a compass cue, nor did they exhibit a specific alignment response towards the axis of polarized light. When given direct view of the sun, migratory monarchs with their polarized light detectors painted out were still able to use their time-compensated compass: non-clockshifted butterflies, with their dorsal rim area occluded, oriented in their typical south-southwesterly migratory direction. Furthermore, they shifted their flight course clockwise by the predicted approximately 90 degrees after being advance clockshifted 6 h. We conclude that in migratory monarch butterflies, polarized light cues are not necessary for a time-compensated celestial compass to work and that the azimuthal position of the sun disc and/or the associated light-intensity and spectral gradients seem to be the migrants' major compass cue.

Global orientation aftereffect in multi-attribute displays: implications for the binding problem.

We investigated the binding problem (e.g. the combination of edge information across attributes), using an orientation aftereffect paradigm (OAE). Horizontal layers of vertical edges were phase-shifted to create a global near-vertical orientation. Multi-attribute displays were created by alternating the attribute defining edges (e.g. luminance, colour, texture or motion) across layers. OAE magnitude was dependent only on the attributes used in the adaptation phase, and the similarity of attributes from adaptation to testing phase had no significant effect. Moreover, compared to single-attribute conditions, the cooperation between attributes is moderate. These results favour segregation models of the binding mechanism.

The effect of tobacco blend additives on the retention of nicotine and solanesol in the human respiratory tract and on subsequent plasma nicotine concentrations during cigarette smoking.

The influence of the tobacco additives diammonium hydrogen phosphate (DAP) and urea on the delivery and respiratory tract retention of nicotine and solanesol and on the uptake of nicotine into venous blood was investigated in 10 smokers under mouth-hold and 75 and 500 mL inhalation conditions. Three cigarettes with identical physical specifications were produced from a common lamina tobacco blend. The control cigarette contained nonammoniated reconstituted tobacco sheet (RTS), whereas DAP and other ammonia compounds were added to the RTS of the second cigarette. Urea was added to the tobacco of the third cigarette. The presence of DAP or urea in the test cigarettes did not significantly influence solanesol retention within the mouth during the mouth-hold condition. Nicotine retention within the mouth during the mouth-hold condition was, however, significantly higher for the DAP cigarette (64.3 +/- 10.5%) than for the urea (53.3 +/- 11.3%) or control cigarette (46.3 +/- 8.6%), but this did not result in an increase in nicotine uptake into venous blood. Solanesol retentions during the 75 and 500 mL inhalation volume conditions and nicotine retentions during the 75 mL inhalation volume condition were not significantly different for the three cigarette types. Although the nicotine retention approached 100% with each cigarette type during the 500 mL inhalation condition, the nicotine retention for the urea-treated cigarette (99.6 +/- 0.2%) was marginally, but statistically, significant, higher than for the control (99.1 +/- 0.5%) and DAP-treated cigarettes (98.8 +/- 0.6%). There were no statistically significant differences between the indices of nicotine uptake into venous blood for the three cigarette types in any of the inhalation conditions.

Waved albatrosses can navigate with strong magnets attached to their head.

The foraging excursions of waved albatrosses Phoebastria irrorata during incubation are ideally suited for navigational studies because they navigate between their Galápagos breeding site and one specific foraging site in the upwelling zone of Peru along highly predictable, straight-line routes. We used satellite telemetry to follow free-flying albatrosses after manipulating magnetic orientation cues by attaching magnets to strategic places on the birds' heads. All experimental, sham-manipulated and control birds, were able to navigate back and forth from Galápagos to their normal foraging sites at the Peruvian coast over 1000 km away. Birds subjected to the three treatments did not differ in the routes flown or in the duration and speed of the trips. The interpretations and implications of this result depend on which of the current suggested magnetic sensory mechanisms is actually being used by the birds.

How owls structure visual information.

Recent studies on perceptual organization in humans claim that the ability to represent a visual scene as a set of coherent surfaces is of central importance for visual cognition. We examined whether this surface representation hypothesis generalizes to a non-mammalian species, the barn owl ( Tyto alba). Discrimination transfer combined with random-dot stimuli provided the appropriate means for a series of two behavioural experiments with the specific aims of (1) obtaining psychophysical measurements of figure-ground segmentation in the owl, and (2) determining the nature of the information involved. In experiment 1, two owls were trained to indicate the presence or absence of a central planar surface (figure) among a larger region of random dots (ground) based on differences in texture. Without additional training, the owls could make the same discrimination when figure and ground had reversed luminance, or were camouflaged by the use of uniformly textured random-dot stereograms. In the latter case, the figure stands out in depth from the ground when positional differences of the figure in two retinal images are combined (binocular disparity). In experiment 2, two new owls were trained to distinguish three-dimensional objects from holes using random-dot kinematograms. These birds could make the same discrimination when information on surface segmentation was unexpectedly switched from relative motion to half-occlusion. In the latter case, stereograms were used that provide the impression of stratified surfaces to humans by giving unpairable image features to the eyes. The ability to use image features such as texture, binocular disparity, relative motion, and half-occlusion interchangeably to determine figure-ground relationships suggests that in owls, as in humans, the structuring of the visual scene critically depends on how indirect image information (depth order, occlusion contours) is allocated between different surfaces.

Virtual migration in tethered flying monarch butterflies reveals their orientation mechanisms.

A newly developed flight simulator allows monarch butterflies to fly actively for up to several hours in any horizontal direction while their fall migratory flight direction can be continuously recorded. From these data, long segments of virtual flight paths of tethered, flying, migratory monarch butterflies were reconstructed, and by advancing or retarding the butterflies' circadian clocks, we have shown that they possess a time-compensated sun compass. Control monarchs on local time fly approximately southwest, those 6-h time-advanced fly southeast, and 6-h time-delayed butterflies fly in northwesterly directions. Moreover, butterflies flown in the same apparatus under simulated overcast in natural magnetic fields were randomly oriented and did not change direction when magnetic fields were rotated. Therefore, these experiments do not provide any evidence that monarch butterflies use a magnetic compass during migration.

Depth generalization from stereo to motion parallax in the owl.

Although many sources of three-dimensional information have been isolated and demonstrated to contribute independently, to depth vision in animal studies, it is not clear whether these distinct cues are perceived to be perceptually equivalent. Such ability is observed in humans and would seem to be advantageous for animals as well in coping with the often co-varying (or ambiguous) information about the layout of physical space. We introduce the expression primary-depth-cue equivalence to refer to the ability to perceive mutually consistent information about differences in depth from either stereopsis or motion-parallax. We found that owls trained to detect relative depth as a perceptual category (objects versus holes) when specified by binocular disparity alone (stereopsis), immediately transferred this discrimination to novel stimuli where the equivalent depth categories were available only through differences in motion information produced by head movements (observer-produced motion-parallax). Motion-parallax discrimination did occur under monocular viewing conditions and reliable performance depended heavily on the amplitude of side-to-side head movements. The presence of primary-depth-cue equivalence in the visual system of the owl provides further conformation of the hypothesis that neural systems evolved to detect differences in either disparity or motion information are likely to share similar processing mechanisms.