Visual physiology of Australian stingless bees

Stingless bees engage in a range of visually guided behaviours that require relatively high spatial resolution and contrast sensitivity. Although the eyes of honeybees, bumblebees, carpenter bees, and sweat bees have been studied extensively, there is limited knowledge of stingless bees. Here, we studied two sympatric Australian species, Tetragonula carbonaria and Austroplebeia australis, which are important crop pollinators. The bigger A. australis had more and larger ommatidial facets compared to T. carbonaria. Using pattern electroretinography, we showed that A. australis had higher contrast sensitivity (13.07) compared to T. carbonaria (5.99), but their spatial resolving power did not differ (0.53 cycles deg-1). We discuss these differences in visual physiology in the context of the distinct foraging behaviours of the two species.

Visual antipredator effects of web flexing in an orb web spider, with special reference to web decorations

Some visual antipredator strategies involve the rapid movement of highly contrasting body patterns to frighten or confuse the predator. Bright body colouration, however, can also be detected by potential predators and used as a cue. Among spiders, Argiope spp. are usually brightly coloured but they are not a common item in the diet of araneophagic wasps. When disturbed, Argiope executes a web-flexing behaviour in which they move rapidly and may be perceived as if they move backwards and towards an observer in front of the web. We studied the mechanisms underlying web-flexing behaviour as a defensive strategy. Using multispectral images and high-speed videos with deep-learning-based tracking techniques, we evaluated body colouration, body pattern, and spider kinematics from the perspective of a potential wasp predator. We show that the spider's abdomen is conspicuous, with a disruptive colouration pattern. We found that the body outline of spiders with web decorations was harder to detect when compared to spiders without decorations. The abdomen was also the body part that moved fastest, and its motion was composed mainly of translational (vertical) vectors in the potential predator's optical flow. In addition, with high contrast colouration, the spider's movement might be perceived as a sudden change in body size (looming effect) as perceived by the predator. These effects alongside the other visual cues may confuse potential wasp predators by breaking the spider body outline and affecting the wasp's flight manoeuvre, thereby deterring the wasp from executing the final attack.

Mallard landing behavior on water follows a -constant braking strategy

Many flying animals use optic flow to control their flight. During landing maneuvers, pigeons, hummingbirds, bats, Draco lizards and bees use the -constant braking strategy. This strategy regulates the approach by keeping the ratio of distance to an object and the rate of change of that distance constant. In keeping this ratio, , constant, a variety of deceleration profiles can lead to different collision avoidance behaviors. The landing behaviors listed above all qualify as controlled collisions, where the animal is decelerating into the object. We examined whether the same regulatory strategy is employed by mallards when landing on water. Video of mallard landing behavior was recorded at a local pond and digitized. Kinematic and τ parameters were calculated for each landing (N=177). The Pearson correlation coefficient for τ with respect to time to land was 0.99±0.02, indicating mallards employ a controlled-collision strategy. This result implies regulation by the birds to fix as constant while landing (on average, 0.90±0.13). In comparison with other active flyers, mallards use a higher value of when landing (0.775±0.109, 0.710±0.132 and 0.702±0.052 for pigeons, hummingbirds and bats, respectively). This higher may reflect physical differences in substrate from solid to liquid. The higher compliance of water in comparison to a solid substrate may reduce impact forces that could be injurious on a solid substrate, thereby enabling mallards to approach faster and expend less energy for costly, slow flight.

Spatial tuning of translational optic flow responses in hawkmoths of varying body size

To safely navigate their environment, flying insects rely on visual cues, such as optic flow. Which cues insects can extract from their environment depends closely on the spatial and temporal response properties of their visual system. These in turn can vary between individuals that differ in body size. How optic flow-based flight control depends on the spatial structure of visual cues, and how this relationship scales with body size, has previously been investigated in insects with apposition compound eyes. Here, we characterised the visual flight control response limits and their relationship to body size in an insect with superposition compound eyes: the hummingbird hawkmoth Macroglossum stellatarum. We used the hawkmoths’ centring response in a flight tunnel as a readout for their reception of translational optic flow stimuli of different spatial frequencies. We show that their responses cut off at different spatial frequencies when translational optic flow was presented on either one, or both tunnel walls. Combined with differences in flight speed, this suggests that their flight control was primarily limited by their temporal rather than spatial resolution. We also observed strong individual differences in flight performance, but no correlation between the spatial response cutoffs and body or eye size.

Spatial resolution and sensitivity of the eyes of the stingless bee, Tetragonula iridipennis

Stingless bees are important pollinators in the tropics. The tremendous variation in body size makes them an excellent group to study how miniaturization affects vision and visual behaviours. Using direct measurements and micro-CT, we reconstructed the eye structure, estimated anatomical spatial resolution and optical sensitivity of the stingless bee Tetragonula iridipennis. T. iridipennis is similar in size to the Australian stingless bee Tetragonula carbonaria and is smaller than honeybees. It has correspondingly small eyes (area = 0.56 mm²), few ommatidia (2451 ± 127), large inter-facet (3.0 ± 0.6°) and acceptance angles (2.8°). Theoretical estimates suggest that T. iridipennis has poorer spatial resolution (0.17 cycles degree^-1) than honeybees, bumblebees, and T. carbonaria. Its optical sensitivity (0.08 µm² sr), though higher than expected, is within the range of diurnal bees. This may provide them with greater contrast sensitivity, which is likely more relevant than the absolute sensitivity in this diurnal bee. Behaviourally determined detection thresholds for single targets using y-maze experiments were 11.5° for targets that provide chromatic contrast alone and 9.1° for targets providing chromatic and achromatic contrast. Further studies into microhabitat preferences and behaviour are required to understand how miniaturization influences its visual ecology.

A new, fluorescence-based method for visualizing the pseudopupil and assessing optical acuity in the dark compound eyes of honeybees and other insects

Recent interest in applying novel imaging techniques to infer optical resolution in compound eyes underscores the difficulty of obtaining direct measures of acuity. A widely used technique exploits the principal pseudopupil, a dark spot on the eye surface representing the ommatidial gaze direction and the number of detector units (ommatidia) viewing that gaze direction. However, dark-pigmented eyes, like those of honeybees, lack a visible pseudopupil. Attempts over almost a century to estimate optical acuity in this species are still debated. Here, we developed a method to visualize a stable, reliable pseudopupil by staining the photoreceptors with fluorescent dyes. We validated this method in several species and found it to outperform the dark pseudopupil for this purpose, even in pale eyes, allowing more precise location of the gaze centre. We then applied this method to estimate the sampling resolution in the frontal part of the eye of the honeybee forager. We found a broad frontal acute zone with interommatidial angles below 2° and a minimum interommatidial angle of 1.3°, a broader, sharper frontal acute zone than previously reported. Our study provides a new method to directly measure the sampling resolution in most compound eyes of living animals.

Light Polarization by Biological Nanocoatings

Light plays paramount functions for living beings in nature. In addition to color, the polarization of light is used by many animals for navigation and communication. In this study, we describe the light polarizing role of special nanostructures coating cuticular surfaces of diverse arthropods. These structures are built as parallel nanoscale ridges covering the eyes of the sunlight-navigating spider Drassodes lapidosus and of the water pond-swarming black fly Simulium vittatum, as well as the light-emitting abdominal lantern of the firefly Aquatica lateralis. Exact topography and dimensions of the parallel nanoridges provide different light polarizing efficiencies and wavelength sensitivity. Optical modeling confirms that the nanoscale ridges are responsible for the spectral polarization dependency. Co-opting from our recent work on the self-assembly of Drosophila corneal nanostructures, we engineer arthropod-like parallel nanoridges on artificial surfaces, which recapitulate the light polarization effects. Our work highlights the fundamental importance of nanocoatings in arthropods for the light polarization management and provides a new biomimetic approach to produce ordered nanostructures under mild conditions.

Distance estimation by Asian honey bees in two visually different landscapes

Honey bees estimate distances to food sources using image motion experienced on the flight path and they use this measure to tune the waggle phase duration in their dance communication. Most studies on the dance-related odometer are based on experiments with Apis mellifera foragers trained into small tunnels with black and white patterns which allowed quantifiable changes in the optic flow. In this study, we determined the calibration curves of two Asian honey bee species, A. florea and A. cerana, in two different natural environments with clear differences in the vegetation conditions and hence visual contrast. We found that the dense vegetation condition (with higher contrast) elicited a more rapid increase in the waggle phase duration with distance than the sparse vegetation in A. florea but not in A. cerana Our findings suggest that contrast sensitivity of the waggle dance odometer might vary among honey bee species.

The neuroecology of bee flight behaviours

By combining functional, ecological and evolutionary perspectives, neuroecology can provide key insights into understanding how behaviour and the underlying sensory and neural processes are shaped by ecology and evolutionary history. Bees are an ideal system for neuroecological studies because they represent a numerous and diverse insect group that inhabit a broad range of environments. Flight is central to the evolutionary success of bees and is the key to their survival and fitness but this review of recent work on fundamental flight behaviours in different species - landing, collision avoidance and speed control - reveals striking differences. We discuss the potential ecological and evolutionary drivers behind this variation but argue that to understand their adaptive value future work should include multidisciplinary approaches that integrate neuroscience, ecology, phylogeny and behaviour.

The buzz around spatial resolving power and contrast sensitivity in the honeybee, Apis mellifera

Most animals rely on vision to perform a range of behavioural tasks and variations in the anatomy and physiology of the eye likely reflect differences in habitat and life history. Moreover, eye design represents a balance between often conflicting requirements for gathering different forms of visual information. The trade-off between spatial resolving power and contrast sensitivity is common to all visual systems, and European honeybees (Apis mellifera) present an important opportunity to better understand this trade-off. Vision has been studied extensively in A. mellifera as it is vital for foraging, navigation and communication. Consequently, spatial resolving power and contrast sensitivity in A. mellifera have been measured using several methodologies; however, there is considerable variation in estimates between methodologies. We assess pattern electroretinography (pERG) as a new method for assessing the trade-off between visual spatial and contrast information in A.mellifera. pERG has the benefit of measuring spatial contrast sensitivity from higher order visual processing neurons in the eye. Spatial resolving power of A.mellifera estimated from pERG was 0.54 cycles per degree (cpd), and contrast sensitivity was 16.9. pERG estimates of contrast sensitivity were comparable to previous behavioural studies. Estimates of spatial resolving power reflected anatomical estimates in the frontal region of the eye, which corresponds to the region stimulated by pERG. Apis mellifera has similar spatial contrast sensitivity to other hymenopteran insects with similar facet diameter (Myrmecia ant species). Our results support the idea that eye anatomy has a substantial effect on spatial contrast sensitivity in compound eyes.

Contrast sensitivity and visual acuity of Queensland fruit flies (Bactrocera tryoni)

This study examines the visual acuity of Queensland fruit flies (Bactrocera tryoni) by analysing their turning responses to an immersive visual stimulus consisting of a pattern of vertical stripes presented at various angular periods and rotational rates. The results infer that these flies possess an interommatidial angle of approximately [Formula: see text], and an ommatidial acceptance angle of approximately [Formula: see text]. This suggests that the visual acuity of Queensland fruit flies is substantially better than that of the classical vinegar fly (Drosophila melanogaster), and is comparable to those of the housefly (Musca domestica) and the honeybee (Apis mellifera). The contrast sensitivity of Queensland fruit flies is comparable to that of the housefly.

Spatial Vision and Visually Guided Behavior in Apidae

The family Apidae, which is amongst the largest bee families, are important pollinators globally and have been well studied for their visual adaptations and visually guided behaviors. This review is a synthesis of what is known about their eyes and visual capabilities. There are many species-specific differences, however, the relationship between body size, eye size, resolution, and sensitivity shows common patterns. Salient differences between castes and sexes are evident in important visually guided behaviors such as nest defense and mate search. We highlight that Apis mellifera and Bombus terrestris are popular bee models employed in the majority of studies that have contributed immensely to our understanding vision in bees. However, other species, specifically the tropical and many non-social Apidae, merit further investigation for a better understanding of the influence of ecological conditions on the evolution of bee vision.

The role of lateral optic flow cues in hawkmoth flight control

Flying animals require sensory feedback on changes of their body position, as well as on their distance to nearby objects. The apparent image motion, or optic flow, which is generated as animals move through the air, can provide this information. Flight tunnel experiments have been crucial for our understanding of how insects use this optic flow for flight control in confined spaces. However, previous work mainly focused on species from two insect orders: Hymenoptera and Diptera. We therefore set out to investigate if the previously described control strategies to navigate enclosed environments are also used by insects with a different optical system, flight kinematics and phylogenetic background. We tested the role of lateral visual cues for forward flight control in the hummingbird hawkmoth Macroglossum stellatarum (Sphingidae, Lepidoptera), which possess superposition compound eyes, and have the ability to hover in addition to their fast forward flight capacities. Our results show that hawkmoths use a similar strategy for lateral position control as bees and flies in balancing the magnitude of translational optic flow perceived in both eyes. However, the control of lateral optic flow on flight speed in hawkmoths differed from that in bees and flies. Moreover, hawkmoths showed individually attributable differences in position and speed control when the presented optic flow was unbalanced.