Ocellar structure of African and Australian desert ants.

Few walking insects possess simple eyes known as the ocelli. The role of the ocelli in walking insects such as ants has been less explored. Physiological and behavioural evidence in the desert ant, Cataglyphis bicolor, indicates that ocellar receptors are polarisation sensitive and are used to derive compass information from the pattern of polarised skylight. The ability to detect polarised skylight can also be inferred from the structure and the organisation of the ocellar retina. However, the functional anatomy of the desert ant ocelli has not been investigated. Here we characterised the anatomical organisation of the ocelli in three species of desert ants. The two congeneric species of Cataglyphis we studied had a fused rhabdom, but differed in their organisation of the retina. In Cataglyphis bicolor, each retinula cell contributed microvilli in one orientation enabling them to compare e-vector intensities. In Cataglyphis fortis, some retinula cells contributed microvilli in more than one orientation, indicating that not all cells are polarisation sensitive. The desert ant Melophorus bagoti had an unusual ocellar retina with a hexagonal or pentagonal rhabdomere arrangement forming an open rhabdom. Each retinula cell contributed microvilli in more than one orientation, making them unlikely to be polarisation detectors.

Ocellar structure is driven by the mode of locomotion and activity time in Myrmecia ants.

Insects have exquisitely adapted their compound eyes to suit the ambient light intensity in the different temporal niches they occupy. In addition to the compound eye, most flying insects have simple eyes known as ocelli, which assist in flight stabilisation, horizon detection and orientation. Among ants, typically the flying alates have ocelli while the pedestrian workers lack this structure. The Australian ant genus Myrmecia is one of the few ant genera in which both workers and alates have three ocellar lenses. Here, we studied the variation in the ocellar structure in four sympatric species of Myrmecia that are active at different times of the day. In addition, we took advantage of the walking and flying modes of locomotion in workers and males, respectively, to ask whether the type of movement influences the ocellar structure. We found that ants active in dim light had larger ocellar lenses and wider rhabdoms compared with those in bright-light conditions. In the ocellar rhabdoms of workers active in dim-light habitats, typically each retinula cell contributed microvilli in more than one direction, probably destroying polarisation sensitivity. The organisation of the ocellar retina in the day-active workers and the males suggests that in these animals some cells are sensitive to the pattern of polarised skylight. We found that the night-flying males had a tapetum that reflects light back to the rhabdom, increasing their optical sensitivity. We discuss the possible functions of ocelli to suit the different modes of locomotion and the discrete temporal niches that animals occupy.

Techniques for Investigating the Anatomy of the Ant Visual System.

This article outlines a suite of techniques in light microscopy (LM) and electron microscopy (EM) which can be used to study the internal and external eye anatomy of insects. These include traditional histological techniques optimized for work on ant eyes and adapted to work in concert with other techniques such as transmission electron microscopy (TEM) and scanning electron microscopy (SEM). These techniques, although vastly useful, can be difficult for the novice microscopist, so great emphasis has been placed in this article on troubleshooting and optimization for different specimens. We provide information on imaging techniques for the entire specimen (photo-microscopy and SEM) and discuss their advantages and disadvantages. We highlight the technique used in determining lens diameters for the entire eye and discuss new techniques for improvement. Lastly, we discuss techniques involved in preparing samples for LM and TEM, sectioning, staining, and imaging these samples. We discuss the hurdles that one might come across when preparing samples and how best to navigate around them.

Light and dark adaptation mechanisms in the compound eyes of Myrmecia ants that occupy discrete temporal niches.

Ants of the Australian genus Myrmecia partition their foraging niche temporally, allowing them to be sympatric with overlapping foraging requirements. We used histological techniques to study the light and dark adaptation mechanisms in the compound eyes of diurnal (Myrmecia croslandi), crepuscular (M. tarsata, M. nigriceps) and nocturnal ants (M. pyriformis). We found that, except in the day-active species, all ants have a variable primary pigment cell pupil that constricts the crystalline cone in bright light to control for light flux. We show for the nocturnal M. pyriformis that the constriction of the crystalline cone by the primary pigment cells is light dependent whereas the opening of the aperture is regulated by an endogenous rhythm. In addition, in the light-adapted eyes of all species, the retinular cell pigment granules radially migrate towards the rhabdom, a process that in both the day-active M. croslandi and the night-active M. pyriformis is driven by ambient light intensity. Visual system properties thus do not restrict crepuscular and night-active ants to their temporal foraging niche, while day-active ants require high light intensities to operate. We discuss the ecological significance of these adaptation mechanisms and their role in temporal niche partitioning.

Compound eye and ocellar structure for walking and flying modes of locomotion in the Australian ant, Camponotus consobrinus.

Ants are unusual among insects in that individuals of the same species within a single colony have different modes of locomotion and tasks. We know from walking ants that vision plays a significant role in guiding this behaviour, but we know surprisingly little about the potential contribution of visual sensory structures for a flying mode of locomotion. Here we investigate the structure of the compound eye and ocelli in pedestrian workers, alate females and alate males of an Australian ant, Camponotus consobrinus, and discuss the trade-offs involved in optical sensitivity and spatial resolution. Male ants have more but smaller ommatidia and the smallest interommatidial angles, which is most likely an adaptation to visually track individual flying females. Both walking and flying forms of ants have a similar proportion of specialized receptors sensitive to polarized skylight, but the absolute number of these receptors varies, being greatest in males. Ocelli are present only in the flying forms. Each ocellus consists of a bipartite retina with a horizon-facing dorsal retina, which contains retinula cells with long rhabdoms, and a sky-facing ventral retina with shorter rhabdoms. We discuss the implications of these and their potential for sensing the pattern of polarized skylight.

Adaptations for nocturnal and diurnal vision in the hawkmoth lamina.

Animals use vision over a wide range of light intensities, from dim starlight to bright sunshine. For animals active in very dim light the visual system is challenged by several sources of visual noise. Adaptations in the eyes, as well as in the neural circuitry, have evolved to suppress the noise and enhance the visual signal, thereby improving vision in dim light. Among neural adaptations, spatial summation of visual signals from neighboring processing units is suggested to increase the reliability of signal detection and thus visual sensitivity. In insects, the likely neural candidates for carrying out spatial summation are the lamina monopolar cells (LMCs) of the first visual processing area of the insect brain (the lamina). We have classified LMCs in three species of hawkmoths with considerably different activity periods but very similar ecology-the diurnal Macroglossum stellatarum, the nocturnal Deilephila elpenor and the crepuscular-nocturnal Manduca sexta. Using this classification, we investigated the anatomical adaptations of hawkmoth LMCs suited for spatial summation. We found that specific types of LMCs have dendrites extending to significantly more neighboring cartridges in the two nocturnal and crepuscular species than in the diurnal species, making these LMC types strong candidates for spatial summation. Moreover, while the absolute number of cartridges visited by the LMCs differed between the two dim-light species, their dendritic extents were very similar in terms of visual angle, possibly indicating a limiting spatial acuity. The overall size of the lamina neuropil did not correlate with the size of its LMCs.

The visual system of the Australian 'Redeye' cicada (Psaltoda moerens).

We investigated the functional anatomy of the visual system in the Australian 'Redeye' cicada Psaltoda moerens, including compound eyes and ocelli. The compound eyes have large visual fields, about 7500 ommatidia per eye and binocular overlaps of 10-15° in the frontal and of 50-60° in the dorsal visual field. The diameters of corneal facet lenses range between 22 and 34 µm and the lenses are unusually long with up to 100 µm in some eye regions. In the posterior part of the eyes, the hexagonal facet array changes to a square lattice. The compound eyes are of the eucone apposition type with 8 retinular cells contributing to a fused rhabdom in each ommatidium. The red eye colour is due to the pigment granules in the secondary pigment cells. We found a small Dorsal Rim Area (DRA), in which rhabdom cross-sections are rectangular rather than round. The cross-sections of DRA rhabdoms do not systematically change orientation along the length of the rhabdom, indicating that microvilli directions do not twist, which would make retinular cells in the DRA polarization sensitive. The three ocelli have unusual lenses with a champagne-cork shape in longitudinal sections. Retinular cells are short in the dorsal and ventral part of the retinae, and long in their equatorial part. Ocellar rhabdoms are short (<10 µm), positioned close to the corneagenous layer and are formed by pairs of retinular cells. In cross-section, the rhabdomeres are 2-5 µm long and straight. The red colour of ocelli is produced by screening pigments that form an iris around the base of the ocellar lens and by screening pigments between the ocellar retinular cells. We discuss the organization of the compound eye rhabdom, the organization of the ocelli and the presence of a DRA in the light of what is known about Hemipteran compound eyes. We note in particular that cicadas are the only Hemipteran group with fused rhabdoms, thus making Hemiptera an interesting case to study the evolution of open rhabdoms and neural superposition.

Neuromechanism study of insect-machine interface: flight control by neural electrical stimulation.

The insect-machine interface (IMI) is a novel approach developed for man-made air vehicles, which directly controls insect flight by either neuromuscular or neural stimulation. In our previous study of IMI, we induced flight initiation and cessation reproducibly in restrained honeybees (Apis mellifera L.) via electrical stimulation of the bilateral optic lobes. To explore the neuromechanism underlying IMI, we applied electrical stimulation to seven subregions of the honeybee brain with the aid of a new method for localizing brain regions. Results showed that the success rate for initiating honeybee flight decreased in the order: α-lobe (or ß-lobe), ellipsoid body, lobula, medulla and antennal lobe. Based on a comparison with other neurobiological studies in honeybees, we propose that there is a cluster of descending neurons in the honeybee brain that transmits neural excitation from stimulated brain areas to the thoracic ganglia, leading to flight behavior. This neural circuit may involve the higher-order integration center, the primary visual processing center and the suboesophageal ganglion, which is also associated with a possible learning and memory pathway. By pharmacologically manipulating the electrically stimulated honeybee brain, we have shown that octopamine, rather than dopamine, serotonin and acetylcholine, plays a part in the circuit underlying electrically elicited honeybee flight. Our study presents a new brain stimulation protocol for the honeybee-machine interface and has solved one of the questions with regard to understanding which functional divisions of the insect brain participate in flight control. It will support further studies to uncover the involved neurons inside specific brain areas and to test the hypothesized involvement of a visual learning and memory pathway in IMI flight control.

Compound eye adaptations for diurnal and nocturnal lifestyle in the intertidal ant, Polyrhachis sokolova.

The Australian intertidal ant, Polyrhachis sokolova lives in mudflat habitats and nests at the base of mangroves. They are solitary foraging ants that rely on visual cues. The ants are active during low tides at both day and night and thus experience a wide range of light intensities. We here ask the extent to which the compound eyes of P. sokolova reflect the fact that they operate during both day and night. The ants have typical apposition compound eyes with 596 ommatidia per eye and an interommatidial angle of 6.0°. We find the ants have developed large lenses (33 µm in diameter) and wide rhabdoms (5 µm in diameter) to make their eyes highly sensitive to low light conditions. To be active at bright light conditions, the ants have developed an extreme pupillary mechanism during which the primary pigment cells constrict the crystalline cone to form a narrow tract of 0.5 µm wide and 16 µm long. This pupillary mechanism protects the photoreceptors from bright light, making the eyes less sensitive during the day. The dorsal rim area of their compound eye has specialised photoreceptors that could aid in detecting the orientation of the pattern of polarised skylight, which would assist the animals to determine compass directions required while navigating between nest and food sources.

Caste-specific visual adaptations to distinct daily activity schedules in Australian Myrmecia ants.

Animals are active at different times of the day and their activity schedules are shaped by competition, time-limited food resources and predators. Different temporal niches provide different light conditions, which affect the quality of visual information available to animals, in particular for navigation. We analysed caste-specific differences in compound eyes and ocelli in four congeneric sympatric species of Myrmecia ants, with emphasis on within-species adaptive flexibility and daily activity rhythms. Each caste has its own lifestyle: workers are exclusively pedestrian; alate females lead a brief life on the wing before becoming pedestrian; alate males lead a life exclusively on the wing. While workers of the four species range from diurnal, diurnal-crepuscular, crepuscular-nocturnal to nocturnal, the activity times of conspecific alates do not match in all cases. Even within a single species, we found eye area, facet numbers, facet sizes, rhabdom diameters and ocelli size to be tuned to the distinct temporal niche each caste occupies. We discuss these visual adaptations in relation to ambient light levels, visual tasks and mode of locomotion.

Anatomical and physiological evidence for polarisation vision in the nocturnal bee Megalopta genalis.

The presence of a specialised dorsal rim area with an ability to detect the e-vector orientation of polarised light is shown for the first time in a nocturnal hymenopteran. The dorsal rim area of the halictid bee Megalopta genalis features a number of characteristic anatomical specialisations including an increased rhabdom diameter and a lack of primary screening pigments. Optically, these specialisations result in wide spatial receptive fields (Deltarho = 14 degrees ), a common adaptation found in the dorsal rim areas of insects used to filter out interfering effects (i.e. clouds) from the sky. In this specialised eye region all nine photoreceptors contribute their microvilli to the entire length of the ommatidia. These orthogonally directed microvilli are anatomically arranged in an almost linear, anterior-posterior orientation. Intracellular recordings within the dorsal rim area show very high polarisation sensitivity and a sensitivity peak within the ultraviolet part of the spectrum.

Lateralization of olfaction in the honeybee Apis mellifera.

Lateralization of function is a well-known phenomenon in humans. The two hemispheres of the human brain are functionally specialized such that certain cognitive skills, such as language or musical ability, conspecific recognition, and even emotional responses, are mediated by one hemisphere more than the other [1, 2]. Studies over the past 30 years suggest that lateralization occurs in other vertebrate species as well [3-11]. In general, lateralization is observed in different sensory modalities in humans as well as vertebrates, and there are interesting parallels (reviewed in [12]). However, little is known about functional asymmetry in invertebrates [13, 14] and there is only one investigation in insects [15]. Here we show, for the first time, that the honeybee Apis mellifera displays a clear laterality in responding to learned odors. By training honeybees on two different versions of the well-known proboscis extension reflex (PER) paradigm [16, 17], we demonstrate that bees respond to odors better when they are trained through their right antenna. To our knowledge, this is the first demonstration of asymmetrical learning performance in an insect.

A neural network to improve dim-light vision? Dendritic fields of first-order interneurons in the nocturnal bee Megalopta genalis.

Using the combined Golgi-electron microscopy technique, we have determined the three-dimensional dendritic fields of the short visual fibres (svf 1-3) and first-order interneurons or L-fibres (L1-4) within the first optic ganglion (lamina) of the nocturnal bee Megalopta genalis. Serial cross sections have revealed that the svf type 2 branches into one adjacent neural unit (cartridge) in layer A, the most distal of the three lamina layers A, B and C. All L-fibres, except L1-a, exhibit wide lateral branching into several neighbouring cartridges. L1-b shows a dendritic field of seven cartridges in layers A and C, dendrites of L2 target 13 cartridges in layer A, L3 branches over a total of 12 cartridges in layer A and three in layer C and L4 has the largest dendritic field size of 18 cartridges in layer C. The number of cartridges reached by the respective L-fibres is distinctly greater in the nocturnal bee than in the worker honeybee and is larger than could be estimated from our previous Golgi-light microscopy study. The extreme dorso-ventrally oriented dendritic field of L4 in M. genalis may, in addition to its potential role in spatial summation, be involved in edge detection. Thus, we have shown that the amount of lateral spreading present in the lamina provides the anatomical basis for the required spatial summation. Theoretical and future physiological work should further elucidate the roles that this lateral spreading plays to improve dim-light vision in nocturnal insects.

Neural organisation in the first optic ganglion of the nocturnal bee Megalopta genalis.

Each neural unit (cartridge) in the first optic ganglion (lamina) of the nocturnal bee Megalopta genalis contains nine receptor cell axons (6 short and 3 long visual fibres), and four different types of first-order interneurons, also known as L-fibres (L1 to L4) or lamina monopolar cells. The short visual fibres terminate within the lamina as three different types (svf 1, 2, 3). The three long visual fibres pass through the lamina without forming characteristic branching patterns and terminate in the second optic ganglion, the medulla. The lateral branching pattern of svf 2 into adjacent cartridges is unique for hymenopterans. In addition, all four types of L-fibres show dorso-ventrally arranged, wide, lateral branching in this nocturnal bee. This is in contrast to the diurnal bees Apis mellifera and Lasioglossum leucozonium, where only two out of four L-fibre types (L2 and L4) reach neighbouring cartridges. In M. genalis, L1 forms two sub-types, viz. L1-a and L1-b; L1-b in particular has the potential to contact several neighbouring cartridges. L2 and L4 in the nocturnal bee are similar to L2 and L4 in the diurnal bees but have dorso-ventral arborisations that are twice as wide. A new type of laterally spreading L3 has been discovered in the nocturnal bee. The extensive neural branching pattern of L-fibres in M. genalis indicates a potential role for these neurons in the spatial summation of photons from large groups of ommatidia. This specific adaptation in the nocturnal bee could significantly improve reliability of vision in dim light.

Retinal and optical adaptations for nocturnal vision in the halictid bee Megalopta genalis.

The apposition compound eye of a nocturnal bee, the halictid Megalopta genalis, is described for the first time. Compared to the compound eye of the worker honeybee Apis mellifera and the diurnal halictid bee Lasioglossum leucozonium, the eye of M. genalis shows specific retinal and optical adaptations for vision in dim light. The major anatomical adaptations within the eye of the nocturnal bee are (1) nearly twofold larger ommatidial facets and (2) a 4-5 times wider rhabdom diameter than found in the diurnal bees studied. Optically, the apposition eye of M. genalis is 27 times more sensitive to light than the eyes of the diurnal bees. This increased optical sensitivity represents a clear optical adaptation to low light intensities. Although this unique nocturnal apposition eye has a greatly improved ability to catch light, a 27-fold increase in sensitivity alone cannot account for nocturnal vision at light intensities that are 8 log units dimmer than during daytime. New evidence suggests that additional neuronal spatial summation within the first optic ganglion, the lamina, is involved.