Matched-filter coding of sky polarization results in an internal sun compass in the brain of the desert locust.

Many animals use celestial cues for spatial orientation. These include the sun and, in insects, the polarization pattern of the sky, which depends on the position of the sun. The central complex in the insect brain plays a key role in spatial orientation. In desert locusts, the angle of polarized light in the zenith above the animal and the direction of a simulated sun are represented in a compass-like fashion in the central complex, but how both compasses fit together for a unified representation of external space remained unclear. To address this question, we analyzed the sensitivity of intracellularly recorded central-complex neurons to the angle of polarized light presented from up to 33 positions in the animal's dorsal visual field and injected Neurobiotin tracer for cell identification. Neurons were polarization sensitive in large parts of the virtual sky that in some cells extended to the horizon in all directions. Neurons, moreover, were tuned to spatial patterns of polarization angles that matched the sky polarization pattern of particular sun positions. The horizontal components of these calculated solar positions were topographically encoded in the protocerebral bridge of the central complex covering 360° of space. This whole-sky polarization compass does not support the earlier reported polarization compass based on stimulation from a small spot above the animal but coincides well with the previously demonstrated direct sun compass based on unpolarized light stimulation. Therefore, direct sunlight and whole-sky polarization complement each other for robust head direction coding in the locust central complex.

Melatonin synthesis in the optic lobes and midbrain of the grasshopper Oedipoda caerulescens.

The pathways of insect melatonin (MEL) biosynthesis apparently follow the same routes as those identified in vertebrates but information on MEL synthesis variations related with serotonin (5-HT), 5-hydroxy-indole acetic acid (5HIAA), and N-acetylserotonin (NAS) levels, as well as 5-HT N-acetyltransferase (NAT) activity throughout the day, is very limited in the insect nervous system. In the present study, the levels of MEL, metabolites (5-HT, NAS, and 5-HIAA) and enzyme NAT were determined in the optic lobes and the midbrain of the grasshopper Oedipoda caerulescens, in conditions of light and darkness. In both tissues, a different pattern of MEL synthesis was observed over the light/dark cycle. Variations in the levels of 5-HT, NAS and NAT activity related to the synthesis of cerebral MEL follow a pattern very similar to that observed in the pineal of mammals, with a peak of synthesis in the first half of the scotophase. Also, we observed differences in the metabolism of 5-HT between the optic lobes and the midbrain light/dark-dependent.

Evidence of vitamin D synthesis in insects exposed to UVb light.

Vertebrates obtain the prohormone vitamin D primarily by endogenous cutaneous synthesis under ultraviolet b (UVb) exposure. To date, endogenous synthesis of vitamin D in insects has never been investigated. In an initial experiment, we exposed four insect species which differ in ecology and morphology (migratory locusts, house crickets, yellow mealworms and black soldier fly larvae (BSFL)) to a low irradiance UVb source. In a second experiment we exposed these species to a higher UV irradiance, and in a third we tested the effect of exposure duration on vitamin D concentrations in yellow mealworms. Low irradiance UVb tended to increase vitamin D3 levels in house crickets, vitamin D2 levels in BSFL and vitamin D2 and D3 in yellow mealworms. Higher UVb irradiance increased vitamin D3 levels in all species but BSFL. Both BSFL and migratory locusts had increased vitamin D2 levels. Longer UVb exposure of yellow mealworms increased vitamin D2 and increased vitamin D3 until a plateau was reached at 6400 IU/kg. This study shows that insects can synthesize vitamin D de novo and that the amounts depend on UVb irradiance and exposure duration.

Exposure to extremely low frequency electromagnetic fields alters the behaviour, physiology and stress protein levels of desert locusts.

Electromagnetic fields (EMFs) are present throughout the modern world and are derived from many man-made sources including overhead transmission lines. The risks of extremely-low frequency (ELF) electromagnetic fields are particularly poorly understood especially at high field strengths as they are rarely encountered at ground level. Flying insects, however, can approach close to high field strength transmission lines prompting the question as to how these high levels of exposure affect behaviour and physiology. Here we utilise the accessible nervous system of the locust to ask how exposure to high levels of ELF EMF impact at multiple levels. We show that exposure to ELF EMFs above 4 mT leads to reduced walking. Moreover, intracellular recordings from an identified motor neuron, the fast extensor tibiae motor neuron, show increased spike latency and a broadening of its spike in exposed animals. In addition, hind leg kick force, produced by stimulating the extensor tibiae muscle, was reduced following exposure, while stress-protein levels (Hsp70) increased. Together these results suggest that ELF EMF exposure has the capacity to cause dramatic effects from behaviour to physiology and protein expression, and this study lays the foundation to explore the ecological significance of these effects in other flying insects.

Neurons in the brain of the desert locust Schistocerca gregaria sensitive to polarized light at low stimulus elevations.

Desert locusts (Schistocerca gregaria) sense the plane of dorsally presented polarized light through specialized dorsal eye regions that are likely adapted to exploit the polarization pattern of the blue sky for spatial orientation. Receptive fields of these dorsal rim photoreceptors and polarization-sensitive interneurons are directed toward the upper sky but may extend to elevations below 30°. Behavioral data, however, suggests that S. gregaria is even able to detect polarized light from ventral directions but physiological evidence for this is still lacking. In this study we characterized neurons in the locust brain showing polarization sensitivity at low elevations down to the horizon. In most neurons polarization sensitivity was absent or weak when stimulating from the zenith. All neurons, including projection and commissural neurons of the optic lobe and local interneurons of the central brain, are novel cell types, distinct from polarization-sensitive neurons studied so far. Painting dorsal rim areas in both eyes black to block visual input had no effect on the polarization sensitivity of these neurons, suggesting that they receive polarized light input from the main eye. A possible role of these neurons in flight stabilization or the perception of polarized light reflected from bodies of water or vegetation is discussed.

[Comparative investigation of locust's phototactic visual spectrum effect and phototactic response to spectral illumination].

To provide theoretical support for determining locust's phototactic spectrum, and explore locust's phototactic mechanism stimulated by light, utilizing AvaSpec fiber-optic spectrometer system and AvaLight-DHS, the investigation of locust's phototactic visual spectrum effect after light energy stimulated locust's vision system was carried out and on this basis, utilizing the investigated device of locust's phototactic response to spectral illumination, the discrepancy of locust's phototactic response to spectral illumination was certificated comparatively. The results show that the degree of locust's vision system absorbing the single spectrum photon of 430, 545 and 610 nm is significant and there exists difference, and the behavioral response to orange, violet, green, and blue spectral light has the difference in selective sensitivity, with the intensity of response to violet light being the strongest. The degree of response to orange light is the maximum, simultaneously, locust's vision system absorbing spectral photon energy has selective difference and requirement of illumination time, moreover, the sensitive degree of locust's visual system to spectrum and the strength of the lighting energy, influencing locust's phototactic response degree, and the micro-response of locust's phototactic vision physiology, led by the photoelectric effect of locust absorbing sensitive photon and converting photon energy, is the reason for locust's phototactic orientation response. In addition, locust's phototactic visual spectrum effect, only when the biological photoelectric effect of locust's visual system is stimulated by spectral illumination, can present the sensitivity of the spectral absorption effect, so, using the stronger ultraviolet stimulation characteristic of violet light, the different sensitive stimulation of orange, green, blue spectral light on locust's phototactic vision, and combining orange, violet, green, blue spectral light field mechanism reasonably, can form the optical field trapping locusts effectively, while, the elaboration of locust's phototactic mechanism lacks the quantification data, the comparative investigation of locust's phototactic behavior, locust's visual electric potential and locust's visual spectrum effect has to be done, to improve and perfect the theory of locust's phototactic mechanism.

Embryonic diapause in the Australian plague locust relative to parental experience of cumulative photophase decline.

The Australian plague locust Chortoicetes terminifera (Walker) exhibits facultative embryonic diapause during autumn. To approximate natural photoperiod changes during late summer and autumn, locust nymphs were reared under different total declines in laboratory photophase (-0.5, -0.75, -1.0, -1.25, -1.5, -1.75, -2 h each lowered in 15 min steps) in a 24 h photoperiod to quantify any effect on the subsequent production of diapause eggs. Induction of diapause eggs was significantly affected by accumulated photoperiod decline experienced by the parental generation throughout all development stages from mid-instar nymph to fledgling adult. The incidence of embryonic diapause ranged from nil at -0.5 h to 86.6% diapause at -2 h. Continued declines in photoperiod for post-teneral locusts (transitioned from -1h until fledging to -1.75 h) produced a further increase in the proportion of diapause eggs. The results were unaffected by time spent at any given photoperiod, despite a previously indicated maximal inductive photoperiod of 13.5h being used as the mid-point of all treatments. Implications for the seasonal timing processes of photoperiodism in C. terminifera, which has a high migratory capacity and a latitudinal cline in the timing of diapause egg production across a broad geographic range, are discussed.

A test of the thermal melanism hypothesis in the wingless grasshopper Phaulacridium vittatum.

Altitudinal clines in melanism are generally assumed to reflect the fitness benefits resulting from thermal differences between colour morphs, yet differences in thermal quality are not always discernible. The intra-specific application of the thermal melanism hypothesis was tested in the wingless grasshopper Phaulacridium vittatum (Sjöstedt) (Orthoptera: Acrididae) first by measuring the thermal properties of the different colour morphs in the laboratory, and second by testing for differences in average reflectance and spectral characteristics of populations along 14 altitudinal gradients. Correlations between reflectance, body size, and climatic variables were also tested to investigate the underlying causes of clines in melanism. Melanism in P. vittatum represents a gradation in colour rather than distinct colour morphs, with reflectance ranging from 2.49 to 5.65%. In unstriped grasshoppers, darker morphs warmed more rapidly than lighter morphs and reached a higher maximum temperature (lower temperature excess). In contrast, significant differences in thermal quality were not found between the colour morphs of striped grasshoppers. In support of the thermal melanism hypothesis, grasshoppers were, on average, darker at higher altitudes, there were differences in the spectral properties of brightness and chroma between high and low altitudes, and temperature variables were significant influences on the average reflectance of female grasshoppers. However, altitudinal gradients do not represent predictable variation in temperature, and the relationship between melanism and altitude was not consistent across all gradients. Grasshoppers generally became darker at altitudes above 800 m a.s.l., but on several gradients reflectance declined with altitude and then increased at the highest altitude.

Effects of parental radiation exposure on developmental instability in grasshoppers.

Mutagenic and epigenetic effects of environmental stressors and their transgenerational consequences are of interest to evolutionary biologists because they can amplify natural genetic variation. We studied the effect of parental exposure to radioactive contamination on offspring development in lesser marsh grasshopper Chorthippus albomarginatus. We used a geometric morphometric approach to measure fluctuating asymmetry (FA), wing shape and wing size. We measured time to sexual maturity to check whether parental exposure to radiation influenced offspring developmental trajectory and tested effects of radiation on hatching success and parental fecundity. Wings were larger in early maturing individuals born to parents from high radiation sites compared to early maturing individuals from low radiation sites. As time to sexual maturity increased, wing size decreased but more sharply in individuals from high radiation sites. Radiation exposure did not significantly affect FA or shape in wings nor did it significantly affect hatching success and fecundity. Overall, parental radiation exposure can adversely affect offspring development and fitness depending on developmental trajectories although the cause of this effect remains unclear. We suggest more direct measures of fitness and the inclusion of replication in future studies to help further our understanding of the relationship between developmental instability, fitness and environmental stress.

A buckling region in locust hindlegs contains resilin and absorbs energy when jumping or kicking goes wrong.

If a hindleg of a locust slips during jumping, or misses its target during kicking, energy generated by the two extensor tibiae muscles is no longer expended in raising the body or striking a target. How, then, is the energy in a jump (4100-4800 µJ) or kick (1700 µJ) dissipated? A specialised buckling region found in the proximal hind-tibia where the bending moment is high, but not present in the other legs, buckled and allowed the distal part of the tibia to extend. In jumps when a hindleg slipped, it bent by a mean of 23±14 deg at a velocity of 13.4±9.5 deg ms(-1); in kicks that failed to contact a target it bent by 32±16 deg at a velocity of 32.9±9.5 deg ms(-1). It also buckled 8.5±4.0 deg at a rate of 0.063±0.005 deg ms(-1) when the tibia was prevented from flexing fully about the femur in preparation for both these movements. By experimentally buckling this region through 40 deg at velocities of 0.001-0.65 deg ms(-1), we showed that one hindleg could store about 870 µJ on bending, of which 210 µJ was dissipated back to the leg on release. A band of blue fluorescence was revealed at the buckling region under UV illumination that had the two key signatures of the elastic protein resilin. A group of campaniform sensilla 300 µm proximal to the buckling region responded to imposed buckling movements. The features of the buckling region show that it can act as a shock absorber as proposed previously when jumping and kicking movements go wrong.

Sparse but specific temporal coding by spikes in an insect sensory-motor ocellar pathway.

We investigate coding in a locust brain neuron, DNI, which transforms graded synaptic input from ocellar L-neurons into axonal spikes that travel to excite particular thoracic flight neurons. Ocellar neurons are naturally stimulated by fluctuations in light collected from a wide field of view, for example when the visual horizon moves up and down. We used two types of stimuli: fluctuating light from a light-emitting diode (LED), and a visual horizon displayed on an electrostatic monitor. In response to randomly fluctuating light stimuli delivered from the LED, individual spikes in DNI occur sparsely but are timed to sub-millisecond precision, carrying substantial information: 4.5-7 bits per spike in our experiments. In response to these light stimuli, the graded potential signal in DNI carries considerably less information than in presynaptic L-neurons. DNI is excited in phase with either sinusoidal light from an LED or a visual horizon oscillating up and down at 20 Hz, and changes in mean light level or mean horizon level alter the timing of excitation for each cycle. DNI is a multimodal interneuron, but its ability to time spikes precisely in response to ocellar stimulation is not degraded by additional excitation. We suggest that DNI is part of an optical proprioceptor system, responding to the optical signal induced in the ocelli by nodding movements of the locust head during each wing-beat.

Gamma radiation consequences on desert locust Schistocerca gregaria (Forsk.) digestive system.

Schistocera gregaria (Forsk.) (Orthoptera, Acrididae) remains a major insect pest in Africa, more particularly in the Sahelian zone. Present control methods are only partially efficient. In a previous study, we tested the potentiality of a sterile insect technique (SIT). Males of S. gregaria appeared to be much radiosensitive as already a dose of 3 Gy limited their survival. Gamma-radiations are known to damages the epithelial tissue of midgut, which affects the alimentation in insects. In this work, we show how digestive system of S. gregaria males is affected when submitted to a dose of 4 gamma rays. Nutrition is affected as males stop feeding soon after irradiation and progressively lose weight. Histological analyses on the midgut showed important epithelium damages. The regenerative cells by which the epithelial cells are replaced were damaged on the first days following irradiation. Consequently, regenerative cells are unable to divide and replace the normal loss of midgut cell. After nine days, the entire midgut epithelium was destroyed and only longitudinal muscles layer remained intact. This indicates that low radiation doses should be used if SIT will be applied.

Coding of azimuthal directions via time-compensated combination of celestial compass cues.

Many animals use the sun as a reference for spatial orientation [1-3]. In addition to sun position, the sky provides two other sources of directional information, a color gradient [4] and a polarization pattern [5]. Work on insects has predominantly focused on celestial polarization as an orientation cue [6, 7]. Relying on sky polarization alone, however, poses the following two problems: E vector orientations in the sky are not suited to distinguish between the solar and antisolar hemisphere of the sky, and the polarization pattern changes with changing solar elevation during the day [8, 9]. Here, we present neurons that overcome both problems in a locust's brain. The spiking activity of these neurons depends (1) on the E vector orientation of dorsally presented polarized light, (2) on the azimuthal, i.e., horizontal, direction, and (3) on the wavelength of an unpolarized light source. Their tuning to these stimuli matches the distribution of a UV/green chromatic contrast as well as the polarization of natural skylight and compensates for changes in solar elevation during the day. The neurons are, therefore, suited to code for solar azimuth by concurrent combination of signals from the spectral gradient, intensity gradient, and polarization pattern of the sky.

Effects of elevated ultraviolet radiation and endophytic fungi on plant growth and insect feeding in Lolium perenne, Festuca rubra, F. arundinacea and F. pratensis.

Plants of perennial ryegrass (Lolium perenne L.), red fescue (Festuca rubra L.), tall fescue (F. arundinacea Schreb.) and meadow fescue (F. pratensis Huds) were exposed at an outdoor facility located in Edinburgh, UK to modulated levels of UV-B radiation (280-315 nm) using banks of cellulose diacetate filtered UV-B fluorescent lamps that also produce UV-A radiation (315-400 nm). The plants were derived from a single clone of each species and were grown both with and without colonization by naturally-occurring fungal endophytes. The UV-B treatment was a 30% elevation above the ambient erythemally-weighted level of UV-B during July to October. Growth of treated plants was compared with plants grown under elevated UV-A radiation alone produced by banks of polyester filtered lamps and with plants grown at ambient levels of solar radiation under banks of unenergized lamps. At the end of the treatment period, sample leaves were collected for feeding trials with the desert locust Schistocerca gregaria (Forsk). The UV-B treatment produced no effects on the aboveground biomass of any of the four grasses. The UV-B treatment and the UV-A control exposure both increased plant height and the number of daughter plants formed by rhizome growth in F. rubra. There were significant effects of endophyte presence on the total fresh and dry weights of F. arundinacea and F. rubra, on fresh weight only in F. pratensis, and on the fresh and dry weights of inflorescence in F. arundinacea and L. perenne. There were no effects of UV treatments on the absolute amounts of leaf consumed or on the feeding preferences of locusts for leaves with or without endophyte in three species: F. rubra, F. arundinacea and L. perenne. In F. pratensis there was no effect of UV treatment on the weight of leaves consumed but a significant UV x endophyte interaction caused by a marked change in feeding preference between leaves with and without endophyte that differed between the UV-B treatment and UV-A control exposures. The alkaloid compounds known as lolines were analysed in leaves of F. pratensis and were only found in plants grown with endophyte. However, there was no significant relationship between total loline content and insect feeding preference. These effects illustrate the potential complexities of species interactions under increasing levels of UV-B. The experiment also demonstrates the importance of appropriate controls in UV lamp supplementation experiments for interpretation of both plant growth and insect feeding effects.

Gamma-rays kill grasshopper primary spermatocytes in groups.

Primary spermatocyte killing by gamma-rays was studied in the grasshopper Heteracris littoralis in which spermatogenic development occurs in cysts containing a maximum of 64 cells during the first meiotic division. Cell killing at this stage is not random and mainly involves the death of whole cysts. The dose-response curve for cell killing has complex kinetics with at least two components but lacks any shoulder at low doses, thus indicating no repair of the lethal damage. Cell loss is apparent from surviving cysts as early as 45 min post irradiation but loss of greater than 24 cells is incompatible with cyst survival. Loss of fewer than 24 cells also is not random since certain values for cell loss are frequently observed while other, interspersed values are not seen at all.