Hoverflies use a time-compensated sun compass to orientate during autumn migration

Lords of the flies: dipteran migrants are diverse, abundant and ecologically important

Insect migrants are hugely abundant, with recent studies identifying the megadiverse order Diptera as the major component of many migratory assemblages. Despite this, their migratory behaviour has been widely overlooked in favour of more 'charismatic' migrant insects such as butterflies, dragonflies, and moths. Herein we review the available literature on dipteran migration to determine its prevalence, identify key migratory routes and elucidate areas that may prove fruitful for future research. Using 13 lines of evidence to determine migratory behaviour, we determined that species from 60 out of 130 dipteran families show evidence of migration, with Syrphidae fulfilling 12 of these criteria, followed by the Tephritidae with 10. By contrast, 22 families met just two criteria or fewer, underlining the need for more research into the migratory characteristics of these groups. In total, 592 species of Diptera were identified as potentially migratory, making them the most speciose group of insect migrants yet described. Despite this, only 0.5% of dipteran species were found to be migrants, a figure rising to 3% for the Syrphidae, a percentage mirrored by other migratory taxa such as butterflies, noctuid moths, and bats. Research was biased to locations in Europe (49% of publications) and while vast regions remain understudied, our review identified major flyways used by dipteran migrants across all biogeographic realms. Finally, we highlight an unsurpassed level of ecological diversity within dipteran migrants, including ecological roles of huge economic value. Overall, this review highlights how little is known about dipteran migration and how vital their migratory behaviour may be to the health of global ecosystems.

Spatiotemporal computations in the insect celestial compass

Obtaining a geocentric directional reference from a celestial compass requires compensation for the sun's movement during the day (relative to the observer), which depends on the earth's rotation, time of year and the observer's latitude. We examine how insects could solve this problem, assuming they have clock neurons that represent time as a sinusoidal oscillation, and taking into account the neuroanatomy of their celestial compass pathway. We show how this circuit could exploit trigonometric identities to perform the required spatiotemporal calculations. Our basic model assumes a constant change in sun azimuth (the 'hour angle'), which is recentred on solar noon for changing day lengths. In a more complete model, the time of year is represented by an oscillation with an annual period, and the latitude is estimated from the inclination of the geomagnetic field. Evaluating these models in simulated migration and foraging behaviours shows the hour angle may be sufficient.

Programmable LED Array for Evaluating Artificial Light Sources to Improve Insect Trapping

We developed a programmable LED array to evaluate different wavelength illumination (UV, blue, green, yellow, amber, and red) and modulation schemes to improve catch rates in insect traps. The device can communicate through Bluetooth® with a simple Android app to update the operational settings to facilitate field experiments, including which LEDs to operate, when to operate (always, night only, or predefined intervals after sunset and/or before sunrise), and to change the LED intensities/modulation during operation. We used the devices to evaluate different wavelengths to improve catches in traps for coconut rhinoceros beetle (CRB; Oryctes rhinoceros Linnaeus) in the field, as well as to evaluate lighting preferences of Asian citrus psyllid (ACP; Diaphorina citri Kuwayama). In both cases, insects were most strongly attracted to constant UV illumination. However, CRB avoided traps with any "visible" wavelength LEDs placed in panels of traps, while ACP was moderately attracted to blue, yellow, and amber. For CRB, UV illumination of cups at the bottom of panel traps reduced catch rates compared to UV illumination higher in the panels of traps, consistent with observations of dorsal orientation towards light observed by other researchers in nocturnal beetles and moths. Finally, we provide some hardware design recommendations to improve the energy efficiency of similar devices for more widespread deployment in insect traps and for controlling the LEDs to evaluate the effects of intensity and modulation with minimal pulsing, which our observations suggest may result in insects avoiding traps.

Multiple factors contribute to female dominance in migratory bioflows

Migration is a widely observed phenomenon supported by morphological, physiological and behavioural traits that vary with season and sex in many species. Recently, the genetic components underpinning migration in the marmalade hoverfly (Diptera: Syrphidae) have been unpacked through detection of differentially expressed genes between migrant and non-migrant females. Males also migrate, but changing sex ratios during autumn migration, from around 50% female in northern Europe to around 90% in southern Europe, suggests males are poor long-distance fliers. To elucidate the mechanisms underpinning this sex difference, we performed morphological, physiological and transcriptomic characterization of actively migrating females and males. Both sexes show similar physiological adaptations including hyperphagia and starvation resistance, but females display higher tolerance to cold, have lower wing loading values and display a greater flight capacity. In addition, females modulate the expression of genes involved in immunity, hypoxia and longevity while suppressing hormonal pathways involved in maintaining reproductive diapause. These traits contribute to the success of female migrants and underlie the diminishing pool of males, influencing population dynamics across huge geographic areas and through the whole migratory and overwintering period.

Enhanced flight performance in hoverfly migrants

Many animals undergo seasonal migrations in which they travel long distances aided by variations in morphology, physiology, and behavior. Here, we compare the flight characteristics, measured in a tethered flight mill, of autumn migratory and summer non-migratory morphs of the marmalade hoverfly Episyrphus balteatus (Diptera: Syrphidae), an ecologically and economically important pollinator, pest predator, and long-distance migrant. Our results show that migratory morphs flew twice as far as the non-migratory morphs. Body condition, reflecting the quantity of energy stores, had an even greater effect as hoverflies with fat abdomens flew almost five times the distance of those with thin abdomens, whereas speed varied only by size. These findings demonstrate enhanced flight capabilities in migratory morphs and underscore the importance of body condition for long-distance flight. Consequently, resource availability, feeding behavior, and the ability to accumulate and utilize fuel are likely to be key factors influencing the migration of hoverflies.

The most remarkable migrants-systematic analysis of the Western European insect flyway at a Pyrenean mountain pass

In autumn 1950 David and Elizabeth Lack chanced upon a huge migration of insects and birds flying through the Pyrenean Pass of Bujaruelo, from France into Spain, later describing the spectacle as combining both grandeur and novelty. The intervening years have seen many changes to land use and climate, posing the question as to the current status of this migratory phenomenon. In addition, a lack of quantitative data has prevented insights into the ecological impact of this mass insect migration and the factors that may influence it. To address this, we revisited the site in autumn over a 4 year period and systematically monitored abundance and species composition of diurnal insect migrants. We estimate an annual mean of 17.1 million day-flying insect migrants from five orders (Diptera, Hymenoptera, Hemiptera, Lepidoptera and Odonata) moving south, with observations of southward 'mass migration' events associated with warmer temperatures, the presence of a headwind, sunlight, low windspeed and low rainfall. Diptera dominated the migratory assemblage, and annual numbers varied by more than fourfold. Numbers at this single site hint at the likely billions of insects crossing the entire Pyrenean mountain range each year, and we highlight the importance of this route for seasonal insect migrants.

The efficiency of varying methods and degrees of time compensation for the solar azimuth

Daytime migrants are known to orientate using the position of the sun, compensating for its changing position throughout the day with a 'time-compensated sun compass'. This compass has been demonstrated in many migratory species, with various degrees of accuracy for the actual movement of the sun. Here, we present a model for differing levels of compensation for the solar ephemeris that shows that a high degree of efficiency, in terms of distance travelled, can be achieved without full time compensation. In our model, compensating for the sun's position had a diminishing return with an accuracy of 80% leading to only a 2% reduction in distance travelled. We compare various modes of time compensation-full, partial, time averaged and step-revealing their directional efficiency in terms of distance travelled under an autumn migration scenario. We find that the benefit of time compensation varies with latitude, with time averaging performing very well, especially at all high latitudes, but step compensation performing better at very low latitudes. Importantly, even rudimentary adjustment can dramatically increase the efficiency of migration, which suggests an easy pathway for the independent evolution of time compensation.

The genome sequence of the pied hoverfly, Scaeva pyrastri (Linnaeus, 1758)

We present a genome assembly from an individual female Scaeva pyrastri (the pied hoverfly; Arthropoda; Insecta; Diptera; Syrphidae). The genome sequence is 320 megabases in span. Most of the assembly is scaffolded into four chromosomal pseudomolecules, including the assembled X chromosome. The mitochondrial genome has also been assembled and is 17.3 kilobases in length. Gene annotation of this assembly on Ensembl identified 11,352 protein coding genes.

Varieties of visual navigation in insects

The behaviours and cognitive mechanisms animals use to orient, navigate, and remember spatial locations exemplify how cognitive abilities have evolved to suit a number of different mobile lifestyles and habitats. While spatial cognition observed in vertebrates has been well characterised in recent decades, of no less interest are the great strides that have also been made in characterizing and understanding the behavioural and cognitive basis of orientation and navigation in invertebrate models and in particular insects. Insects are known to exhibit remarkable spatial cognitive abilities and are able to successfully migrate over long distances or pinpoint known locations relying on multiple navigational strategies similar to those found in vertebrate models-all while operating under the constraint of relatively limited neural architectures. Insect orientation and navigation systems are often tailored to each species' ecology, yet common mechanistic principles can be observed repeatedly. Of these, reliance on visual cues is observed across a wide number of insect groups. In this review, we characterise some of the behavioural strategies used by insects to solve navigational problems, including orientation over short-distances, migratory heading maintenance over long distances, and homing behaviours to known locations. We describe behavioural research using examples from a few well-studied insect species to illustrate how visual cues are used in navigation and how they interact with non-visual cues and strategies.

Autumnal migration patterns of hoverflies (Diptera: Syrphidae): interannual variability in timing and sex ratio

Background

The migration of hoverflies (Diptera: Syrphidae) is a well-known phenomenon, with growing interest due to the ecosystem services provided by migrants. However, we still lack fundamental data on species composition, timing of migration, or sex ratio of migrants. To address this gap, we focused on the southward autumnal migration of hoverflies through central Europe.

Methods

To recognize migrating individuals from resident ones, we used a pair of one-side-blocked Malaise traps, exposed in a mountain pass in the Jeseníky mountains, Czech Republic, where a mass migration of hoverflies takes place annually. Traps were set for 4 years, from August to October.

Results

In total, we recorded 31 species of migrating hoverflies. The timing of migration differed between the years, taking place from the beginning of September to the end of October. Differences in phenology were observed in the four most common migrant species, where larger species seemed to migrate earlier or at the same time compared to the smaller ones. The sex ratio was strongly asymmetrical in most common species Episyrphus balteatus, Eupeodes corollae, and Sphaerophoria scripta, and varied between years for each species. Weather conditions strongly influenced the migration intensity at ground-level: hoverflies migrate mainly during days with south wind, high temperature, high atmospheric pressure, and low precipitation.

Migratory hoverflies orientate north during spring migration

Migratory hoverflies are long-range migrants that, in the Northern Hemisphere, move seasonally to higher latitudes in the spring and lower latitudes in the autumn. The preferred migratory direction of hoverflies in the autumn has been the subject of radar and flight simulator studies, while spring migration has proved to be more difficult to characterize owing to a lack of ground observations. Consequently, the preferred migratory direction during spring has only been inferred from entomological radar studies and patterns of local abundance, and currently lacks ground confirmation. Here, during a springtime arrival of migratory insects onto the Isles of Scilly and mainland Cornwall, UK, we provide ground proof that spring hoverfly migrants have an innate northward preference. Captured migratory hoverflies displayed northward vanishing bearings when released under sunny conditions under both favourable wind and zero-wind conditions. In addition, and unlike autumn migrants, spring individuals were also able to orientate when the sun was obscured. Analysis of winds suggests an origin for insects arriving on the Isles of Scilly as being in western France. These findings of spring migration routes and preferred migration directions are likely to extend to the diverse set of insects found within the western European migratory assemblage.

Genome-wide transcriptomic changes reveal the genetic pathways involved in insect migration

Insects are capable of extraordinary feats of long-distance movement that have profound impacts on the function of terrestrial ecosystems. The ability to undertake these movements arose multiple times through the evolution of a suite of traits that make up the migratory syndrome, however the underlying genetic pathways involved remain poorly understood. Migratory hoverflies (Diptera: Syrphidae) are an emerging model group for studies of migration. They undertake seasonal movements in huge numbers across large parts of the globe and are important pollinators, biological control agents and decomposers. Here, we assembled a high-quality draft genome of the marmalade hoverfly (Episyrphus balteatus). We leveraged this genomic resource to undertake a genome-wide transcriptomic comparison of actively migrating Episyrphus, captured from a high mountain pass as they flew south to overwinter, with the transcriptomes of summer forms which were non-migratory. We identified 1543 genes with very strong evidence for differential expression. Interrogation of this gene set reveals a remarkable range of roles in metabolism, muscle structure and function, hormonal regulation, immunity, stress resistance, flight and feeding behaviour, longevity, reproductive diapause and sensory perception. These features of the migrant phenotype have arisen by the integration and modification of pathways such as insulin signalling for diapause and longevity, JAK/SAT for immunity, and those leading to octopamine production and fuelling to boost flight capabilities. Our results provide a powerful genomic resource for future research, and paint a comprehensive picture of global expression changes in an actively migrating insect, identifying key genomic components involved in this important life-history strategy.