Insect guts present distinctive environments for microbial colonization, and bacteria in the gut potentially provide many beneficial services to their hosts. Insects display a wide range in degree of dependence on gut bacteria for basic functions. Most insect guts contain relatively few microbial species as compared to mammalian guts, but some insects harbor large gut communities of specialized bacteria. Others are colonized only opportunistically and sparsely by bacteria common in other environments. Insect digestive tracts vary extensively in morphology and physicochemical properties, factors that greatly influence microbial community structure. One obstacle to the evolution of intimate associations with gut microorganisms is the lack of dependable transmission routes between host individuals. Here, social insects, such as termites, ants, and bees, are exceptions: social interactions provide opportunities for transfer of gut bacteria, and some of the most distinctive and consistent gut communities, with specialized beneficial functions in nutrition and protection, have been found in social insect species. Still, gut bacteria of other insects have also been shown to contribute to nutrition, protection from parasites and pathogens, modulation of immune responses, and communication. The extent of these roles is still unclear and awaits further studies.

The gut microbiota of insects - diversity in structure and function.

The flight of insects has fascinated physicists and biologists for more than a century. Yet, until recently, researchers were unable to rigorously quantify the complex wing motions of flapping insects or measure the forces and flows around their wings. However, recent developments in high-speed videography and tools for computational and mechanical modeling have allowed researchers to make rapid progress in advancing our understanding of insect flight. These mechanical and computational fluid dynamic models, combined with modern flow visualization techniques, have revealed that the fluid dynamic phenomena underlying flapping flight are different from those of non-flapping, 2-D wings on which most previous models were based. In particular, even at high angles of attack, a prominent leading edge vortex remains stably attached on the insect wing and does not shed into an unsteady wake, as would be expected from non-flapping 2-D wings. Its presence greatly enhances the forces generated by the wing, thus enabling insects to hover or maneuver. In addition, flight forces are further enhanced by other mechanisms acting during changes in angle of attack, especially at stroke reversal, the mutual interaction of the two wings at dorsal stroke reversal or wing-wake interactions following stroke reversal. This progress has enabled the development of simple analytical and empirical models that allow us to calculate the instantaneous forces on flapping insect wings more accurately than was previously possible. It also promises to foster new and exciting multi-disciplinary collaborations between physicists who seek to explain the phenomenology, biologists who seek to understand its relevance to insect physiology and evolution, and engineers who are inspired to build micro-robotic insects using these principles. This review covers the basic physical principles underlying flapping flight in insects, results of recent experiments concerning the aerodynamics of insect flight, as well as the different approaches used to model these phenomena.

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The aerodynamics of insect flight

Dispersal costs can be classified into energetic, time, risk and opportunity costs and may be levied directly or deferred during departure, transfer and settlement. They may equally be incurred during life stages before the actual dispersal event through investments in special morphologies. Because costs will eventually determine the performance of dispersing individuals and the evolution of dispersal, we here provide an extensive review on the different cost types that occur during dispersal in a wide array of organisms, ranging from micro-organisms to plants, invertebrates and vertebrates. In general, costs of transfer have been more widely documented in actively dispersing organisms, in contrast to a greater focus on costs during departure and settlement in plants and animals with a passive transfer phase. Costs related to the development of specific dispersal attributes appear to be much more prominent than previously accepted. Because costs induce trade-offs, they give rise to covariation between dispersal and other life-history traits at different scales of organismal organisation. The consequences of (i) the presence and magnitude of different costs during different phases of the dispersal process, and (ii) their internal organisation through covariation with other life-history traits, are synthesised with respect to potential consequences for species conservation and the need for development of a new generation of spatial simulation models.

Costs of dispersal

Recent progress in flapping wing aerodynamics and aeroelasticity

/pdf/the-dance-language-and-orientation-of-bees-3jdlph0pp0.pdf

The Dance Language and Orientation of Bees

Flying insects generate forces that are too large to be accounted for by conventional steady-state aerodynamics. To investigate these mechanisms of force generation, we trained red admiral butterflies, Vanessa atalanta, to fly freely to and from artificial flowers in a wind tunnel, and used high-resolution, smoke-wire flow visualizations to obtain qualitative, high-speed digital images of the air flow around their wings. The images show that free-flying butterflies use a variety of unconventional aerodynamic mechanisms to generate force: wake capture, two different types of leading-edge vortex, active and inactive upstrokes, in addition to the use of rotational mechanisms and the Weis-Fogh 'clap-and-fling' mechanism. Free-flying butterflies often used different aerodynamic mechanisms in successive strokes. There seems to be no one 'key' to insect flight, instead insects rely on a wide array of aerodynamic mechanisms to take off, manoeuvre, maintain steady flight, and for landing.

Unconventional lift-generating mechanisms in free-flying butterflies.

SUMMARY Here we show, by qualitative free- and tethered-flight flow visualization,
that dragonflies fly by using unsteady aerodynamic mechanisms to generate
high-lift, leading-edge vortices. In normal free flight, dragonflies use
counterstroking kinematics, with a leading-edge vortex (LEV) on the forewing
downstroke, attached flow on the forewing upstroke, and attached flow on the
hindwing throughout. Accelerating dragonflies switch to in-phase wing-beats
with highly separated downstroke flows, with a single LEV attached across both
the fore- and hindwings. We use smoke visualizations to distinguish between
the three simplest local analytical solutions of the Navier–Stokes
equations yielding flow separation resulting in a LEV. The LEV is an open
U-shaped separation, continuous across the thorax, running parallel to the
wing leading edge and inflecting at the tips to form wingtip vortices. Air
spirals in to a free-slip critical point over the centreline as the LEV grows.
Spanwise flow is not a dominant feature of the flow field – spanwise
flows sometimes run from wingtip to centreline, or vice versa –
depending on the degree of sideslip. LEV formation always coincides with rapid
increases in angle of attack, and the smoke visualizations clearly show the
formation of LEVs whenever a rapid increase in angle of attack occurs. There
is no discrete starting vortex. Instead, a shear layer forms behind the
trailing edge whenever the wing is at a non-zero angle of attack, and rolls
up, under Kelvin–Helmholtz instability, into a series of transverse
vortices with circulation of opposite sign to the circulation around the wing
and LEV. The flow fields produced by dragonflies differ qualitatively from
those published for mechanical models of dragonflies, fruitflies and
hawkmoths, which preclude natural wing interactions. However, controlled
parametric experiments show that, provided the Strouhal number is appropriate
and the natural interaction between left and right wings can occur, even a
simple plunging plate can reproduce the detailed features of the flow seen in
dragonflies. In our models, and in dragonflies, it appears that stability of
the LEV is achieved by a general mechanism whereby flapping kinematics are
configured so that a LEV would be expected to form naturally over the wing and
remain attached for the duration of the stroke. However, the actual formation
and shedding of the LEV is controlled by wing angle of attack, which
dragonflies can vary through both extremes, from zero up to a range that leads
to immediate flow separation at any time during a wing stroke.

/pdf/dragonfly-flight-free-flight-and-tethered-flow-35vxeiljgv.pdf

Dragonfly flight: free-flight and tethered flow visualizations reveal a diverse array of unsteady lift-generating mechanisms, controlled primarily via angle of attack.

The morphological traits, flight speeds, and temperatures of 54 species of common Neotropical rain-forest butterflies in Corcovado National Park, Costa Rica, correlate with their palatabilities. Bu...

Predation and the flight, morphology, and temperature of neotropical rain-forest butterflies

We test whether palatability of Neotropical butterflies is associated with distribution of mass to the thorax and abdomen. Thoracic mass is predominantly muscle mass, whereas abdominal mass includes organs of digestion, food storage, and reproduction. To escape from predation, butterflies palatable to the rufous-tailed jacamar (Galbula ruficauda) use fast, erratic flight, whereas unpalatable butterflies have defensive chemicals and slow, regular flight patterns. We adjusted for effects of phylogeny and report partial correlations for two levels of analysis: 1) comparisons among-lineage means, which test for correlations between traits of distantly related lineages, and 2) comparisons among deviations from lineage means (or within lineages), which test for correlations between traits of more closely related species. Among lineages for both males (n=10 lineages) and females (n=9), palatability and thoracic mass were positively correlated, whereas palatability and abdominal mass were negatively correlated. An inverse correlation between thoracic and abdominal mass is a consequence of the two segments composing 75% of the total body mass. Predation, indexed by palatability, may select for increased flight speed and thoracic mass at the expense of the abdomen, but relative flight speed and thoracic mass were not significantly correlated. Within lineages (n=45 species for each sex), thoracic mass was uncorrelated with palatability in both sexes. Relative flight speed correlated positively with thoracic mass and negatively with body mass. Palatability and abdominal mass were negatively correlated for males but not females. Hence differences between the sexes in mass distribution suggest differences in reproductive constraints and predation stress.

Flight morphology of neotropical butterflies : palatability and distribution of mass to the thorax and abdomen

Airspeed measurements during natural free flight were made on a total of 270 neotropical butterflies representing 62 species. Morphological data were obtained from the same individuals for which airspeeds had been determined. Flight speed was positively correlated with body mass, thoracic mass and wing loading. Controlling for body mass, higher wing loadings were correlated with increased flight speed. Flight speed and wing aspect ratio were negatively correlated. No consistent correlations were found between airspeed and wing length, wing area or body length. Released butterflies and butterflies encountered in natural free flight did not differ substantially in flight speed allometry. The observed scaling of flight speeds was similar to that derived for a much smaller sample of butterflies flying in an insectary, although absolute values of flight speed were approximately three times higher in natural flight and correlation coefficients of allometric regressions were typically lower. These results suggest that butterfly airspeeds under natural conditions can reasonably be predicted from morphological measurements, and that studying flight in enclosed spaces preserves the allometry of flight speeds.

Robert B. Srygley

Papers

Unconventional lift-generating mechanisms in free-flying butterflies.

Dragonfly flight: free-flight and tethered flow visualizations reveal a diverse array of unsteady lift-generating mechanisms, controlled primarily via angle of attack.

Predation and the flight, morphology, and temperature of neotropical rain-forest butterflies

Flight morphology of neotropical butterflies : palatability and distribution of mass to the thorax and abdomen

Flight physiology of neotropical butterflies: allometry of airspeeds during natural free flight