Flight
• insects were the first organisms to
develop active flight
• insects were flying c 100My before
pterosaurs!
importance of flight
• flight was the breakthrough underlying the
evolutionary success of insects
• about 99% of insect species belong to the
‘Pterygota’ – the winged insects
• flight has enabled these relatively small
animals to overcome the effects of distance
• can use ‘rare’ or dilute resources, therefore
can specialise, can find mates over large
distances
origins of flight
• selection pressures?
– gliding (or at least righting)
– thermoregulation?
– sexual displays?
• structures?
– paranotal processes
– gills
– leg-base sclerites
selection pressures?
• gliding (or at least righting)
– insects as herbivores - hitting fruiting bodies
– long way down …
– presence of (large numbers) of chelicerate
predators
• thermoregulation?
– … Kingsolver … temp. control IS important
• sexual displays?
– ? can justify anything ...
structures?
• paranotal processes
– historical explanation … discredited
– can’t explain articulations, muscles etc
• gills
– gills aren’t aerofoils, selection pressures ‘wrong’
– flight preceeds aquatic larvae
• leg-base sclerites
– currently accepted as best explanation
Paranotal processes
– conceptual model: parallels many vertebrate gliders
Wings derived from larval gills
- based on serial gills of Ephemeroptera larvae
Wings derived from leg-base sclerites
- based on muscle attachments, nerve circuitry
flight capabilities
• Prodigous flight capacity of insects
– Common eggfly, Painted Lady, Meadow
Argus: regularly fly from Australia to N.Z
– NZ Red Admiral to near Palmer Pen.
– Pantala flavescens - circumtropical migrant
… Australia/Pacific Is to NZ
– Aphids, other 'aerial plankton’, cross
oceans
mechanisms that drive insect
wings
• direct and indirect flight muscles
• innervated and fibrillar muscles
• energy preserving elastic processes
direct and indirect flight
muscles
• 2 totally different forms of flight muscle
organisation
– direct … Odonata, Orthoptera, etc. etc
– indirect … Diptera, Hymenoptera etc. etc
• direct flight muscles work the wing
bases
• indirect flight muscles distort the thorax
as an elastic box
Direct flight muscles
Indirect flight muscles
Weis Fogh
‘click’ mechanism
How it fits
together
innervated and fibrillar
muscles
• two totally different ways of operating
flight muscle
innervated - synchronous
fibrillar - asynchronous
• synchronous - Lepidoptera, Odonata etc
wing beat frequency ~ 12 - 30 Hz
• asynchronous - Diptera, Hymenoptera
wing beat frequency 190 - 1100 Hz
fibrillar muscles
• contract in response to being stretched
• contracting dorso-ventrals stretch
longitudinals
• contracting longitudinals stretch dorso-
ventrals
• 1 nerve pulse -> 40 (or more) muscle
contraction cycles
• nerve pulse can switch off ‘engine’
energy preserving elastic
processes
• Insect muscles are supposed to be
about 8% efficient cf 15% in
homeotherms … how do they do it?
• energy-preserving elastic processes -
resilin
distortion of thoracic sclerites
- both store and return energy to the
flight system
how?
• … 'scientists have proved that the
bumblebee can't fly' - refers to some
'back of an envelope' calculations done
by an aerodynamicist in the 1930s
• classical ‘steady-state’ aerodynamics
classical aerodynamics
• calculations used to design planes
• ‘steady-state’
• aerofoils and Bernoulli's ppl …
• critical angle and breakdown of lift
insect wings as aerofoils
• traditional method of analysis
• supination/pronation
• arc of wing movement
• under steady-state aerodynamics an
insect wing can provide lift for ~85% of
the stroke cycle
insect wings as dynamic
structures
• turbles forming aerofoil
• effects of setae/scales
• flexing of wing
• dragonfly ... nodus, pterostigma
• changing aerofoil shape through stroke
or along wing (or both)
Slick air-air interface -
reduces friction, postpones
onset of turbulent drag
Some of the dynamic flexing axes in a dragonfly wing
Butterfly wing rigidity caused by discoidal cell
Stick insect - no transverse bracing
problems
• 'spoiling' of second aerofoil …
– link with hooks (Hymenoptera,
Lepidoptera)
– flap out of phase (Orthoptera)
– one functional pair of wings (Diptera,
Strepsiptera, some Ephemeroptera, some
Hymenoptera, Coleoptera)
scale effects
• insects are flying in a different physical
environment to (say) aircraft, or even
birds
• scale effects
• Reynolds’ number:
length * speed * density / viscosity
• can visualise flow by operating at same
Reynolds’ number
different ways of flying
• above critical angle turbulence doesn't
destroy lift until aerofoil has travelled
several chord lengths
• unsteady flows can generate rotational
flows (vortices) which generate very
great lift
unsteady state aerodynamics
• very high lift generated by vortices
• strongly implicated in insect flight
• known mechanisms:
‘clap and fling’ - Weis Fogh 1973
‘peel’ - Ellington 1984
leading edge vortices - Ellington 1996
others suspected
• quantitative analysis at front end of
computing envelope ...
ANTERIOR VIEW …
Clap-and-fling, wings clap together at top of stroke, then
fling apart … this generates strong circulation about wing.
Originally proposed for small wasps, now widely
recognised (e.g. pigeons taking off)
DORSAL VIEW
Peel – wings peel apart from front edge (peel maintains a
constant angle). Like the ‘fling’ this generates air
circulation around the wing.
Easiest place to see: Big greasy butterfly
Leading edge vortex – vortex established over front edge
of wing, part of toroidal vortex. Generates very significant
lift. Can also recover energy from vortex of preceding
stroke.
different flight mechanisms
• … ref Wootton 1990 Sci Am article
• … document dragonfly flight
mechanisms
• note capacity to ‘switch’ physical lift-
generating processes - animal doesn’t
care about theory … selected for results
• many insects are grossly over-equipped
for flying
downdraft
Bound vortex Trailing vortex
Vortices around a dragonfly wing - X-section
flight envelope
• a dragonfly can switch from forward
flight at 100 body-lengths/s to
backwards at 3 body-lengths/s within a
few body lengths
• dragonflies can hover with their wings
beating vertically
• dragonflies are unstable in all axes -
allows enormous manoeuvrability
Flier type dragonfly – wing stroke perp to body
Percher type dragonfly – note acute angle
References
• Ellington C.P. 1984 The aerodynamics of hovering
insect flight. (parts I - VI) Phil. Trans. R. Soc. Lond. B.
305
• Ellington, C.P., van den Berg, C., Willmott, A.P. and
Thomas, A.L.R. (1996). Leading-edge vortices in
insect flight. Nature 384: 626-630.
• Somps, C. Luttges, M. 1985 Dragonfly flight: novel
uses of unsteady separated flows. Science 228:
1326-1329
• Wootton, R.J. 1990. The mechanical design of insect
wings. Sci. Am. 263(5): 66-72
• Dickinson papers 2000, 2001, 2002 and
web site (hovering flight of Drosophila)
• Srygley + coauthor – free flight in a
butterfly (Nature, Dec 2002) … but see
also German work 1986 on free flying
hawk moths
• Rüppell dragonfly flight – analysis of
high-speed film