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Butterfly Anatomy
1 - Head
2 - Thorax - legs & abdomen
3 - Wings
-
venation & scales
4
- Wing scales - scanning electron microscope images
5
- Hearing organs, flight,
thermoregulation
Hearing organs
Some butterflies, including the
Hamadryas Crackers and
Heliconius Longwings can detect sound,
using an "ear" near the base of the underside of their wings. The
ear can only be seen with the aid of a powerful microscope. It takes
the form of a funnel shaped sac, covered with a very thin membrane.
This vibrates in response to high frequency sound, and stimulates
nerve cells called scolopidia, which send a message to the
butterfly's brain.
Hamadryas
butterflies use their ears to detect crackling noises made by
territorial males. The sound is made by twanging 2 tiny prongs on
the tip of the abdomen against bristles on the valvae. Males
habitually bask on tree trunks, where they wait to intercept passing
females. It has been speculated that the sound might deter competing
males from occupying the same territory but this seems to be
unlikely as a single tree trunk will often host 3-4 males perching
in close proximity. It seems more likely that the sounds act act as
a trigger to initiate responses from females during courtship.
In
Morpho helenor the eardrum is located
at the base of the wing.
© Kathleen
Lucas
Kathleen Lucas of the University of Bristol used a laser beam to
scan the membrane of the eardrum of Morpho
peleides ( = helenor ). She
found that lower frequencies between 1000 - 5000 Hz caused
vibrations to focus on a spot on the outer membrane, but that
frequencies above 5000 Hz caused the entire membrane to vibrate,
including the "fried egg" dome structure arrowed in the photo. Moth
ears respond equally to all frequencies, but
Morpho butterflies seem able to differentiate between low and
high pitched sounds. Lucas speculated that this could help the
butterflies figure out if birds are about to attack. If e.g. they
could tell apart the sounds of flapping bird wings and those of bird
song, it might trigger different escape responses by the butterfly.
Some
scientists
believe that when butterflies first evolved they were nocturnal, and
that their ears originally served to detect and avoid predatory
bats. Bats emit acoustic pulses when flying at night, and use their
highly sensitive ears to detect the echo reflected back by solid
objects. This way they avoid hitting unseen obstacles, and are able
to locate moving prey in the dark.
Noctuid moths ( and certain other
groups )
are able to hear a bat's acoustic pulses.
The
frequency and volume enable the moth to detect how far away the bat
is. Furthermore
the relative positions of the moths hearing organs enable it to
determine the direction of approach. The moth initially reacts by
steering away from the bat, but if it gets within striking distance
the moth instantly dive-bombs to avoid being eaten.
Nerve cells
similar to those in the "ears" are also found in enlarged veins at
the base of the fore-wings of many butterflies. These are
particularly well developed in Satyrines such as
Oressinoma,
Maniola, Pararge and
Hipparchia, all of which react
instantly to the sound made as dry leaves are crunched underfoot, or
to the noise made by the shutter of a camera.
Flight
Insect
flight evolved at least 90 million years ago, long before it
appeared in birds or bats, so its original function must have been
for something other than predator avoidance. The most likely
explanation is that it evolved to enable insects to reach food
sources by the most direct and rapid route. What is not currently
understood is the method by which the evolution took place.
Some have suggested that wings
evolved from nodes on the thorax. Another possibility is that they
may have originally appeared as short flexible thoracic hairs, akin
to cat whiskers, which enabled the insects to find there way through
burrows. Once insects began to climb plants they may then have
evolved further as a way of cushioning the landing of falling
insects, and ultimately as a means of easier dispersal, mate
location and food location.
Skippers tend to have
a buzzing moth-like flight, and other small butterflies such as
Lycaenids and Riodinids need to beat their wings rapidly to propel
themselves through the air. Larger species such as Nymphalids,
Pierids and Papilionids fly by a combination of flapping and
gliding. When gliding, the wings are held so as to create a concave
under-surface, producing a parachute effect which slows the rate of
descent. These larger species also make use of thermals to gain or
maintain height when gliding above the forest canopy, or when
migrating.
Males of many
species adopt a "perch and wait" mate locating strategy, and need to
be able to take flight rapidly to intercept potential mates.
Examples include
Skippers ( Hesperiinae ), Metalmarks
( Riodinidae ), and
Graylings ( Satyrinae ).
These species often tend to have triangular forewings with a
particularly thick and straight costa. The springy qualities of the
costa, in combination with their powerful flight muscles, enables them
to accelerate rapidly at take off.
Other species, such as
Whites ( Pierinae ), Swallowtails ( Papilionidae ), Blues (
Lycaenidae ) and Morphos ( Morphini ) adopt a "patrolling" mate
location strategy. Thus they have no need for such rapid
acceleration. They tend therefore to have rounder and less robust
wings, which are larger in relation to their thinner and less
muscular bodies. Consequently their flight is much lazier.
Eurybia
species, probably molochina, Madre de
Dios, Peru ©
Adrian Hoskins
In the neotropics,
Eurybia
butterflies ( Riodinidae ) habitually spend long periods
resting upside down and with wings spread open, beneath the leaves
of low growing vegetation. Flight analysis has shown that by doing
so they are able to take off much more rapidly than they could if
they rested the "right" way up. From their hiding place they keep a
watchful eye on passing insects. Periodically they dash out to
intercept and investigate other butterflies, but instantly return to
settle under a nearby leaf. The speed of flight is remarkable, and
the degree of agility apparent when they fly into the vegetation,
flip upside-down and settle under another leaf is quite amazing to
behold.
Thermo-regulation
Butterflies are cold-blooded. If
they are too cold they cannot fly. If they get too hot they become
dehydrated and die. They have no internal means of regulating their
body temperature, so they need to use behavioural strategies
instead.
In
cool conditions butterflies need to raise their body temperatures
before they are able to fly. To do so they use a technique known as
dorsal basking, whereby they use the upper surface of their wings as
solar panels to absorb heat and give them energy.
Often they settle to bask on pale,
heat-reflecting substrates such as stones, tree-trunks or patches of
bare ground. Heat is reflected back from the substrate and absorbed
by the dark undersides of the wings, speeding up the warm-up
process. Males in particular use this method, to ensure that they
always have sufficient energy available to enable them to instantly
fly up to intercept passing females.
Some butterflies, such as Clouded
Yellows, Graylings & Green Hairstreaks, always keep their wings
closed when at rest, and adopt another technique known as lateral
basking. In cool conditions they bask by tilting their wings over to
one side, so as to present the maximum area of wing surface to the
sun. Conversely, when they get too hot, they tilt in the opposite
direction so that their wing surfaces are parallel to the sun's
rays, and present the minimum surface area to the sun.
Grayling Hipparchia semele, lateral
basking at Arnside Knott, Cumbria, England ©
Adrian Hoskins
The Whites, Blues and
Coppers have wing surfaces which reflect, rather than absorb solar
energy. Consequently they bask with their wings half open, so that
the heat produced by sunlight falling on the dark thorax is
contained within the "cage" of the half-open wings, rather than
being dispersed on the breeze. This behaviour is called reflectance
basking.
Another method used to raise body
temperatures is "shivering". Many Nymphalid species, including
Peacocks, Small Tortoiseshells & Red Admirals prepare themselves for
flight by rapidly shivering the wings ( which are held closed during
this process ). Even on the coolest day, a minute or two of this
activity generates enough friction to heat up the thoracic muscles
enough to enable them to fly short distances. Nocturnal moths often
adopt the same technique.
Butterflies can only operate within
a limited temperature range, so on hot days they need to find ways
of keeping cool. Forest-dwelling species simply hide beneath leaves,
while species that inhabit open areas often fly into bushes to seek
shade, or enter rabbit burrows.
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