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Butterfly Anatomy
Page 5
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
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
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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