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Eyes
Butterflies and most other adult insects have a pair of spherical compound eyes, each
comprising of up to 17000 "ommatidia" - individual light receptors with
their own microscopic lenses. These work in unison to produce a
mosaic view of the scene around them.
Structure
Each ommatidium consists of a cornea and cone,
which together function as a lens. Emerging from the back of each
cone is a rod down which light travels to reach a cluster of 2-6 sensory cells, each of which is sensitive to a particular part
of the visual spectrum.
The eyes of Skippers are
different from those of other butterflies. They have a space between the cones and rods
which allows light from each
ommatidium to spill into neighbouring rods, effectively
increasing their resolution and sensitivity. As a result Skippers
can fly very accurately from one spot to another. This different type of eye structure is one of
the reasons why taxonomists place them in a different
super-family to all other butterflies - the Hesperioidea.
Capabilities
The laws of
optics show that it's
likely that everything from about one centimetre to 200 metres will be rendered
in sharp focus by butterflies, as their ommatidia are of very
short focal length.
The butterfly's brain can
instantly detect whether the image formed by each ommatidium is dark
or light. If a predator approaches or if the butterfly moves its
head a tiny fraction, the amount of light
hitting each receptor changes instantly because of it's very narrow
angle of view. This sensitivity to changes in its surroundings
means that a butterfly is
extremely efficient at detecting movement and at gauging the
distance of
an approaching predator, enabling it to
take immediate
evasive action.
The sensitivity to changes in their visual field, combined with a
high flicker-vision frequency of about 150 images per second, may also
help butterflies to piece together the thousands of elements of the mosaic image
produced by the compound eye. It is not known whether butterflies
and other insects are able to merge these mosaic elements into a
single image. If are able to do so, it would render them capable
of distinguishing patterns at close distances.
Vertebrates
need to move their
eyes and heads to scan their surroundings, but the compound eyes
of butterflies provide them with
almost 360 degree vision. They can see everything at the same
time, so they can accurately probe into flowers in front of them, and at
the same time devote equal concentration to detecting threats from
behind.
Butterflies can see polarized light, enabling them to determine the position
of the sun, even when it is partly hidden by cloud. This
lets them relate their position to
the sun and use it as a compass when moving around their habitats.
Colour
perception
Humans and birds perceive colours in a
different way to butterflies, as the latter are ultra-sensitive to
UV as well as visible radiation. Flowers have
ultra-violet patterns that are invisible to humans but which can be
recognised by butterflies. These UV patterns guide butterflies to the
source of nectar in much the same way that runway lights guide an
aircraft in to land.
Experiments on Colias butterflies dyed orange,
red, green, blue and black have shown that females don't
discriminate between males of different colours. Most biologists
agree that visible colours and patterns are NOT used for
butterfly-to-butterfly communication. Their primary function is to
convey survival-related signals to birds ( i.e. camouflage, aposematic
colour, mimetic patterns etc ).
Butterflies can
communicate with each-other visually, but they use a "private
channel" of ultraviolet patterns which are overlaid on the visible
patterns, and cannot be seen by vertebrates. They enable butterflies
to recognise conspecifics during the initial "approach"
phase of mate location. It has been proven by experimentation that
males which have had their UV-reflecting patterns obliterated suffer
a significant drop in mate-location success.
As well as being
sensitive to UV patterns, butterflies are also alert to
the iridescent colours produced when sunlight refracts from the wings of other butterflies.
Many species have also
evolved selective colour response, i.e. they are "tuned" to react to colours
that are dominant in the wing patterns of their own species.
Examples include
Heliconius erato
which is sensitive to red, Morpho helenor
which reacts very strongly to blue,
and Philaethria dido
which
is responsive to green.
Shape perception
Male butterflies
will intercept and chase any insect of approximately the same size
and colour as the female of their own species during the approach
phase of mate-location. Experiments using dummy cardboard females
have however shown that males respond equally to square, circular,
triangular, or butterfly-shaped dummies.
Females of some
species however seem capable of recognising plants purely on the
basis of leaf-shape and colour. This ability varies from one species
to another, and is most highly developed in monophagous butterflies
- those whose larvae will only eat one type of plant.
Polyphagous
butterflies ( those which utilise several families or genera of
larval foodplant ) tend to rely almost exclusively on chemical cues.
I have e.g. often
observed Pieris napi females searching
for oviposition sites. They
alight momentarily on various plants, sampling each by
puncturing the leaf cuticle with spurs on the legs, to release
chemicals in the leaf which are then tasted using the olfactory
receptors in the feet. Leaves which were tested included
bracken, ivy and oak leaves, all of which are very different in
shape from the crucifers needed for oviposition. This appears to
indicate that in this species sight plays little or no role in
selecting plants for egg-laying.
Vision in nocturnal moths
Elephant Hawkmoths
Deilephila elpenor have been
studied to determine whether or not nocturnal moths can perceive
colour. It seemed unlikely, but Kelber et al found that this species has 9 light
receptors in each ommatidium ( compared to between 2-6 in
butterflies ); and used behavioural experiments to prove that the
moths can discriminate coloured stimuli at intensities
corresponding to dim starlight.
Optical maintenance
Insects are
unable to blink, so need other ways to protect their eyes. In many
butterflies and moths the eyes are shielded by the labial palpi,
which act as dust filters. Butterflies in the
Satyrine genus
Lethe have a dense
layer of fine setae or "hairs" on their compound eyes.
Studies by the author
of these butterflies in Sri Lanka and Borneo indicate that they are strongly attracted to
wet dung, and spend long periods probing into it. It seems plausible
therefore that the setae could function in the same way as a
cat's whiskers, acting as tactile sensors that warn them when their eyes get too close to the dung, which would
blind them if it
stuck to the eye surface.
Antennae
From between the eyes emerge a pair of segmented antennae. These
can be voluntarily angled at various positions, and are best thought of as a
form of radar. They have many functions including pheromone
detection, which is used for mate location and recognition.
Essex Skipper Thymelicus lineola (
England ) frontal view of antennae
The antennae of Monarchs
Danaus plexippus are covered in over 16000 olfactory ( scent
detecting ) sensors - some scale-like, others in the form of hairs
or olfactory pits.
The scale-like sensors, which number about
13700 in total, are sensitive to sexual
pheromones, and to the honey odour which enables them to locate
sources of nectar.
Butterfly
antennae, like those of ants and bees may also used to
communicate physically - e.g. it is common to see male
Small
Tortoiseshells Aglais urticae drumming their antennae on the hindwings of females during
courtship, possibly to "taste" pheromones on the
female's wings. Similar activity can be found in Wood Whites
Leptidea sinapis and many other
species.
Butterflies are
often observed
"antenna dipping" - dabbing the antennal tips onto soil or
leaves. In this case they are sampling the substrate to detect
it's chemical qualities. They do this to establish whether soil
contains essential nutrients. Male butterflies often drink
mineralised moisture to obtain sodium, which they pass to the
females during copulation.
Differences between butterfly and
moth antennae
Butterfly antennae are always clubbed at the tips. In most butterfly
subfamilies e.g. Nymphalinae, Heliconiinae and Pierinae the shaft is straight and the
club very pronounced, but in the Ithomiinae the antennae thicken
progressively towards the tip. The clubs of Skippers ( Hesperiidae )
taper to a fine point and are hooked at the tip, but most
other butterflies have rounded ends to the clubs.
Some moths including Burnets ( Zygaenidae ) and
Cane Borers ( Castniidae ) also have antennae that are clubbed just
like those of butterflies. This is one of many reasons why the
"convenience" division of Lepidoptera into butterflies and moths is difficult to
justify
scientifically.
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6-spot Burnet
Zygaena filipendulae
( Zygaenidae ), England. Burnet moths have antennae
that are clubbed even more than those of true butterflies. |
Male moths from the
Saturniidae, Lasiocampidae and a few other families
have plumed "pectinate" antennae which are covered in tens of
thousands of olfactory sensors, and can detect the scent of females from
distances of up to 2km away. The females have no need to detect
pheromones, so their antennae, although similar in structure, have
very much shorter plumes.
Antenna of male
American Oak Silkmoth
Antheraea polyphemus
( image © Emily Halsey )
Johnston's organ
At the base of the
antennae is a "Johnston's organ". This is covered in nerve
cells called scolopidia, which are sensitive to stretch, and are
used to detect the position of the antennae, as affected by gravity
and wind. Thus they are used to sense orientation and
balance during flight, and enable the butterflies to finely adjust
their direction or rate of ascent / descent. It is also thought
possible that they are able to detect magnetic fields when migrating.
Palpi
Protruding from the front of the head are
a pair of small projections called labial palpi, which are covered
in olfactory ( scent detecting ) sensors. Similar sensors are also
located on the antennae, thorax, abdomen and legs.
These sensors
are present in a variety of forms, and it is likely that each type
fulfils a different role. Sensors on the antennae for example might
be "tuned" to locate sexual pheromones, while those on the legs may
be sensitive to chemicals exuded by larval foodplants. Logic would
indicate that those on the labial palpi and proboscis, due to their
position, might be tuned to detect adult food sources
such as nectar, urine, carrion or tree sap.
Alternatively it is possible that they might function
to detect the "smell" of air which emanates from particular
locations - incoming dry desert air for example might be detected
and act as a trigger to stimulate migration.
Some biologists
argue that in addition to their olfactory functions, palpi have
other functions such as shielding the proboscis. Logically this
would mean a short proboscis would be associated with small palpi,
and a long proboscis associated with larger palpi. In fact this is
not the case - species with very long proboscises, such as
Saliana skippers and
Eurybia Underleafs
have average sized palpi, while
Libythea Beaks and other species with prominent
palpi have unremarkable proboscises.
Another theory is that the
palpi may serve as dust filters to protect the surface of the eyes. DeVries
states that the most well developed palpi are found in butterflies
which feed as adults on rotting fruit or dung, where there is a
greater probability of soiling the eyes or becoming infested with
mites, but this theory certainly does not hold true for the Beak
butterflies
( Libytheinae ) which have extremely long palpi but from my
observations feed mainly on mineralised moisture at the edge of
puddles.
Beak
butterfly
Libythea myrrha
( Malaysia
), showing labial palpi projecting from head.
Proboscis
The proboscis consists of a pair of interlocking
c-section channels that when linked
together form a tube, much like a drinking straw. This tube can be coiled up like a spring for
storage, or extended to enable the butterfly to reach deep into flowers
to suck up nectar. If the proboscis
gets clogged with sticky fluids, the 2 sections can be uncoupled and
cleaned.
Olfactory sensors near the tip of the
proboscis and in the food canal, together with similar sensors on
the tarsus and tibia of the legs, enable butterflies to "taste"
nectar, pollen, dung, and minerals.
The "BD" butterfly
Callicore
cynosura,
using its proboscis as a drinking straw to imbibe dissolved minerals from the surface of a damp rock on the
shore of an Amazonian tributary.
Feeding behaviour
In temperate zones
most butterflies obtain their sustenance from flowers, but
there are exceptions - male Purple Emperors for example never visits
flowers; they live entirely on fluids which they obtain
from dung, carrion, urine-soaked ground, tree sap, and on "honey
dew" - sugary aphid secretions which coat the upper surface of
tree foliage in mid-summer.
In the
Alps and Pyrenees mountain ranges of Europe males of many species, particularly
Lysandra, Pyrgus,
Thymelicus,
Cupido & Mellicta often
aggregate in groups of several dozen ( and sometimes several hundred
) to imbibe mineralised moisture from the edges of puddles, urine
soaked ground or cattle dung. This phenomenon is common in alpine
regions throughout the northern hemisphere.
In
the tropics the majority of males from all families follow the behaviour described above for the Purple
Emperor.
Females of many
species appear not to feed at all, and rely on proteins and amino
acids transferred via the sperm of males during copulation. In the
case of Papilionidae, Pieridae and Lycaenidae however females
commonly obtain sustenance from flower nectar.
In Central & South America female Heliconius
butterflies
visit
Lantana and various other flowers for nectar.
They also sequester pollen from
Psiguria,
Anguria and Gurania flowers in the
rainforest.
The pollen collected from
the flowers is processed by the females to extract amino acids
which increase longevity and enable them to produce eggs for up to
9 months.
The butterflies have acquired the ability to learn
and remember the locations of individual pollen plants. They visit these
every day, following a predefined circuit through the forest.
Phoebis argante and
Rhabdodryas trite aggregating to
imbibe mineralised moisture, Peru
Swarms of butterflies, e.g. males of Eurema,
Phoebis,
Marpesia, Adelpha and
Callicore habitually aggregate on river beaches to filter-feed,
drinking mineralised water from damp sand. Numerous other
species such as Doxocopa, Rhetus &
Caria also gather in lesser numbers in
similar situations.
Males
from
subfamilies such as Charaxinae and Apaturinae are commonly attracted to dung, rotting fruit or carrion.
DeVries has estimated that at least 40 percent of
all Nymphalidae in Costa Rica feed exclusively on rotting fruit.
The carrion feeders vary enormously in their choice of foodstuff.
In Ecuador I have commonly seen Glasswings
feeding on the decomposing corpses of robber flies, and in Venezuela
I watched a male
Rhetus periander sucking fluids from
the corpse of a giant tarantula.
At Pululuhua Crater in
Ecuador I once found scores of high-altitude Satyrines including
Lymanopoda,
Lasiophila & Junea feeding on a snake corpse; and
at Maquipucuna Cloudforest I stumbled upon a stunning Necyria
avidly feeding on the corpse of a bullfrog.
In temperate regions carrion-feeding is
far
less common than in the tropics, but
I
fondly
remember finding 6 male
Purple Emperors
Apatura iris
feeding at the
carcass of a deer that was floating in an open cesspit in
a thicket
in southern
England.
The butterflies were so
stupefied by their
unsavoury meal that 2 of them remained on the carcass as I lassoed
a rope around the antlers and hauled it to the edge of the cesspit
to take photographs !
In the
rainforests of South America many butterflies form associations with ant-bird colonies.
The birds follow armies of marauding soldier ants, feeding on insects
that scatter as the ants approach. In turn the butterflies follow the ant-birds, feeding on their liquefied
droppings. Biologists studying butterflies in rainforests commonly
place tiny wads of dampened white tissue, designed to simulate
bird-droppings, on leaves to attract butterflies from the families
Hesperiinae and Ithomiinae.
The
feeding behaviour of butterflies is discussed in greater detail in
the individual species accounts, which can be accessed from the
galleries or the
Species
Index.
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