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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 small 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 narrow angle seen by each ommatidium means that a detailed
image of their surroundings is probably only possible at very
close distances when the individual elements merge together.
The lenses cannot focus, but due to
their extremely short focal length they don't need to - the laws of
optics mean that everything between about 5mm to 100 metres is
probably rendered sharply.
Unlike vertebrates, which need to move their
eyes / heads to scan their surroundings, butterflies have
almost 360 degree vision. They can see everything at the same
time, e.g. they can accurately probe into flowers in front of them, and at
the same time devote equal concentration to detecting threats from
behind.
The butterfly's brain can
instantly detect whether the image formed by each ommatidium is
dark or light. If a predator approaches, the amount of light
hitting each receptor changes instantly because of it's narrow
angle of view. This, in combination with binocular vision, means
that the compound eyes
are
extremely efficient at detecting the movement and distance of
approaching predators, enabling them to
take immediate
evasive action.
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.
Humans and birds perceive colours and patterns in a
different way to butterflies, as the latter have the ability to
detect ultra-violet as well as visible radiation. Flowers have
ultra-violet patterns that are invisible to humans but can be
recognised by butterflies. The patterns guide the butterflies to the
source of nectar in much the same way that runway lights guide an
aircraft in to land.
The wings of butterflies have visible patterns that can be
seen by mammals and birds, but they also have separate ultra-violet
patterns which can only be detected by other butterflies. These
allow them to quickly distinguish their own species from others
with similar visible patterns.
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Visual
selection of plants for egg-laying
At Stansted Forest in May 2008,
I watched a female Green-veined White as she spent
several minutes searching for places to lay her eggs. Every 4 -
5 seconds she would alight momentarily on a leaf, "tasting" it
using olfactory sensors on her feet to check whether or not it
was the correct foodplant for her offspring.
Surprisingly she tested bracken, ivy and oak leaves ( all
visually very different from the crucifers she needed to locate
for oviposition ), a fact which appears to indicate that sight
plays little or no role in selecting plants for egg-laying. |
How well can moths see at night ?
Elephant Hawkmoths have been
studied to determine whether or not nocturnal moths can perceive
colour. Kelber et al found that this species has 9 light
sensors in each ommatidium ( compared to between 2 - 6 in
butterflies ); and used behavioural experiments to prove that the
moths are able to discriminate coloured stimuli at intensities
corresponding to dim starlight.
Antennae
From between the
eyes emerge a pair of segmented antennae. These can be angled at
various positions, and are best thought of as a form of radar - they
are used to detect pheromones ( scents used for mate location and
selection ), and guide the butterfly towards potential mates.
Essex Skipper Thymelicus lineola
( England ) frontal view of antennae.
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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 - I have often watched male
Small
Tortoiseshells Aglais urticae drumming their antennae on the hindwings of females during
courtship, possibly to "taste" pheromones on the
female's wings.
It is also
common to see butterflies "antenna dipping" - dabbing the
tips of their antennae onto soil or leaves. In this instance they are
sampling the substrate
to detect it's chemical qualities. Males use this method to establish whether soil
contains essential nutrients ( males of many species drink
mineralised moisture to obtain sodium ). Female butterflies use
the same technique to assess the chemical qualities of foliage,
which helps them determine whether the plant is of the correct
species for egg-laying.
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 in many species are hooked at the tip, but most
other butterflies have rounded ends to the clubs.
Some moths, such as the Burnets ( Zygaenidae ) and
Cane Borers ( Castniidae ) have antennae that are clubbed, much
like those of butterflies. This is one of several reasons why any
distinction between butterflies and moths is difficult to define
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 many other families
have strongly feathered 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.
The
antennae of the male Fox moth
Macrothylacia
rubi
are strongly feathered
Most other moths have very thin tapering antennae. In
some such as the tiny day-flying Nemophora degeerella from England, these are incredibly
long, but their function appears to be unknown.
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Nemophora degeerella
has antennae 5 times the length of it's
wings |
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
likely that they are able to detect magnetic fields when migrating.
Labial 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 attuned to the detection of 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 wipers to clean 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
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 into flowers
to suck up the liquids on which they feed. 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, rotting
fruit and the
aphid secretion "honey dew".
In the
Alps mountains of Europe males of many species, particularly
Lysandra, Pyrgus,
Thymelicus,
Cupido and 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 tropical
regions it is common to see Swallowtails, Heliconiines and Pierids
visiting flowers but the majority of species from other
families, particularly males, follow the behaviour described above for the Purple
Emperor.
Often dozens or even
hundreds of butterflies ( e.g. males of Eurema,
Phoebis, Pyrgus,
Trogonoptera
Marpesia, Adelpha, Callicore, Graphium and
Eunica ) gather to filter-feed,
drinking mineralised water from puddles or damp sand. Many other
species such as Doxocopa,
Perisama and
Lymanopoda also gather in lesser numbers to imbibe moisture
in the same way.
Butterflies of
subfamilies such as Charaxinae and Apaturinae are more
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 - I have often seen Glasswings
feeding on the decomposing corpses of flies, and once saw
Rhetus periander sucking fluids from
the corpse of a giant tarantula
in Venezuela.
In Ecuador I found scores of high-altitude Satyrines including
Lasiophila, Junea and
Lymanopoda
feeding on a snake corpse; and a Necyria
metalmark feeding on the corpse of a bull frog.
In temperate regions carrion-feeding is
less common than in the tropics, but
I once found 6 male
Purple Emperors
Apatura iris
feeding at the
carcass of a roe 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 Ithomiines ( Tiger-mimics,
Glasswings ) and Skippers form associations with ant-bird colonies.
The birds follow marauding soldier ant armies, feeding on insects
which scatter as the ants approach. In turn the Skippers and
Ithomiines follow the ant-birds, feeding on their liquefied
droppings.
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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