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Eyes
Butterflies and most other adult insects have a pair of spherical compound eyes, each
comprising of up to 17000 facets - individual light receptors
"ommatidia" 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 ( Hesperiidae ) are more
efficient than those of other butterflies because they have a clear
area between the cone and the rods. This allows the light from each
ommatidium to spill into neighbouring rods, which effectively
increases their resolution and sensitivity. Thus skippers are able
to fly speedily and very accurately from one spot to another.
This different type of eye structure is one of
the reasons why taxonomists place the Hesperiidae in a different
super-family to all other butterflies - the Hesperioidea.
Capabilities
Butterfly
eyes are probably incapable of resolving detailed shapes or
patterns although some biologists speculate that butterflies might
be able to build up a composite image by means of "flicker
vision".
What we know for sure is that
the narrow angle seen by each ommatidium makes the compound eye
extremely efficient at detecting movement, enabling butterflies to
very quickly detect the approach of a predator, and take immediate
evasive action.
Unlike vertebrates, which have 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 on detecting threats from
behind.
In addition to their capabilities at detecting
movement, butterfly eyes are also very adept at detecting
slight
changes in light levels. They can see polarized light, which enables them to detect the position
of the sun, even when partly hidden by cloud. This enables them to
find their way around their habitats, and relate their position to
the sun when migrating.
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A male Orange
tip Anthocharis cardamines as
seen by human eyes. Birds and reptiles see things in much
the same way. |
The same insect
as it might look to another butterfly. The white areas of
the wings absorb UV while the orange areas reflect it. |
It is interesting
to note that 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. The white wings of the Large White
Pieris brassicae reflect high
levels of UV, and look much the same when viewed by visible or
ultra-violet light. The white areas of the Orange tip
Anthocharis cardamines
however absorb almost all UV, while the orange wing tips are
highly reflective. To another butterfly, the male Orange tip would
probably appear to be a very dark insect, with highly reflective
wing-tips.
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Sight apparently plays
little or no role in selecting plants for egg-laying, as
indicated by my observation of a female Green-veined White
at Stansted Forest in May 2008. The butterfly 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. |
Dingy Skipper Erynnis tages ( England ) showing
segmented antennae.
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 to the source from which they
are emanating.
Essex Skipper Thymelicus lineola
( England ) frontal view of antennae.
The antennae of the Monarch
Danaus plexippus
have been found to be covered in about 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, were found to be sensitive both to sexual
pheromones, and to the honey odour which enables them to locate
sources of nectar.
The
antennae of the male Fox moth Macrothylacia
rubi are strongly feathered
Some moths, such as the Burnets ( Zygaenidae ) and
the Cane Borers ( Castniidae ) have antennae that are clubbed, much
like those of butterflies.
Male moths from other families ( e.g.
Saturniidae and Lasiocampidae )
have strongly feathered antennae that are covered in tens of
thousands of olfactory sensors, and can detect the scent of females from
distances of up to 2km away.
Most other moths have very thin tapering antennae. In
some species such as the tiny day-flying
Nemophora degeerella from England, these are incredibly
long, but their function appears to be unknown.
The tiny moth
Nemophora degeerella has antennae 5 times the length of it's
wings
In addition to their olfactory functions,
butterfly antennae ( like those of ants and bees ) are also
used to communicate physically - I have for example often watched Wood
Whites
Leptidea sinapis
and Small Tortoiseshells
Aglais urticae
"talking" to the opposite sex by
exchanging chemical messages with their antennae.
It is common
also to see butterflies "antenna dipping" - periodically dabbing the
tips of their antennae onto the soil or foliage on which they are
sitting. In this instance they are clearly "tasting" the substrate
to detect it's chemical qualities. By this means they can establish whether the soil
contains essential nutrients, or whether the foliage is of the
correct species for egg-laying.
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 entomologists
postulate that in addition to their olfactory functions, palpi have
secondary 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.
In most butterflies the palpi are small - no more than 1-2mm long.
"Beak" butterflies
( Nymphalidae : Libytheinae ) have very prominent palpi that are
much longer - about 5mm, more than half as long as the antennae.
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.
Anteos menippe ( Peru ) with proboscis extended to imbibe
mineralised moisture.
Feeding behaviour
In temperate zones
most butterflies obtain their sustenance from flowers, but
there are exceptions - the male Purple Emperor for example never visits
flowers; it feeds instead on liquefied minerals which it obtains
from dung, carrion, urine-soaked ground, tree sap, rotting
fruit, and the
aphid secretion "honey dew".
In tropical
regions it is common to see Swallowtails, Acraeas, Heliconiines and Pierids
visiting flowers, but the majority of species from other
families follow the behaviour described above for the Purple
Emperor.
Often large
numbers of butterflies ( e.g. male Papilionidae,
Pieridae and Limenitidinae ) gather to filter-feed, drinking
mineralised water from puddles or damp sand. Butterflies of other
subfamilies such as Charaxinae and Apaturinae however are more
commonly attracted to dung, rotting fruit or carrion.
The carrion feeders vary
enormously in their choice of foodstuff - I have often seen
Glasswings feeding on the decomposing corpses of flies, and have
seen the Blue Doctor
Rhetus periander
feeding on a road-kill giant tarantula
in Venezuela.
In Ecuador I once found scores of high-altitude Satyrines including
Junea,
Lasiophila
and
Lymanopoda
feeding on a snake corpse.
In England 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
Hampshire
thicket.
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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