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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
Thorax - legs &
abdomen
The middle section of the body, the thorax, is best thought of as a
muscular anchor to which the head, legs, segmented abdomen, and wings
are attached ©
Adrian Hoskins
Thorax
The thorax consists of 3 body segments which are fused together,
forming a chitinous cage which contains the flight muscles, and acts
as an anchor point for the legs.
Within the thoracic cavity of flying insects are very powerful
muscles which lever on the wings. The rapid
expansion and contraction of the muscles causes the wings to rise
and fall at rates of up to 1000 beats per second in bees and
hoverflies, and about 200 beats per second in hawkmoths.
Amongst the butterflies,
Skippers have the fastest wing beats. Their wings whirr audibly at a
rate of about 20 beats per second as the butterflies dart rapidly from
place to place. Other butterflies such as Swallowtails, Pierids and
Satyrines can only manage about 5-10 beats per second. Slower still
are the Ithomiines which have very deep beats at about 4 per second.
Slowest of all are the Caligo Owl
butterflies which struggle to achieve more than 2 or 3 beats per
second.
Legs
All
adult butterflies have 3 pairs of legs, except in the Nymphalidae and
in males of certain other groups, where the front pair are reduced to
brush-like stumps and modified as chemoreceptors.
The tibia of each leg has a subgenual (
under the knee ) organ, which detects and amplifies small vibrations.
This alerts butterflies to ground vibrations caused by the approach of
animals or birds, and ready to instantly respond. In most cases
butterflies take flight, but some species such as the
Peacock
Inachis io
and
Bullseye moths
Automeris randa
react by suddenly flashing open their wings to display "false
eye" markings that startle the predator.
The tibia on the forelegs
of Pieridae, Hesperiidae, Papilionidae and Lycaenidae are often
equipped with a flexible spur through which the antennae can be drawn
for cleaning.
The spur also functions as
a spike with which a female can puncture the cuticle of a leaf,
causing it to bleed minute quantities of chemicals. The
butterfly then checks the chemical composition of the leaf, using
olfactory sensors on her legs and feet. This enables her to determine
whether the plant is of the correct species to support her offspring.
Thus a female will spend long periods flitting from leaf to leaf,
"tasting" each one with her feet to assess its suitability prior to
egg laying.
Wood
White Leptidea sinapis,
Surrey, England ©
Adrian Hoskins
The
Wood White Leptidea sinapis,
like all Pieridae species, has
6 fully functional legs. Males of some Pieridae such as the Brimstone
Gonepteryx rhamni often rest with the
forelegs held tight against the body, so at first glance can appear to
have only 4 legs.
Euphydryas aurinia, a Nymphalid with only 4 functional legs in
both sexes ©
Adrian Hoskins
Abdomen
The abdomen contains the digestive system, breathing apparatus, a long
tubular heart, and the sexual organs. The abdominal exoskeleton is
multi-segmented. Each of the 10 segments is comprised of a ring of a
hard material called chitin. The segments are linked by flexible
tissues, allowing the abdomen to bend, a necessity for copulation and
egg-laying.
Reproductive organs
The genitalia are
at the tip of the abdomen. Each species has uniquely shaped genital
armature - the male "key" only fitting the correct female "lock".
Because the armature is unique to each species,
taxonomists
have traditionally relied heavily on microscopic examination of
genitalia to determine species and their relationship with other taxa.
The advent of DNA analysis and advances in phylogenetics however now
mean that genitalia study is just one of many techniques adopted.
Females are equipped with an ovipositor, used to release and deposit
the fertilised eggs. In most species this is short and not normally
visible, but in certain moths it is modified into a long "sting-like"
tube so that the eggs can be inserted into chinks in the bark of
trees.
Creatonotos transiens, male with coremata
extended
Coremata
The males of many
neotropical Arctiid moths, including Creatonotos
transiens ( above ) possess at the tips of their abdomens an
extraordinary eversible organ called a coremata. An unmated female
"call" to males by releasing pheromones from the tip of her abdomen.
Males are attracted by the scent and arrive on the scene, forming a
lek, often comprising of a dozen or more individuals.
Experiments have
demonstrated that males which have accumulated plant-derived
pyrrolizidine alkaloids ( PAs ) then respond by everting their
coremata and releasing pheromones. The PAs are passed to females in a
spermatophore during copulation, conferring them with toxic qualities
that protect them from predation, and also increasing their longevity
and fecundity.
Captive males
that have been deprived of PAs do not evert their coremata or release
pheromones. It seems likely therefore that the females are able to
select which males to mate with on the basis of the strength of their
pheromones - i.e. choosing the male with the highest PA delivering
ability.
Spiracles
On the sides of each segment are microscopic holes called spiracles,
through which air enters and leaves the body. Slight rhythmic
movements of the body, coordinated with the opening and closing of the
spiracles, causes air to be drawn into tiny lung-like sacs, and later
expelled.
Digestive system
Butterflies feed
exclusively on liquids which may according to species include nectar,
dissolved pollen, mineralised water, liquefied dung, urine, sweat,
bodily fluids from decomposing animal corpses, and in some cases even
tears from the eyes of alligators ! After digestion and extraction of
proteins and other minerals the waste matter is expelled from the anus
either in liquid form, or as tiny faecal pellets.
Sound producing organs
Insects
such as cicadas and grasshoppers are well known for producing
courtship songs, but most people only associate other insects with
"incidental" sounds such as the buzzing of wings. There is a great
deal of evidence however that insects in general, including
lepidoptera, produce sounds that fulfil a variety of functions. Many
of these sounds are beyond the range of human hearing, and can only be
detected with specialised acoustical equipment. In some butterflies
however the sounds are clearly audible.
Hamadryas butterflies
can produce a crackling sound by twanging 2 tiny prongs on the tip of
their abdomens against bristles on the valvae. This is discussed
further on the next page.
Nocturnal moths are commonly preyed
upon by bats, which project a series of ultrasound clicks and listen
to their echoes in order to locate flying moths. Many moths have
developed "ears" on their wings or thorax which can alert them to
approaching bats, enabling them to take evasive action. The
neotropical tiger moth Bertholdia trigona
goes a stage further - it actively jams the bats "radar" by producing
its own ultrasound, by vibrating a tympanal organ located on its
metathorax.
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