|
The butterfly
lifecycle
The Lepidoptera
( butterflies, skippers and moths ), belong to a group of insects
called Endopterygotes, all of which go through 4 distinct phases in
their lifecycles :
Section 1 - EGG
( ovum / ova )
Section 2 - CATERPILLAR
( larva / larvae )
Section 3 - CHRYSALIS
( pupa / pupae )
Section 4 - ADULT
( imago )
Imago
emergence
|
feeding
|
mate
location
|
courtship
|
reproduction
|
mate
location
perching,
patrolling and territories
|
the daily routine
|
adult lifespan |
anatomy
Eueides lybia, newly emerged from
chrysalis, Rio Madre de Dios, Peru
Emergence
Emergence of the
fully developed adult butterfly from the pupa is triggered by
factors including humidity, temperature, light level, and time of
day. Most butterflies emerge shortly after dawn.
Prior to emergence the spiracles ( breathing holes )
of the butterfly within are linked by short tubes to the spiracle
openings on the pupa shell. Air is drawn in through these tubes,
enabling the butterfly to pump up it's body, which causes the shell
of the pupa to split, just behind the head.
The butterfly then forces it's way out, and uses it's
legs to pull itself clear of the empty pupa. If the pupa was
attached to a twig or leaf, the butterfly usually suspends itself
from the pupa shell while it expands it's wings.
Skippers and many moths pupate within some sort of
silken nest or cocoon. Prior to emergence they usually eject
solvents from the proboscis which dissolve away the silk and enable
them to emerge. Other butterflies ( e.g. certain Satyrines and
Lycaenids ) pupate in subterranean cells. In these cases they have
to crawl about until they can find a twig or firm stem from which
they can suspend themselves.
Having settled into position, the butterfly then
spends several minutes hanging virtually
motionless. During this time it pumps fluids into the wing veins,
causing the wings to expand to their full size.
After allowing
time for the wings to harden, male butterflies usually fly off to
seek nectar or other forms of sustenance. Females of many species however tend to
stay within a few metres of the emergence site until mated.
After
drying the wings, and before taking their first flight, butterflies
and moths expel the metabolic waste product meconium from their
abdomens, in the form of a pinkish liquid.
In most species males emerge at
least a day or two before females. The reason usually given for this
is that females usually mate on the day they emerge, so it is
advantageous if there are already plenty of males available to them.
An equally likely explanation that
is not mentioned in literature is that males are not usually capable
of mating until they are 2 or 3 days old, as they often need to feed
in order to accumulate alkaloids that are vital to reproduction.
Well known examples of this include the
Purple Emperor
whose males feed at dung,
Swordtails and
Daggerwings
which feed at urine,
Longwings
which feed on pollen, and
Glasswings such as
Pteronymia sao
which feed on decomposing plant material. The latter derive
pyrrolizidine alkaloids from the plants, which are used in the
production of pheromones and defensive toxins, as well as for
reproductive purposes.
Feeding behaviour
Males of many
tropical species are regularly observed imbibing dissolved minerals from damp
mud, bird droppings, aphid secretions, sap runs, or even from
carrion. The minerals are later passed to the females during
copulation, and may contain vital nutrients necessary for the
production of fertile eggs.
In
temperate regions both sexes of most species feed primarily at
nectar, but males of several species, e.g.
Apatura iris,
Lysandra coridon,
Pyrgus alveus,
Thymelicus lineola
and
Mellicta athalia
commonly imbibe mineralised moisture, or visit dung.
Anteos menippe
( Peru ), male imbibing dissolved
minerals from damp sand.
Mate location
Butterflies generally have short lives, females often having less
than a week to find a mate, copulate, search for oviposition sites,
and lay their eggs.
Rapid mate location and recognition is vital - butterflies cannot afford to waste time on too many fruitless
encounters with species other than their own.
|
 |
|
Dryas iulia
- a single male on the ground quickly attracts
others to a feeding site |
Butterflies can see
all the colours of the visible spectrum, plus ultra violet. Males of
brightly coloured species such as the orange
Julia
Dryas iulia,
and yellow
Phoebis
and
Eurema species are able to detect others of their
own kind from several meters away, and use this ability to home in
on feeding grounds.
Experiments with bright blue
Morpho
species show that waving blue foil in the air is very
effective at attracting them towards observers.
These facts prove
that butterflies use colour to locate others of the same species,
and it seems logical to assume that colour plays a significant role
in mate location.
Papilio
machaon
- a species with an instantly recognisable pattern
Pattern recognition is a different matter, and only comes into
play when the butterflies are in close proximity. Very conspicuous
patterns such as the black and yellow pattern of
Papilio machaon,
or the black, red and white of
Vanessa atalanta
may be recognised at close distances, but it is obvious that subtle
patterns cannot be.
To illustrate this point, simply imagine an alpine meadow with a
mixed population of
Erebia
species - all are plain drab brown insects of similar size and
shape, differing only in minute details that could not possible be
seen with the poor resolution of a butterfly's eyes.
The same argument applies to the
Pyrgus
skippers, the
Melitaea
Fritillaries and the numerous Polyommatine blues, where many very
similarly patterned and closely related species share the same
habitat. Colour
and pattern can therefore play only a fairly minor role in mate
recognition.
It quickly becomes apparent that pheromones ( scents ), rather than
visual stimuli, are the major factor enabling a butterfly to
recognise a potential mate.
Males of many
species have specialised scales on their forewings called
"androconia", which have sacs at their bases containing
pheromones.
These are disseminated into the atmosphere via tiny hairs or plumes on the edges of the
scales, and are used to attract females and entice them to copulate.
In the Danainae the
androconia are on the hindwings. The males are equipped with tufts
of hair-pencils at the tips of their abdomens, which they
brush against the scales to collect the pheromones, and these are
later disseminated by expanding the tufts when in the presence of
females.
Wood Whites
Leptidea sinapis
engaged in courtship ritual
Courtship and Reproduction
In some species
courtship is very brief, in others it is protracted and involves
complex rituals. Male and female Wood Whites
Leptidea sinapis
for example are often seen sitting facing each other, exchanging
chemical messages with their antennae. The male repeatedly flicks
out his long proboscis, whipping the female alternately on the
underside of her left and right wings. Both sexes periodically flick
open their wings, and it is clear that the butterflies are
communicating something.
The ritual does not appear to instigate copulation, so the nature of
the "messages" is unclear.
The courtship
behaviour of butterflies in general are poorly studied, but it is
clear that in most species the females will not permit males to
copulate until they have completed an often complex ritual. This
typically begins with the males releasing pheromones, which leads to
the female settling on foliage. The male might then begin a
"courtship dance" around the female, or use some other form of
physical stimulus to which the female reacts in a particular way.
Each stimulus must elicit the "correct" response, producing a chain
reaction of events that culminates in copulation.
Small
Tortoiseshells
Aglais urticae
have a protracted courtship which can last for several hours. The
male follows the female as she flies from place to place, and when
she settles and opens her wings, he walks onto her hindwings,
tapping them with his antennae. This process is repeated many times
over several hours, during which the male will drive off any other
intruding males. Eventually the female leads him to a
sheltered spot, typically underneath a small bush, where copulation
takes place.
Brimstone
Gonepteryx rhamni
males are also
often seen "wing-walking" on females, but this is
invariably followed by the female inverting her wings and raising
her abdomen - a signal to the male that she is rejecting his
advances, and a sure sign that she has already
mated ( females of most butterflies only mate once ).
Brimstone
Gonepteryx rhamni,
female raises abdomen to signal rejection to male.
When female
Brimstones
are receptive, copulation takes place almost instantly, without any
observable pre-nuptial ritual. Most butterflies remain copulated
only for an hour or so, but the Brimstone is quite remarkable in
this respect - I once found a pair of Brimstones which remained
copulated beneath a bramble leaf for an amazing 17 days before
finally parting company !
Lysandra bellargus
copulating
pair ( female on left )
Males usually
mate with several females in the course of their lives. Females of
certain long-lived species such as the Monarch
Danaus plexippus
will mate with
several males, but in the majority of species females normally only mate once. After
mating, the genital opening on the females of
Marsh Fritillaries
Euphydryas aurinia
and some
other species becomes sealed, physically preventing them from mating
with other males. Males of the Apollo butterfly
Parnassius apollo
seal the female
genital opening with a structure called a sphragis, preventing other
males from copulating with them.
The behaviour of
Heliconius erato, as observed by myself in Costa Rica, is
fascinating to watch. Female pupae, when close to emergence, often have several male adults in
very close
attendance. A frantic battle takes place the instant the female
hatches, as all the males struggle to copulate with her, not even
allowing her time to expand and dry her wings. The mated pair then
have to endure the attention of the remaining males, which are often
extremely persistent, trying to prise the pair apart.
Eventually, with the approach of dusk, the unsuccessful males disperse,
allowing the pair to remain copulated until the next morning.
The butterflies can live for many months, but spend their entire
lives within a restricted area. As a result of this minimal contact
with other populations, the species has developed a vast number of
localised "races" and colour forms.
Female
Brimstones
Gonepteryx rhamni go to the opposite extreme - they emerge
in July,
feed for a few weeks, and then go into hibernation for several
months. After awakening the following April, they are
mated, and then fly many miles, often across inhospitable
terrain, stopping to lay their eggs on any buckthorn plants that they
encounter on their travels. The butterflies thus spread throughout
the countryside, interbreeding with other Brimstones that may have
originated a considerable distance away. The resulting high genetic
diversity is probably a major factor in the remarkable success of
the species, which is able to exploit an enormous range of habitats
and climatic extremes, being found at altitudes from sea level to
2800m, and having a range that encompasses north Africa, the whole
of Europe, and extends across temperate Asia to Siberia and
Mongolia.
Perching, patrolling, and territories
Entomologists have traditionally divided the pre-nuptial behaviour
of butterflies into two groups - those that "patrol" and those that
"perch".
Patrolling species are those where the male actively patrols a
regular or random route through it's habitat in order to locate
females.
Perching species are those where the male spends long periods
sitting on a prominent projecting leaf, or on a particular rock or
patch of ground, which it uses as a base from which to intercept
passing females. This group of butterflies are generally regarded as
being territorial, because males will intercept other males of the
same species, which after a brief encounter move out of the area.
In
truth, any male, of either a patrolling or perching species, will
fly towards any other flying insect of similar size and colour, to
investigate it and determine whether it is a female of it's own
species. Most males will try to fly over or around a female in a
particular direction,
Silver-washed Fritillary males
for example loop over and under females as they
fly along a track, showering them with pheromones.
If two
males meet, they both attempt to fly in the same way around each
other, and end up spiralling upwards to a great height until they
realise that they are both males, at which time perching males
usually return to their preferred leaf or rock, while the
"intruding" male ( usually a newly emerged individual which has not
yet found it's own perch ) carries on in search of it's own perching
place. This activity is often wrongly interpreted as being a defence
tactic, but in reality is nothing more than a static butterfly
meeting a roving individual, and then returning to it's perch.
A
perch is simply a vantage point from which males can get a good view
of all passing insects. Each species has it's own preferred type of
perch however -
Purple Emperors
will perch at the top of a prominent tree, usually on a hilltop,
while the Duke of Burgundy will perch on the leaf of a bush,
typically in a small glade or at the sunny intersection of forest
paths.
In
both cases the butterflies have homed in on a spot where they have a
good view in all directions. Both sexes of each species
instinctively head towards their perching sites, and thereby have
the greatest chance of meeting members of the opposite sex. In the
case of the
Duke of Burgundy
the male further increases his chances of
meeting a female by setting up camp at an intersection, where
butterfly "traffic" meets.
The
daily routine
Male and female butterflies of any given species usually behave very
differently. In most species the males are highly active, and their
behaviour follows a predictable cycle of feeding, basking, and
patrolling in search of females. Males of other species are often
highly territorial, and will defend their territories against other
insects including wasps, flies, and flying beetles. If another male
of the same species enters their territory, they engage in an aerial
sortie, spiralling high above the trees until the intruding
butterfly is ousted.
Female
butterflies live entirely different lives. Prior to mating they are
often sedentary, remaining very close to the spot where they emerged
from the pupa. After mating they seek places to lay their eggs, but
usually fly only short distances between bouts of egg laying.
These
differences in behaviour are reflected in their appearance - males
need to be noticed, so are generally more colourful than females.
The Adonis Blue
Lysandra bellargus
from Europe is a good
example : males are a brilliant iridescent blue colour, but the
females are drab dark brown creatures - they spend most of their
time crawling about on the ground amongst short grasses, looking for
places to lay their eggs, but need to bask occasionally so they can
raise their body temperatures. When basking on the ground, their
drab colouration helps them to escape the notice of insectivorous
birds.
Adonis
Blue
Lysandra bellargus, male, Ballard
Down, Dorset
Adonis Blue
Lysandra bellargus, female, Ballard Down, Dorset
Lifespan
The whole
lifecycle from egg to adult can take just 3 weeks to complete in
many tropical species.
In temperate regions the lifecycle
of the summer generation may be complete within about 6 weeks,
but many species only produce a single generation in a year. In
sub-arctic and alpine zones some species ( e.g.
Parnassius eversmanni,
Oeneis alpina
and
Boloria natazhati
) take 2 years to complete their lifecycle.
The lifespan of the
adult butterfly varies greatly from species to species. Butterflies
kept in captivity, if fed regularly can live for quite long periods.
Wild butterflies are subject to predation and the extremes of
climate, so while they have the potential in most cases to live for
3 or 4 weeks, in practice the average lifespan is just 7 or 8 days.
There
are however several notable exceptions to this general rule. Some
butterflies, e.g. Monarchs, Commas and Tortoiseshells, hibernate as
adults, and these species often live for several months. The longest
lived European species are the Brimstone and
Peacock - both emerge
in early July, and often survive until the following June.
Certain tropical species are also capable of surviving for equally
long periods - in South America for example,
Taygetis mermeria
and certain Ithomiine and Danaine species are known to sometimes
live for up to 11 months. Various
Heliconius
species, when reared in captivity can live for even longer, but the
absence of faded and worn specimens in the wild would seem to
indicate that longevity in nature is substantially less. |
