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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 )
Ovum
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( caterpillars )
anatomy
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fertilisation
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oviposition
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incubation
Anatomy
The
shape, size, colour and texture of butterfly eggs varies enormously
from one species to another.
Those
of Satyrines and Heliconiines are typically domed or barrel-shaped,
adorned with between 8-30 vertical ribs, between which can be seen
dozens of lateral ridges. Other butterflies, including most
Hesperiidae, Papilionidae and Riodinidae, produce smooth globular
eggs. Those of the Polyommatinae
( Blues ) have a finely reticulated surface, and are shaped like
flattened do-nuts. Pierines ( Whites and Sulphurs ) produce eggs
which are tall and skittle-shaped, with fine vertical ribbing.
All butterfly and moth eggs have a depression at the top, in the
centre of which is a hole known as a micropyle, through which sperm
enters during fertilisation.
The egg shell is peppered with
thousands of microscopic pores, through which air enters to sustain
the developing larva within.
Thecla betulae
egg details
Fertilisation
In
the case of Nymphalidae ( and probably other butterflies ) the
eggs are already formed within the body of females when they
emerge. They grow in size over a period of 2 or 3 days, as they mature within the female's abdomen. Egg-laying
is triggered when they reach a certain size, at which time they pass
from the ovariole to the egg chamber. They are fertilised just prior to egg-laying, the male's sperm
having been stored until this time within a receptacle in the female
abdomen.
Oviposition
Butterflies lay their eggs either singly or in batches, on or near
the foodplants that will be used by the caterpillars.
Some species,
e.g. the Marbled White
Melanargia galathea, drop their eggs randomly as they fly amongst tall grasses,
but most species have very precise requirements. Pearl-bordered
Fritillaries
Clossiana euphrosyne
for example lays their eggs singly on dead bracken or dry
grass stems that are within a metre of their caterpillar's
foodplant, dog violet. The White-letter Hairstreak
Satyrium w-album
is even fussier, always laying it's eggs on elm twigs, at the precise point
where the new year's growth and old growth meet.
Silver-washed
Fritillaries
Argynnis paphia lay their eggs
in chinks on the bark of oak trees, but the larvae don't eat oak - they begin by eating their own
egg-shells, and then go into hibernation until the following spring,
when they descend the tree trunks to feed on the leaves of nearby
violets.
Orange tip
caterpillars
Anthocharis cardamines
normally feed on cuckoo flower or garlic mustard
leaves, but if they encounter another caterpillar they become
cannibalistic. It would therefore be wasteful if the butterflies
laid more than one egg on each plant, so nature has endowed the
females with the ability to detect eggs that have already been
laid, and thus avoid laying more than one egg per plant.
In the tropics, eggs
are often glued underneath the leaves of trees and bushes, where
they are protected from rain, and from the desiccating effects of
hot sunshine. In the Amazonian rainforests Heliconiine butterflies
often lay their eggs on Passiflora
tendrils, presumably to place them as far out of the reach of
marauding ants as possible.
Foodplant
selection
The caterpillars of most species will only eat the leaves of one or
two species of plant, and will die if they find themselves on the
wrong type of tree, bush or herb.
Butterflies
therefore spend a great deal of time "tasting" foliage, using
olfactory sensors on their feet, abdomens and antennae to determine whether
the plant is of the right species for egg-laying. All female
butterflies have spines on the underside of their forelegs. When
they land on a leaf these spines puncture the surface, releasing
aromas that are detected by the olfactory sensors.
In Peru I watched a female
Methona confusa
which spent half an hour flitting around a particular
nightshade plant
( Solanaceae ). She
momentarily settled and tasted at least 20 individual leaves,
several of which were revisited many times, before finally settling
to lay a single egg.
From this it can be seen that it is not just
enough to locate the correct species of plant. The eggs usually have
to be laid on tender young leaves or buds, as the older leaves often
contain toxins that can kill them. They also have to be laid on
plants that are growing in very precise conditions - just the right
degree of shade, just the right conditions of temperature and
humidity, and at a height on the plants where they will not get
eaten by browsing herbivores. Eggs are often laid on the tips of
buds, usually quite high up on the tree or bush. This way they are
less likely to be found by ants.
Laying in
batches
egg
batch under
leaf, species unknown, Peru
The caterpillars
of most butterflies live solitarily, but some, particularly amongst
the Melitaeini, Morphini and Pierini, live gregariously for the
majority of their lives. The females of these species lay their eggs
in batches, sometimes as many as 500 at a time. The Marsh Fritillary
Euphydryas aurinia
is a typical example - it lays about 300 eggs in a single batch, on
the underside of a leaf of devil's bit scabious, always within a few
metres of the spot where the female emerged from it's pupa. Having
laid most of it's eggs the female then disperses and sometimes lays
another much smaller batch of eggs slightly further afield.
egg
batch of Marsh Fritillary Euphydryas aurinia, Wiltshire, England
Incubation
The incubation
period varies greatly from species to species. Eggs of tropical
butterflies usually hatch within a week, but in temperate areas 10-14
days is more typical. There are however many species, such as Purple
Hairstreak
Quercusia quercus,
Chalkhill Blue
Lysandra coridon
and High Brown Fritillary
Argynnis adippe, in which the
eggs hibernate over winter, and in these cases the incubation period
can last for several months.
Natural enemies
The eggs of butterflies and moths are valuable sources of protein,
and prone to predation by snails, slugs, and various bugs and
beetles. They are also eaten accidentally by mammalian herbivores.
It may
seem surprising that something as small as a butterfly egg has its
own parasitoids, but these cause high losses. The main parasitoids
are wasps in the families Scelionidae and Trichogrammidae - as many
as 60 of these microscopic wasps can emerge from a single butterfly
egg !
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