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Strategies for
Survival
Page 1
1 - Mate recognition
2 - Camouflage & Disguise
3 - Aposematic
& Diematic
colouration
4
- Mimicry
5
- Sexual dimorphism
6 - Roosting behaviour
7 - Seasonal dimorphism
8 - Chemical warfare
Mate recognition
Butterflies
occur in an incredible variety of shapes, colours and
patterns. Each design serves a dual role. Firstly it acts as an
identity badge, advertising the butterfly to potential mates.
Secondly and equally importantly
it functions to protect the butterfly from predators, e.g. by
using camouflage, disguise, aposematic or diematic coloration, mimicry or transparency.
It is possible of
course to read too much into the "colour for survival" theme.
There doesn't have to be a survival-related reason for every
nuance of pattern or colour. After all what possible difference
could it make if a caterpillar has a brown head instead of a black
one ? Having said this however it is quite obvious that a
butterfly that is camouflaged as a dead leaf has a considerably higher
chance of avoiding predation than one that is not.
Butterflies
have short lives - typically less than 2 weeks as an adult, so they need to quickly locate
mates. Many species lose
their attraction to the opposite sex within just a day or two of
emergence because their pheromones become exhausted. Rapid mate recognition is
therefore vital.
The first stage in the recognition process
consists of intercepting any flying object of roughly the same size
and colour as their own species - in
the neotropics for example it is easy to attract blue
Morpho butterflies by waving a
piece of blue foil in the air.
Purple Emperor Apatura
iris, male
In many genera
e.g.
Apatura,
Morpho
and
Doxocopa
the wings of males have a highly reflective blue sheen. During flight their
wings glint brilliantly in the sunshine and probably play a major
role in mate location and recognition.
Once initial contact has been made, a
combination of olfactory and visual factors hold the interest of
both sexes as a precursor to the courtship ritual. Visual stimuli
include holding the wings or body at particular angles, flicking the
wings, or simply displaying the patterns by opening the wings.
During the courtship process a
series of exchanges takes place during which various stimuli
trigger either negative or positive responses. A negative response
might indicate that the potential mate was not of the same
species, or was unwilling to mate - females of many Pierids for
example signal an unwillingness to mate by raising their abdomen
and outspreading their wings.
A positive response invariably
triggers another visual, olfactory and tactile stimulus,
which leads to another response. By going through these stimulus /
response sequences the butterflies are able to determine firstly
that they are of the same species,
then that they of opposite sexes, and finally that both are ready
and willing to to mate. In some species this process is very brief, but in
others such as the Small Tortoiseshell
Aglais urticae,
it is protracted and may take several hours during which the male
doggedly follows the female from place to place, persuading her to
copulate.
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Small Tortoiseshell Aglais urticae |
Male
butterflies will usually intercept both sexes of their own
species. The purpose of male-male sorties is to challenge and oust an
intruding male from the territory of another, thereby increasing
the territory owner's chances of success with any female that
passes by.
Males also
"know" that they can often find food by following other males :
In tropical
regions males of many species visit sandbanks and damp paths to
indulge in "mud-puddling". Their purpose is to imbibe mineralised water,
from which they obtain vital salts. These are passed to females
during copulation and are believed to be essential for the
production of fertile eggs.
Typically just one or two males
will chance upon a suitable
feeding spot, but other butterflies flying past
seem able to recognise their brethren on the ground, and swoop
down to join them. The bright patch of colourful butterflies
quickly becomes a magnet to every passing male of the same
species.
Often
several different species may be present at these feeding places.
In these circumstances one might expect each species to be spread
randomly within a large group, but in fact each butterfly
polarises very strongly to it's own brethren, so that each species
congregates as a discrete group.
In calm weather the
butterflies in each group are positioned randomly, but on
riverbanks there is usually a constant gentle breeze, so all the
butterflies in each group tend to face into the breeze as this way
they are less likely to be blown about and lose their feeding spot
to a competitor.
Phoebis argante and
Rhabdodryas trite aggregating at Rio
Shima, Satipo, Peru
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