Strategies for
Survival
Page 8
Chemical Defences
Chemical mimicry
Visual mimicry is
targeted at vertebrate predators - primarily birds and small
reptiles. Butterflies and moths also use mimicry to protect
themselves against insect predators, but in this instance chemical
rather than visual mimicry is used.
Take for example
the case of a caterpillar such as that of the Large Blue
Maculinea arion, which is carnivorous
during the latter part of its life, feeding on ant grubs within
the underground nests of Myrmica sabuleti
ants. The caterpillar had to evolve a way of protecting itself
from the aggressive adult ants. It does this partly by appeasing
the ants with "gifts" of sugary fluids that it exudes from a
dorsal "newcomers gland", and partly by emitting chemical odours
that fool the ants into thinking that the caterpillar is another
ant.
The study of this
pheromonal mimicry in butterflies is still in its infancy, but it
will probably be found to be commonplace among Lycaenid
butterflies, a high percentage of which spend the larval and pupal
stages of their lifecycle in association with ants.
Cosmosoma myrodora, Manu cloudforest, 1400m, Peru
As in the case of
thousands of other nocturnal species, males of the wasp mimic moth
Cosmosoma myrodora are attracted by
pheromones emitted by their females. As a myrodora male approaches
a female, he hovers above her, and discharges a burst of very fine
filaments which swirl in the air around her, and envelop her body.
American
biologists Conner and Boada investigated the lifecycle and ecology
of this moth. They found that Cosmosoma
males are attracted to Eupatorium
plants, and sequestered alkaloids from juices seeping from the
stems. Such pyrrolizidine alkaloids are sequestered by a wide
variety of butterfly genera including
Ithomia, Pteronymia,
Oleria, Lycorea
and Danaus; and by several genera of
moths in the family Arctiidae. The PAs stored in the bodies of the
insects render them toxic or unpalatable to birds, and are a
primary defence method in aposematic species.
The researchers
found that in the case of Cosmosoma
the toxins seemed to be directed mainly at predatory spiders.
Moths caught in the webs of Nephila clavipes
were cut free from the webs by the spider, but moths which had
been deprived of the opportunity to sequester PAs were consumed.
Conner and Boada
found that PAs were passed to females via the discharged
filaments, and also via spermatophores delivered during
copulation. The PA toxins conveyed to females were found to
provide
them with protection against predators during the following
nights, enabling them to lay their eggs unmolested. It was also demonstrated
that the toxins were passed to the eggs, and provided them with
protection against ants, Coccinellid beetles ( ladybirds
) and Chrysopid larvae.
Stinging
Butterflies
and moths do not sting in the sense that a wasp or bee does, but a
significant number of species, particularly tropical ones, have
spikes or setae ("hairs") which have urticating properties.
If touched, the hairs break and release formic acid, or strong
alkaloids that can cause irritation or cause a skin rash on
humans. This type of defence is found mainly in the caterpillars
of the moth family Lasiocampidae, and is probably targeted against
insectivorous birds and small reptiles.
A much more
dangerous chemical defence system is found in the caterpillars of
Lonomia obliqua,
which can be found clustered in groups of
up to 100 on tree trunks in the Amazon rainforest. There have been
numerous incidents where people have
unwittingly touched
or rubbed
their arm against these caterpillars.
The effects
of a dose from multiple caterpillars can be
very severe, including massive intercranial
haemorrhaging and
kidney failure.
Lonomia obliqua caterpillars are a frequent
cause of death in southern Brazil - 354 people died between 1989
and 2005. The fatality rate is about 1.7% - roughly equivalent
to that of rattlesnake bites.
Reflex bleeding
The
caterpillars of several butterflies including the Large White
Pieris brassicae will reflex-bleed if
alarmed, and exude toxins from their mouthparts. These fluids are
sufficiently noxious to dissuade insectivorous birds, which
invariably drop them and then frantically wipe their beaks to
remove any trace of the toxins.
A similar form of
chemical defence is used by certain Pericopiine moths in South
America, which exude a hemolymph foam from their mouthparts and
spiracles if gripped. Any bird attempting to grip the moth in its
beak will experience a very unpleasant taste, and will instantly
drop the moth, allowing it to escape.
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