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| Keeled Skimmer - Orthetrum coerulescens |
Earlier this month we went on a walk with Sussex Wildlife
Trust looking for Devil’s darning needles and Horse-biters in Ashdown
Forest. And just in case you don’t
recognize those old country names for those creatures, we were looking for
Damselflies and Dragonflies. (Note that Damselflies are included in the general
term Dragonfly, so I shall just use the latter to save words.) Ashdown Forest, despite its name is mostly
open heathland where there are plenty of muddy pools and slow streams that are
ideal habitat for those insects. There
are plenty of dragonflies there and for details of the rarities and photos of
what we found see www.pettnats.blogspot.co.uk.
You don’t have to go to Ashdown Forest to see dragonflies,
of course. Anybody with a pond, or who
lives near anybody with a pond will see plenty of them in their garden at this
time of year. Despite their somewhat
sinister country names they are harmless and beautiful creatures that have
fascinating life cycles. (Even the
largest Emperor Dragonfly would be unable to break human skin, let alone a
horse’s, though you may get a deservedly painful nip should you provoke one
enough.)
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| Blue-tailed Damselflies - in tandem |
Their life-cycle starts with a fertilized egg being
deposited in or on a reed stem or even just placed under the water by the
female. If you see one dipping her
abdomen into the water or crawling backwards down a reed stem into the water,
she will be laying eggs. She will be
doing that with or without the ‘help’ of the male. The male is not helping, he will just be
protecting his investment either by clasping her round the neck to prevent
another male mating with her, or by flying close by to chase off any males with
similar ideas.
Each male has a pair of
claspers on the end of his abdomen that fits with a specially shaped shield
just behind the female’s head called a pronotum. These are connected during mating and any
subsequent egg protection behaviour.
Each species has its own special shape of clasper and pronotum which
(mostly) prevents inter-species mating.
Specially shaped genitalia or associated equipment is common in the
insect world and is often used by entomologists to distinguish between species
when other differences are too subtle or too variable.
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| Emperor Dragonfly - Egg Laying |
When the fertilized eggs hatch out, the larvae will skulk
about on the bottom of the pond and prey on any smaller invertebrates that they
find there and when they get big enough they will even take tadpoles or small
fish. Because they have a hard external
skeleton, they have to moult several times in order to grow to their adult
size, and they may take two or more years before they are ready to emerge as
adults. The larva will then wait for
suitable weather and summer temperatures, before crawling up a reed stem above
the water where it splits its external skeleton for one last time and emerges
as the adult insect. It will stretch out
its abdomen and thorax and inflate its wings before the skeleton hardens.
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| Azure Damselflies in the 'Wheel Position' |
The adult Dragonfly will then take to the air, feeding on other
flying insects and looking for a mate.
Adult dragonflies live only for a few days, or a few weeks at most. When the male finds a receptive female, he
transfers a packet of sperm from the end of his abdomen to the second segment
where his ‘equipment’ resides. As the
second segment is near the front, this involves making a complete loop of his
abdomen. He then straightens out and
clasps the female behind the head. She
then loops her abdomen round so that the tip connects with the second segment
of his abdomen making what is called the ‘wheel’ shape. They will then either fly in that position,
or will find a convenient perch until the fertilization is complete, whereupon
the female will detach her abdomen. They
will then either separate completely, or will fly together with the male still
clasping the female until the fertilized eggs have been laid. And then it all starts over again. It sounds really complicated written down
like that, but fortunately dragonflies can’t read so they manage very successfully
with what evolution has provided for them.
