Why do some dragonflies have showy females?

Dijkstra, K.D.B. Dijkstra@naturalis.nnm.nl
Wed, 12 Jan 2000 16:48:46 +0100

Odonata - http://www.listbot.com/cgi-bin/subscriber

Dear dragonhunters,

...and why are they gliding around for everyone to see their striking wings?

I was recently struck by certain analogies between the Asian Chlorogomphidae
and the African speciosa-group of the genus Zygonyx (Libellulidae). Both are
large dragonflies that inhabit running waters in a forest setting and both
have females with amazingly patterned wings (in the case of chlorogomphids
they are strongly enlarged and have many more cells than males). The males
are not so showy, having clear wings, although some male chlorogomphids also
have marked wings. Four years ago I observed a female of Zygonyx
regisalberti in Western Uganda. She flew about four or five metres above the
ground in a small flock of large dragonflies, above the edge of a deep
gorge. This gorge lies in savannah, but is itself vegetated with dense
jungle. The torrential river, potential habitat for the species, runs
through it. The dragonflies glided to and fro, remaining high. The
accompanying individuals were similarly sized and had clear wings. These
were probably conspecific males. 

Remembering this scene, I thought of what I had heard about chlorogomphid
behaviour from Keith Wilson. Asking him whether this might have to do with
reproductive behaviour he replied: "Chlorogomphids fly high over ravines and
around mountain tops. I have watched females with highly coloured wings
exhibit a rapid wing flutter, which may be an invitation to males. I have
certainly seen males attracted to females doing this and attempting to pair
but always rejected! I have never observed successful coupling over forest.
I would not rule out the possibility that coloured-winged chlorogomphid
females display to males, but have no conclusive evidence." Keith also
writes: "In China young Zygonyx males and females swarm high over forest
shortly after emergence. I don't think they indulge in any courtship
behaviour in these swarms. When mature they no longer swarm. The males hold
territories low over riffles and waterfalls and actively search out and
seize any females that approach the streams. However, the Chinese Zygonyx do
not have coloured wings." It could be an option that the individuals I saw
were swarming immatures. But what is such a showy female doing in such a
showy place, surrounded by several (probable) males?

Are there any ideas about the possible function of such striking
appearance/behaviour in females? How may this be linked to the ecology and
morphology of the species involved? Has anyone observed such behaviour? I
would reckon that the large, colourful wings have some signalling function.
That's what makes this form of sexual dimorphism so intriguing. If mating
were involved, it would seem a rather unusual strategy within the Odonata. I
believe a Neotropical genus with striking females was referred to in the
e-mail discussions recently, but I have forgotten the details.

KD Dijkstra
Dijkstra@naturalis.nnm.nl <mailto:Dijkstra@naturalis.nnm.nl>

Status: N