floating on water surface (was Wing Position) [ODO]

Dave McShaffrey mcshaffd@marietta.edu
Tue, 18 Jan 2000 18:28:01 -0500

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----- Original Message -----
From: "Dennis Paulson" <dpaulson@ups.edu>
To: <dragonflies@listbot.com>
Sent: Tuesday, January 18, 2000 2:58 PM
Subject: Re: floating on water surface (was Wing Position) [ODO]

> Odonata - http://www.listbot.com/cgi-bin/subscriber
> Dave McShaffrey wrote:
> >
> >1.  The males are trying to demonstrate the relative density of fish in
> >stream.  Males that defend territories with a high density of fish may
> >all of their offspring eaten by those fish; thus that territory is not
> >suitable for oviposition.  While getting eaten is probably not adaptive
> >the male), if males are abundant then a few males getting eaten at
> >territories full of fish probably won't hurt the population in the long
> >Further, if the male is attacked immediately - but escapes - that may be
> >sign for him to move on to a site that would be more hospitable for his
> >offspring (or his mate while she is ovipositing).
Dennis Paulson replied:

> Dave, I would argue vigorously against this hypothesis, as it goes against
> natural selection on individuals as the driving force behind evolution.
> eaten male does not contribute his genotype to the next generation, thus
> strongly selecting against males that perform such behavior.  It doesn't
> matter whether the female benefits or not, there can't be selection for
> such behavior (except in the context of the Handicap Principle).  Also,
> from my observations (we used to feed the sunfish in our pond), I would
> that few insects that are struck by a fish at the surface escape, so it's
> very large risk.

I don't think it necessarily goes against individual selection - if the
predation is very strong, evolution might favor males who take this chance
and test the waters.  Males who don't test the water run the risk of losing
their mate (and thus, their sperm) or their offspring to predation.  And, of
course, for those species that oviposit in tandem, it could be that it is
safer for the male to test the water when he is unencumbered and more able
to escape, as opposed to when he has the female in tow. Over time, evolution
would favor males who tested the waters (and survived).  Now, of course,
this risky behavior would only be selected for if the effects of predation
are high (and vary between suitable oviposition sites) or if the behavior
isn't as risky as it first appears to us.  Please note the "ifs" and
"mights" - this is purely a theoretical construct.

Also, if this is what the males are doing, it might be bringing us full
circle to the idea of a display, and Carl Cook's recent post really seems to
suggest a display of some sort. The key here would be to find out if the
male does this as he establishes a territory (possibly when no females are
present), or if he does it only in the presence of females.  And, what if
there is a group of males?  Is there some contest to see who tests the water
first?  As Dennis points out, we probably spend too much time collecting and
cataloging and not enough just watching.

Again, Dennis Paulson replied:

> Just to add some natural history to my argument, it may be very different
> fish that attack floating insects and benthic insects.  Trout are
> as surface feeders but probably aren't especially effective predators
> against calopterygid larvae, which are very cryptic and nestled among
> vegetation and rootlets.  Mark McPeek has stated that trout don't seem to
> place the same predator pressure on damselfly larvae as the much more
> effective centrarchids (bass and sunfish).

But the bass and the sunfish are both pretty good at picking insects off the
surface, so some fish at least are attacking both floating and benthic
insects.  I agree though, that fish predation in the microhabitats inhabited
by calopterygid larvae is probably low.  Also, given the mobility of fish, a
stream habitat that is fish free one minute could be full of fish a moment

> Your second hypothesis, about reflection fighting, is interesting.  I've
> often wondered if odonates saw their reflections on the surface.  But most
> of them are differently colored above and below, so a view of their
> underside might not stimulate interaction.

I wonder what role the iridescence pattern of the calopterygids plays in

One final thought - mayfly subimagos have a hydrofuge surface to prevent
them from getting trapped on the water surface.  The wings and body both
repel water.  Apparently, this waterproof surface encumbers flight to some
degree (although flight might also be encumbered by the mere presence of the
imago stage right under the subimago skin).  Subs are thus quick to escape
the water, fly to the shore, and molt to the more aerodynamic adult stage.
Now they are free to disperse long distances, or, in the case of males,
pursue females more easily.  Interestingly, adult mayflies which oviposit in
turbulent water may have extensive patches of hydrofuge skin.  Presumably
this keeps them from getting stuck in the water if they misjudge a  wave.
Mayfly adults without this feature end up spread on the surface of the water
(which is perfectly adaptive for females who use the "crash and burn"
oviposition method).

Are Calopterygid wings hydrofuge?  I just dunked one and it didn't appear
so, but it's hard to say with a specimen which has been in ethanol as long
as this one has been.


Status: N