[Odonata-l] FW: nymph update and other stuff

[Dennis Paulson]

Linda and Bob,

This is a great question. Philip Corbet, in his 1999 book, has some  
information on this. It sounds as if eggs typically hatch when  
inundated, but there doesn't seem to be a lot of information species  
by species.

It seems to me that because the prolarva, by its flipping behavior,  
is adapted to getting the larva into the water if it's not already  
there, eggs do indeed hatch even if they are in standing vegetation.  
Corbet writes about unsubmerged eggs hatching but also about getting  
wet as a stimulus for hatching. He notes that the egg needs to  
respond to the presence of water but that rainfall shouldn't be an  
inappropriate trigger of hatching. And the question of whether the  
eggs of all exophytically ovipositing species need to be moist to  
hatch is far from answered.

Spreadwings (Lestes spp.) oviposit well up in the leaves of emergent  
aquatic vegetation over dry pond basins at numerous Washington  
locations, and when I see those places in spring, filled with water,  
that vegetation is still standing (Red-winged Blackbirds would have a  
tough time finding nest sites if it wasn't). Then new growth comes up  
alongside it, and there is a mixture of both for a while, until last  
year's growth finally dies back and falls into the water. This can  
take a month or more, mostly happening in May in some areas.

In these situations, I have assumed the eggs will hatch while the  
vegetation is still above water level so that the larva can begin its  
growth in early spring as the water warms, but I have no proof of  
that. I also wonder if even in leaves/stems that seem dry, there is  
moisture inside because it is wicked up from the water. Another  
alternative is that the eggs indeed hatch only after the vegetation  
becomes submerged, in which case there would be a tremendous  
variation in hatching time even in the same population, as there is  
much variation in when individual leaves become submerged. The third  
alternative is that most of the vegetation will have become submerged  
by the time the water warms sufficiently for most hatching to take  
place on average, and that the two are more or less synchronized over  
evolutionary time. Much more information is needed about this in  
species that oviposit above the water level.

This is an important phenomenon to understand, and anyone with an  
interest in it could relatively easily perform experiments to test  
these hypotheses. Linda, it sounds as if you already have some idea  
of what is going on with your Aeshna.

Dennis

On Mar 21, 2007, at 7:50 AM, Bob Glotzhober wrote:
> I am forwarding to the List Server a note from Linda Gilbert, along  
> with some questions. Linda is an interpretive naturalist with the  
> Geauga County Park District in northeastern Ohio and has really  
> gotten into dragonflies in the last couple of years. Linda has  
> hatched out some Aeshna eggs and reared them for 60 days so far.  
> She will not be there, but Larry Rosche will be sharing her images  
> at our Ohio Odonata Society annual meeting in Findlay, Ohio this  
> Saturday. She has some neat pictures.
>
> Her questions have to do with eggs laid in cattail stems that do  
> not get pushed down into the water over winter. If you have any  
> insights or facts to share, please be sure to send them both to her  
> directly (she is not on the list server), as well as to the list  
> server.
>
> Bob
>
>
> From: Linda Gilbert [mailto:lkg38 at adelphia.net]
> Sent: Thursday, March 15, 2007 9:34 PM
> To: Undisclosed-Recipient:;
> Subject: nymph update and other stuff
>
> Greetings everyone-
>
> The nymphs are fine.  They are in their 8th instar and a little  
> over 6mm long.  60 days old.  So far so good.
>
> On another related matter, I decided to check the clump of rush  
> stems from which I collected the dragon eggs last August.  The  
> stems have been exposed to the elements and burried under a foot or  
> more of snow for the past couple months.  Most of the snow has  
> melted with this last milder spell of weather, and I was interested  
> to see if the remaining stems that contained eggs had actually  
> fallen into the water.  I relocated one stem that had fallen over  
> but was not submerged and took it in to look at under the scope.   
> It was a surprise to see that at least one egg showed signs of  
> development.  An interesting thing I observed about this particular  
> stem was that it was moist inside--at least in the areas  
> surrounding the eggs where stems had been peeled back when the eggs  
> were laid.  The sections of the stem that did not have peelings was  
> more dry, though the inside stem tissue was still sort of spongy.   
> So moisture must be the (or one of the)  requirement for  
> development to begin whether or not the stem is actually  
> submerged.  What happens if the stems don't fall in the water--do  
> the eggs develop and hatch anyway as long as they are moist and the  
> nymphs crawl out and drop???  Do we know?  Attached is a powerpoint  
> slide with pictures of the clump and the embedded egg.
>
> Cheers,
>
> Linda
>
> <RushClump.ppt>
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-----
Dennis Paulson
1724 NE 98 St.
Seattle, WA 98115
206-528-1382
dennispaulson at comcast.net



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