[Odonata-l] sexual dimorphism

[Dennis Paulson]

This just occurred to me to add to the message I just sent about  
odonate mating behavior.

I mentioned species that mate away from the water, and this may be  
the norm in at least some species, while others seem likely to mate  
either at or away from the water.

I also wrote about females choosing oviposition sites, but in fact, a  
tremendous number of damselflies and some anisopterans oviposit in  
tandem, so it is the MALE that chooses the oviposition site. I assume  
many of you have watched Sympetrum (meadowhawks, darters)  
ovipositing, and it is quite evident that the male not only takes the  
female to the appropriate site but also takes an active part in  
swinging the female downward as she drops her eggs. This must be  
hardwired, as I've seen males that lost their mates continue to make  
ovipositing movements.

So tandem oviposition probably explains mating away from water! If  
the female is not doing the choosing, it doesn't matter if she sees  
the oviposition sites, and there is no reason to come to the water  
until she is mated.

And in fact, those genera that often mate away from water (Enallagma  
and Sympetrum as two examples) also oviposit in tandem. It would be  
interesting to see if there is a significant (or total?) correlation  
between these two characteristics. In my head, there is, especially  
as I think of the libellulids that oviposit in tandem. They are all  
genera in which you see many mature individuals away from the water,  
for example Celithemis and Macrodiplax (pennants) out in fields.  
Erythrodiplax berenice (Seaside Dragonlet) seems rather  
nonterritorial, and this is the only species of the genus that  
oviposits in tandem. Another hypothesis that emerges from this is  
that in species that mate away from the water, there ought to be good  
species-specific color patterns. I wonder if tandem-ovipositing  
libellulids may have a higher proportion of species with colored wing  
markings, for example, and that fits for Celithemis.

Probably Philip Corbet has a discussion of this in his book, and I'm  
too lazy to look it up, but it is more fun to make one's own  
deductions than to read about someone else's!
-----
Dennis Paulson
1724 NE 98 St.
Seattle, WA 98115
206-528-1382
dennispaulson at comcast.net



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