[NatureNS] Flies on the water - Hydrophorus

Date: Mon, 12 Sep 2011 14:33:41 -0300
From: "Stephen R. Shaw" <srshaw@Dal.Ca>
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Hi Paul,
Probably you had quite a few people bothered by your '4 cm' flies.

Small flies that scoot around on water are pretty much all  
Dolichopodidae, aka 'long-legged flies', though these particular ones  
are probably not all that long-legged.  Furthermore, within the  
dolichos, those associated with water surfaces are all (almost all?)  
in the subgroup Hydrophorinae, in the genus Hydrophorus.

We observed and caught a few such in October-November last year living  
in extended waterlogged deep ATV ruts carved alongside the Halifax end  
of the Old St Margarets Bay Road, now a hiking/ATV trail (about the  
only useful 'naturalistic' thing I have yet experienced to have come  
out of that mode of transportation).  The interest was that initially  
they seemed able to fly directly off the water surface, like you say.   
My colleague visiting from Cambridge at the time photographed one of  
them with his ultra-high speed camera while prodding it to take off,  
and even made a short video clip of this.  This seems to show that  
they don't fly off, they actually jump up off the surface before  
starting to fly, so it's a power jump.  This was sufficiently  
interesting to my bug-jump-expert colleague that we managed to get the  
species identified by the dolicho specialist in Ottawa, Scott Brooks.   
All dolichos are known or believed to be carnivorous, so these  
presumably feed on minute insects and other invertebrates living in  
the surface water film.

Most dolichos have a metallic green sheen somewhere on their bodies  
but ours didn't and were drab brownish-grey, and about the same size  
as yours. You say yours were black and I guess they may have looked  
black in certain light conditions.  If yours looked like ours on  
closer inspection, they might have been the same species, Hydrophorus  
alboflorens.

Steve, Halifax
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Quoting Paul MacDonald <paulrita2001@yahoo.com>:
> Hi All
> Sorry - Mistake!
> Flies were perhaps 4 mms in length not cms!
> Have a nice day
> Paul

> ________________________________
> From: Paul MacDonald <paulrita2001@yahoo.com>
> To: "naturens@chebucto.ns.ca" <naturens@chebucto.ns.ca>
> Sent: Friday, September 9, 2011 7:54:02 PM
> Subject: [NatureNS] Flies on the water
>
>
> Hi All
> This afternoon BB and I went paddling to see about a Ladies-Tresses bed.
> As we were getting in the canoe, I saw a flock of ducks feeding on  
> the surface
> of the lake. Looked like Mallard ducks but might have been a few Blacks also.
> I thought flies right away.
> When we got out in the lake, I realized there was plenty of small flies
> on the water - maybe up to 10 - 12 per square meter.
> The flies were just sitting on the surface tension, perhaps 4 cms in  
> length as a guess.
> They were all black and lively - didn't seem dopey. Would fly off if I tried
> to catch them. There were a few much larger ones - perhaps 1 in 1000 or
> even less.
> No fish were feeding on them. I know there are fish in the lake as I caught
> a couple but they didn't seem interested in the ones on the surface.
> In the lee of the Islands there was a big flock of them all gathered together
> out of the wind. I'm not certain they were the same species.
> I've seen these flies before but not often.
> Glad they didn't bite!
> Enjoy the last of the summer
> Paul


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