MadSci Network: General Biology

Re: In the middle Why are there thousands of flies in mid Lake Ontario?

Date: Mon Apr 20 22:27:44 1998
Posted By: Neala MacDonald, Grad Student, MSc in Zoology, University of Western Ontario
Area of science: General Biology
ID: 891613870.Gb

A very unique observation about the flies in the middle of the the 
Great Lakes. I am not too sure of your identification of the species of fly 
- because it sounds like you are describing a larval aquatic fly such as a 
certain 'midges'. This would explain why they occur in swarms, and many 
kilometres away from a solid land mass. These flying insects likely emerge 
from their aquatic life stage when the conditions (temperature, time since 
hatching, etc.) are right, which is not always in what would look to be an 
ideal location. But many species of insect can rest on the surface of the 
water - partly because of their tiny mass and partly because they may have 
body and leg hairs that increase their surface area and make it more likely 
that they can make use of the natural surface tension of the water. 
The apparent lack of food in the middle of a lake may not be an issue for 
this species - as the adult form of many insects do not feed (many don't 
even have mouth parts) - the sole purpose of this lifestage may be to 
I am curious to know if these insects are biting the people in your 
sailboat. I would suspect that they are not (which confirms the lifestage 
hypothesis), and if they are only interested in reproduction these insects 
may be using you and your boat as a reference point for the swarm. Several 
species of insect use a strategy called 'hill-topping' where they 
congregate together on the tallest object around. This helps form a mating 
swarm (which increases the likelihood of any individual meeting up with a 
member of the opposite sex - especially given a space as vast as an open 
lake). And this would explain the behaviour of the swarm if it seems to be 
following your sailboat on a fairly calm day. Very interesting stuff...
Keep up the astute observations!
Compiled with the expertise of Rod Hallum, Entomology Technician, 
University of Western Ontario, London ON.
-Neala MacDonald 

Genevieve Hanna wrote:

> Hi Neala
> Thank-you for the reply about the flies in the middle of lake Ontario.
> I would to add some points...
> Yes they do bite, and they look just like a common house fly but they
> are very slow and sluggish we can actually catch them with our bare
> hands.  In addition when we kill some we can observe other flies
> gathering around the dead fly and  I suspect they are feeding?
> I think you do wonderful work and thanks for your patience
> Yours truly,
> Genevieve Hannah

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