Flies

[Henry55]

why don't flies fly in a straight line? And is there any reason why they fly in that erratic way.
Many thanks.

[Clanad]

On another site, the answer to this question is "...The reason is because flies have "compound eyes".

This type of eye cannot cope with a moving image, it can see only stationary things. Hence when a fly flies straight, it can see an image, but when it turns (changes direction) that image is moving relative to the fly's compound eye. This is then no longer a stationary image, maybe the fly would call it a blur.

Hence the fly flies in a series of short straight lines, during each short straight part of the flight the fly can see one image well.

If it notices something at the edge of its vision, and wants to take a proper look, it adjusts its flight path so that the desired image is more in the centre of its field of view, in this way the image appears stationary to the fly while it flies in this new direction".

[wildwood]

I find it somewhat hard to grasp that a fly's eyes can not cope with a moving image. It is very good at detecting my fly-swatter coming in at supersonic speed. Other insects with compound eyes manage to fly in direct lines.

My guess would be that as a fly's life-depending sense is smell, it weaves through the air covering a wider field where a possible pong from a dead mouse might be coming from.

[heathfield]

I reckon it's because they have lousy binocular vision, and have to move about to be able to judge the size and distance of stationary objects so they don't crash into them.

As to moving objects, well, there's Wildwood's swatter, (and my rolled-up newspaper), and I'm sure you've waved a fly or two away from a beer-glass in your time, Clanad!
And bees, with their compound eyes, can quite happily land on a flower that's waving in the breeze.

[Clanad]

What I have found to be true, Heathfield is that a fly on a flat surface can easily and consistently be killed by clapping your hands together just above him. He cannot process the movement of both hands from opposite directions. Additionally, flies tend to fly directly upward (and slightly rearward) as they lauch themselves into flight. Try it sometime (OK, if your squeamish about fly spatter on your hands, put on latex gloves)... works every time...

[Clanad]

Additionally, recent experiments in fly's flight and sight senses seem to support the first suppostion I posted. (In the scheme of things, I agree the importance of any of this is limited)... Check here for a brief discussion of the findings...

http://www.pubmedcentral.nih.gov/articlerender .fcgi?artid=1110911

[xud]

On the subject of a fly's vision, am I correct to assume that they don't have binocular vision.
Yes, they have two eyes like us, but are they able to focus in the same manner?
It seems to me that fly's, with their compound eyes can neither focus or gauge distance accurately. However, detecting movement is their speciality it would seem.
Why is it they are so difficult to swat? Wasps and Bees are easy, but little ol' flys....

[heathfield]

I was unaware of the two-handed approach, Clanad. I'll try it with two rolled-up newspapers next time! I did know about the up-and-back leap, and try to strike slightly behind them.
I also know that if they're on your screen door, and you strike from the other side, they don't see it coming!
Xud, maybe flies just have to move faster. Unlike wasps and bees, they don't have any defence mechanism.

[kwicky]

I always thought that flies detect the air pressure of a swatter. That is why they have holes in.

[heathfield]

I guess the holes are there since the flow of air from a compression wave in front of a swatter without holes in it would be likely to push the fly out of the way.