I have been wondering why would insects in general, not be scared when important changes happen on your monitor where they have landed...

If you move even just a finger, they are scared to death and fly away, but anything on my monitor and they remain completely placid.

  • $\begingroup$ Ha ha interesting question. Maybe 0.5 second of no change in graphics on your monitor is enough for a fly/mosquito/other to infer that the thing is not moving. And once they landed they may not see what under their feet and can't detect further movement. Or they might just understand that the thing they're walking on is not moving and therefore whatever they see, they grasp there is no danger. Also are you sure that the insects that land on your monitor are those that are really scared of movement such as mosquitoes for example. $\endgroup$
    – Remi.b
    May 25, 2014 at 8:00
  • $\begingroup$ Today it was a large fly. I've mainly got flies, but saw some other insects there too. Note that I, on purpose, would say... move a window under the insect and saw absolutely no reaction. So I can say that there was rather heavy movement. $\endgroup$ May 25, 2014 at 10:05
  • 1
    $\begingroup$ Hypothesis: Flies can resolve each individual frame and recognize that the screen is just changing color/don't notice that some of the flashing is changing colors. Hypothesis 2: The fly isn't looking at your finger but feeling your finger move with pressure senses. $\endgroup$
    – Resonating
    Jan 31, 2015 at 10:57
  • $\begingroup$ Hmmm... my finger is on the mouse, the fly is on the screen, so I'm not too sure about hypothesis 2... $\endgroup$ Jan 31, 2015 at 12:26
  • $\begingroup$ My guess: A fly's eyes are probably half a centimeter away from the screen, which means that they see pretty much all of the screen from a 90 degree angle. Try looking at your screen in a 90 degree angle and you wont see any changes either. $\endgroup$
    – ChrKoenig
    Jan 31, 2015 at 12:39

2 Answers 2


Flies see motion. When the monitor screen changes, the visible spectrum of the pixel is changing and nothing is moving.

Flies have limited color vision. Each color has its own wave frequency, but flies have only two kinds of color receptor cells. This means they have trouble distinguishing between colors, for instance discerning between yellow and white. Insects cannot see the color red, which is the lowest color frequency humans can see. However, houseflies have the ability to see polarized light, but humans cannot differentiate between polarized and unpolarized light. Polarized light is light in which the waves travel only in one plane. [1]

Monitors have polarized film on them so you could test the color hypothesis by taking an old monitor you are willing to sacrifice and removing the polarized film. Then you could test if the flies jump when the screen changes. See here for removing the polarized film on the monitor, or if you have access, this article on A behavioural study of polarization vision in the fly, Musca domestica may hold the answer without needing to ruining a monitor.

  • 2
    $\begingroup$ I can't reconcile your answer with the well-studied optomotor response of fruit flies (e.g., ncbi.nlm.nih.gov/pmc/articles/PMC2846117), where in flies are tethered in place and their visual field is changed via LED screens, simulating movement. Photo here: depts.washington.edu/flyarama $\endgroup$
    – kmm
    Feb 2, 2015 at 1:36
  • $\begingroup$ @kmm here it discussed the flicker affect. That is, insects with compound eyes respond better to moving stimuli then stationary. I am about to eat a cupcake so I will read your link in a bit. $\endgroup$
    – dustin
    Feb 2, 2015 at 1:39
  • $\begingroup$ That's what I can't figure out. The OP describes things moving on the screen, which should activate the flicker effect. $\endgroup$
    – kmm
    Feb 2, 2015 at 1:42
  • 1
    $\begingroup$ @kmm movement on the screen isn't 3D movement since nothing actually moves. What is happening is the pixels are changing colors. Since flies cannot distinguish nearly as many colors, they won't perceive the change. However, if I wrapped my hand in red (which is hardest for them to see) and moved it towards them, the movement will be picked up since their is depth perception amongst other characteristics in play as well as background colors being removed from view by my movement. $\endgroup$
    – dustin
    Feb 2, 2015 at 1:46
  • 2
    $\begingroup$ @kmm did you read LED Arena and Visual Behavior Assay section at the end? They used UV, blue, and green light. Flies can see UV light. Your computer screen has polarizating film blocking UV. So the experiment seems to be geared to benefit the limited visual ability of the fly. Also, they were started by vibration before each experiment to heighten their visual senses. $\endgroup$
    – dustin
    Feb 2, 2015 at 1:53

Flies monitor the space above them:

Flies have a very strong innervation of the back of the head, which contacts the surface ( http://flybase.org/reports/FBim0000875.html ). While working in a fly lab, which studied behavior, I was told that these nerves sense changes in the light intensity above a fly - and it is this "third eye", which usually triggers the escape response of flies.


You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .