why is it that insect are sensitive to light?

Answer 1

If you look really close at the heads of some bugs (I know, there ugly), you can see three little dots between their eyes. Those are simple eyes that are only sensitive to the presence or absense of light. Without them, many insects, such as flies and grasshoppers, will not move at all.

In addition, many nocturnal insects use the moonlight for navigation. When they encounter a bright light, they think that it's the moon and start "correcting" their course constantly as the "moon" seems to move.

Finally, some bugs just use lightbulbs to stay warm. That's why ladybugs will often crowd into cheap school lights during the winter.

Answer 2

Light heats things up. If you give an insect enough light, the heat will kill them.

Additionally, insects have eyes, which detect light. If you shine light on an insect from a certain direction, they may mistake it for the Sun, and try to orient themselves relative to it. This is why bugs get confused around lights at night.