Butterflies need warmth

Butterflies and moths are cold-blooded, like all other insects. That is, their body temperature follows that of their surroundings. When it is cold (below 12°), they are sluggish and find it hard to fly. When it is warm, they are active. By basking in the sun, butterflies can reach a temperature higher than that of the air.

On an overcast day, body and air temperature are the same.

Moths, lacking sunshine, can shiver their wings for several minutes and so build up the body temperature they need.