How is earthworm behavior affected by external stimuli?"

Earthworms are very sensitive towards the odor.

They use it for locating the food in the soil. Because earthworms are dependent upon microorganism in the soil, they must be sensible for the presence of any food around them if they do not respond the odor then they may die.

Earthworms have the escape response so they quickly respond the external stimuli at the part they are stimulated. With the increasing frequency of stimuli at the same part they slowly decrese the response towards it.

They tend to move away from any stimulus be it light or mechanical.

Explanation:

The earthworm, upon encountering a stimulus that is mechanical in nature will turn its anterior end slightly away from the stimulus. Persistence of the stimulus may cause the worm to contract the longitudinal muscles of the opposite side to move the body around taking a new direction. And they also have the behaviour of tending to move away from any light stimulus.