What can the Euglena detect with it's eye spot?
The eyespot of Euglena detects light.
The eyespot is not an eye, but it reacts to changes in the brightness of a light source. The Euglena can carry out photosynthesis to prepare food for itself. The eyespot helps it to move towards light, the source of energy for photosynthesis.
The eyespot lies at the front end of the organism and appears to be reddish/brownish in colour. The Euglena turns to where the eyespot detects the smallest amount of light.