Regen began to study insects in great detail. He was one of the first Slovenian scientists to work in another country after World War I. He was the first scientist to study the grasshopper in a lot of detail. He studied katydid and cricket sounds, he proved that insects did react to sounds from each other. He was able to make crickets react to artificial sounds using a loudspeaker. In one experiment, he used a telephone to send sounds from one cricket to another in different rooms. Later, he showed that insects hear with their ear drum. He was the first person to work out what the ear drum does. He is the person who started the study of bioacoustics. He also studied the insects breathing, their hibernation, coloring, and how they shed their skin.