An air puff stimulus to the cerci of a cricket (Gryllus bimaculatus) evokes flying when it is suspended in air, while the same stimulus evokes swimming when it is placed on the water surface. After bilateral dissection of the connectives between the suboesophageal and the prothoracic ganglia or between the brain and the suboesophageal ganglion, the air puff stimulus evokes flying even when the operated cricket is placed on the water surface. A touch stimulus on the body surface of crickets placed on the water surface elicits only flying when the connectives between suboesophageal and prothoracic ganglia are dissected, while the same stimulus elicits either swimming or flying when the connectives between the brain and the suboesophageal ganglion are dissected. These results suggest that certain neurons running through the ventral nerve cords between the brain and the suboesophageal ganglion or between the suboesophageal and the prothoracic ganglia play important but different roles in the initiation and/or switching of swimming and flying. In the suboesophageal ganglion, we physiologically and morphologically identified four types of "swimming initiating neurons". Depolarization of any one of these neurons resulted in synchronized activities of paired legs with a similar temporal sequence to that observed during swimming.