THE PILOT REPORTED THAT HE HAD OVERFLOWN THE FROZEN LAKE TO DETERMINE THE CONDITION OF THE ICE AND TO LOOK FOR ANY CRACKS IN THE ICE PRIOR TO LANDING. NO CRACKS WERE SEEN AND THE AIRPLANE LANDED WITHOUT INCIDENT. WHILE TAXIING FOR TAKEOFF, THE AIRPLANE'S LEFT MAIN LANDING GEAR DROPPED INTO A CRACK ON THE ICE AND THE LANDING GEAR SEPARATED. THE AIRPLANE'S SKIS WERE NOT INSTALLED.