DURING A WX BRIEFING AT 1433 CDT, THE PLT WAS ADVISED TO EXPECT 500 FT CEILINGS, 2 MI VISIBILITIES, RAIN SHOWERS & THUNDERSTORMS. HE THEN TOOK OFF FROM BISMARK, MO WITH NO FLT PLAN. REPORTEDLY, WX WAS ENCOUNTERED DURING THE FLT WHICH DETERIORATED AS HE PROCEEDED ALONG THE ROUTE. SUBSEQUENTLY, THE ACFT CRASHED INVERTED, IN A CORN FIELD NEAR RAYMOND, IL. A WITNESS SAID THAT HE SAW THE ACFT 'GOING OR SPINNING STRAIGHT DOWN; HOWEVER, HE DID NOT SEE THE ACTUAL CRASH. AN INVESTIGATION REVEALED THE THE OUTER 4-FT SECTION OF THE LEFT WING & THE LEFT AILERON HAD SEPARATED IN FLT. THERE WAS EVIDENCE THAT THEY HAD SEPARATED UPWARD & AFT, NO EVIDENCE OF FATIGUE OR CORROSION WAS FOUND IN THE FRACTURE AREA. THE RIGHT WING TIP HAD ALSO FAILED, BUT IT HAD NOT SEPARATED FROM THE ACFT. AN NTSB WX SUMMARY SHOWED THAT THE CLOUD BASES WERE AT APPX 700 FT AGL & THE CLOUD TOPS WERE AT ABOUT 14,000 FT MSL. ABOUT 50 MI NORTHEAST AT DECATUR, IL, THERE WAS A 2000 FT OVERCAST. THE ACFT WAS NOT EQUIPPED WITH CERTIFIED TURN & BANK OR ATTITUDE INDICATOR.