This has happened twice to my S. purpurea also, and
on both occasions I've discovered that it was because
I'd let the temperature go above 35 degrees C. Above
this temperature you basically roast the tops off of
them. It's a good idea to keep an electronic
thermometer in the terrarium which has memories for
the minimum and maximum temperatures reached during
the day. This way you can easily check if this was
what happened. Radio Shack has a very good electronic
thermometer/hygrometer with minimum and maximum memories
for both variables. It costs about $20.00.
Gordon Wells
Instituto de Cibernetica
Diagonal 647, planta 2
Barcelona 08080
SPAIN