Polytene chromosome staining solution
Lactic acid (85% [w/w] syrup, Sigma L1250)
Orcein, powdered (Sigma O7380)
Mix three parts acetic acid, two parts lactic acid, and one part H2O. Add 1 g orcein per 50 mL solution. The final concentrations of acetic acid and orcein are 50% and 2%, respectively. Heat the solution (but do not boil) and filter. The staining solution will last many years if stored tightly capped.
