Unbuffered solutions (about 0.1%) of the indicator dyes Bromothymol Blue or Bromocresol Purple were prepared in 10% sucrose solution. The Bromothymol Blue and Bromocresol Purple solutions were adjusted to pH 7.0 and 6.5,respectively, and immediately offered, soaked onto pieces of cotton, to recently blood-fed females. Twenty hours later, the females were dissected and the colours in the thoracic midgut (TM) were compared with standard solutions prepared with the same dyes at different pH values covering 0.5 unit intervals