A Weibel reticule (Multipurpose test system M-168) was used to calculate the
volumetric fractions of the different cell types in the digestive tubes. Two sections cut
from different layers, and five randomly selected areas per section were counted for each
individual (i.e. 10 measurements per individual). The volumetric fractions of digestive
(VF ) and basophilic (VF ) cells were determined. In addition, we determined the D B
volumetric fraction corresponding to apical cell fragments detached from digestive cells
of disintegrating tubules (VF ).