Toxicity and Permeability: II. The Entrance of Arsenic into Larvae of the Flesh Fly, Sarcophaga securifera, Vill

Abstract

Larvae of this fly were used to study the rate of penetration of trivalent as in aqueous soln. through insect integument at pH 5 to 11.5, within which the solute changes from undissociated arsenious acid to highly dissociated sodium arsenite. The rate of penetration is highest in young larvae, decreasing to a steady value with larvae 100-180 hrs. old and rising somewhat at pupation. The rate was not appreciably affected by exposure up to 24 hrs., except at the highest pH, and was directly proportional to the concn. Fat solvents either applied first or included in the arsenic soln. increased the rate of its penetration. No detectable difference was found in this pH range between larvae ligated at both ends and detached integuments. Hence no "living" effect occurred. If larvae were ligated only at the head end, excretion occurred during exposure and continued until an extremely small amt. of arsenic was left in pupae from surviving larvae. To study the correlation of rate of penetration with concn. of molecular arsenious acid, both were taken as 100% at pH 5. A plot of these 2 quantities versus pH gave the ordinary sigmoid titration curve, which was relatively low over the neutral range, was in agreement with the titration curve at about pH 10 and was relatively high in stronger alkali. It is concluded that rate of penetration would be proportional to concn. of non-ionized arsenious acid, except for alteration of the lipid layer of the integument in the presence of fairly strong alkali and some unknown change in the cuticle, probably in the protein constituents, occurring near neutrality. Rate of penetration is less well correlated with relative solubility over the pH range concerned, but arsenite does not appear to be a good toxicant for use in choosing between the 2 hypotheses.