Acute toxicity of ammonia against larvae of Anopheles gambiae molecular forms. (A) Observed mortality (±95% confidence limits) after 12 hours exposure at increasing concentrations of ammonia (mg·L-1 total-N). The points and error bars corresponding to each molecular form are slightly shifted from each other for presentation purposes. Sigmoid curves represent the fitted minimal adequate regression lines plotted on the linear scale (red: S-form sites; blue: M-form sites), from which the lethal concentrations presented in Table 2 were calculated. (B) The same data of panel (A) drawn on the logit scale. The explanatory variable (i.e. ammonia concentration) of the minimal adequate logistic regression model for the S form (red curve) is log-transformed, hence the line plots as a curve because the abscissa on the graph is on the linear scale. The dotted red line represents the alternative model for the S-form data, with ammonia concentration expressed on the linear scale, as in the case of the M form (blue line); this regression line has been used for statistical inference purposes, to compare the functional response of the M form with respect to that of the S form (cf. Results). Diamonds at the bottom right of the scattergram indicate the estimated 95% lethal concentration (LC95 ± SE) for M-form sites (blue) and S-form sites (red). Data points for which observed mortality was 0% or 100% are not shown because they map to ± ∞ on the logit scale. (C) Probability density functions (PDF) of individual tolerance thresholds in populations of the two forms estimated from probit analysis (blue: M form, red: S form). Vertical dotted lines identify the ammonia concentration tolerance thresholds where the two PDF curves intersect; above or below these tolerance thresholds, the M population contains a greater proportion of individuals compared to the S form.