Implications of tick size on the quantifications of engorgement in female Dermacentor variabilis

Log-log plots of unfed and fed tick weight on corresponding values of the scutal index (SI = scutal length x scutal width) show similar slopes for female D. variabilis from two laboratory populations. As derived from these plots, the relative engorgement state of a tick at any time during feeding can be expressed as the ratio of the cube roote of weight to the square root of the scutal index (RES = W1/3 divided by SI 1/2), where SI serves as an independent measure of tick size. After feeding for specified periods, mean population fed-weights are not statistically different, but differences between mean population RES ratios are highly significant. When SI is used as a covariate, differences between mean population fed-weights are also highly significant. Tests of mean log-fed-weight and mean RES values show that the effects of individual rabbit hosts on feeding performance are highly significant, but the statistic associated with differences between RES means is higher than that between log-fed-weight means. We conclude that differences in tick engorgement behaviors are measured more sensitively by RES ratios than by engorgement weights.