Developmental period, and the shape of the curve representing it in stored products beetles

Abstract

In order to investigate the shape of the “emergence” curve that represents the distribution of the developmental period of stored products beetles, about 800 Tribolium castaneum were bred from hatching to pupation at 22·5°C and 70 per cent r.h. with wheatfeed as food. The sex and weight as well as the developmental period were recorded for each freshly formed pupa and the resultant emergence curve proved to be bimodal. The weights were therefore plotted against developmental period as a scatter diagram for each sex. A cumulative curve in order of weight for each sex for each day of emergence was drawn on arithmetic probability paper and when this curve was not a straight line the positions of all clear changes of slope were chosen subjectively and marked on the scatter diagram. By this means the population was resolved into two fractions for each sex of about 320 individuals growing quickly and 80 growing more slowly. Within each group weight was negatively correlated with developmental period but the individuals in the fraction of the population that pupated first were lighter than the others and females were heavier than males. For both fractions of the population for both sexes γ- and log-normal curves were computed and found to fit adequately. γ-curves computed for a homogeneous set of published values for the egg period of Tribolium confusum also fitted well. An emergence curve for the developmental cycle of Sitophilus granarius with a long tail bearing a small hump was arbitrarily split into two and satisfactory γ-curves fitted to both fractions.