Growth and Duration of Larval Instars of the Cowpea Weevil, Callosobruchus quadrimaculatus

Abstract

Four geographical strains of the cowpea weevil Callosobruchus quadrimaculatus were reared under the constant condition of temperature and humidity. Head width of the larvae was measured in each successive day and the number of larval instar was determined as four. The growth ratio between two successive instars does not take a constant value and decreases gradually with advancing the instar. The growth of head width can be precisely described by the formula, logY=a+bx+cX² (Y is the measurement of each successive instar, X and a, b and c are the constants) which was intended of its validity by GAINES and CAMPBELL. The strain of the largest size of adult weevil is smallest in its size at the hatching of the egg, and vice versa. This strain shows also high rate of initial growth of larvae and low rate in its later stages of larvae. Namely, he compensating relation is shown between the initial size of larvae and their growth rate. This relation can also be seen in the numerical relations between the constants a, b and c in the formula of GAINES and CAMPBELL. As shown in Fig. 2, the constant a, which is the index of the size of embryo, is inversely proportional to the growth ratio, b, which is also inversely proportional to c, the modifying factor of the growth in old stage. The duration of each larval stage was determined, but any clear relation could not be seen.