Linear Increment in Width of the Head Capsule of two Species of Sawflies

Abstract

Interest in the growth of insect head capsules was aroused by the observation of Dyar (2) that for a number of lepidopterous larvae the ratio of the width of the larval head capsule in a given instar to that in the following instar tends to be constant throughout development, This observation, generally known as “Dyar's Law” or “Dyar's Rule”, has proven valid for many lepidopterous larvae, for whch it was proposed. Many attempts have been made to apply Dyar's Rule to insects of other orders. Various authors, notably Miles (6) and Taylor (71, have considered the application of Dyar's Rule to sawfly larvae, and although they report some inconsistencies, they conclude that the rule is useful if its application is restricted to checking upon the number of feeding instars. The head capsules of non-feeding prcpupal sawfly larvae are generally of the same size as those of the last feeding instar, so that this stage is not expected to satisfy Dyar's Rule.