The Brooks‐Dyar Rule and Morphometrics of the Processionary Caterpillar Ochrogaster Zunifer Herrich‐Schäffer (Lepidoptera: Thaumetopoeidae)

Abstract

Various insect species display a uniform geometric increase in size during the larval stage (that is they follow the Brooks‐Dyar rule). Here, results of larval development are presented for the bunny‐tailed moth, Ochrogaster lunifer. The processionary larvae of this species live in a communal cohort, and moult en masse in the silken nest spun at the base of their host tree (usually a phyllodinous acacia). The exuviae, which remain buried in the accumulated silk and frass of the nest, provide a life history record of the larval cohort. Larval exuviae were collected from 773 cohorts at 37 localities in southeastern Queensland between November 1993 and May 1994. The 6,948 exuviae examined were from cohorts feeding on Acacia concurrens Pedley. Head‐capsules showed a strongly uniform geometric increase in size through eight larval instars, supporting the Brooks‐Dyar rule. The number of instars did not vary between trees or localities. A bimodal distribution of final instar head‐capsule widths was shown to be a sexual dimorphism, and similar bimodal distributions were found for instars V‐VII. Pupal size was also sexually dimorphic. The geometric size increase from one larval instar to the next holds for both males and the larger females. The geometric rule was tested using larval cohorts reared on A. concurrens in the greenhouse through instars I‐IV; development was remarkably similar to that in the field. Larval growth patterns of O. lunifer are very different from the structurally similar bag‐shelter moth. The ability to distinguish different instars of O. lunifer with a high degree of precision from field‐collected exuviae will allow accurate comparisons of development, survival and dispersal of larvae in different group sizes, on different trees and in different localities.