Empirical laws of growth have a wide application in entomology. According to Przibram's law the weight of larvae doubles and linear dimensions increase by 1.26 in succeeding instars. But growth in insects is allometric (Wigglesworth 1953) and application of Przibram's law to such growth is of little practical value. Dyar's law of linear growth holds that the head capsule of caterpillars grows in geometrical progression by a constant ratio for the species. However, the difficulty encountered in spotting a small head capsule, shed in the midst of food particles and silk spun by the larva, or even in holding a larva stationary for microscopical measurements, justifies resorting to an easier index than that of head-capsule size. Change in size of the many fecal pellets excreted in succeeding instars provides an easy and reliable index.
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