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Body Length (mm)

FIGURE 1 Allometric relationships between body part length and body length for P. albimanus (Diptera: Chironomidae) (Data from Ward and Cummins, 1978.)

immature insects prevents expansion and results in discontinuous growth of highly sclerotized cuticular parts (e.g., head capsule, legs). Thus, these structures increase in size incrementally in a stepwise manner following a molt. It is these distinct increases in the size of sclerotized structures that allows for the distinguishing of different instars with little overlap occurring between size classes.

The publication of Dyar's observations was in response to two previous papers that presented contradictory data concerning the number of molts in other species. Dyar studied the number of molts in 39 individuals of 28 species of caterpillars and chose the head capsule as the structure to measure for ease of measurement and because it was not subjected to growth during each stadium as was body length. The taxa chosen by Dyar ranged from 4 to 10 instars. Dyar calculated head-width ratios of successive instars and found that the progression was often nearly constant for a given species (mean = 1.5; range 1.3—1.7), what one would expect from a geometric progression. Dyar then calculated expected head capsule widths of each instar by multiplying the width of the final instar by this ratio and then back-calculating to the first instar. To test the applicability of the ratio, Dyar compared calculated head widths to those observed from reared specimens. Using this approach, it was possible to detect whether some instars had been missed or mismeasured. The most common method for detecting these problems is to plot the logarithm of the head capsule width measurement (or a measurement of another highly sclerotized structure) against the appropriate instar (Fig. 2). Conformity to Dyar's Law results in a straight line the slope of which is constant for a given species. According to Dyar's Law, deviations from a straight line indicate potentially missed instars or errors in measurement.

Although Dyar's Law has been widely used in entomological studies, the progression in the size of sclerotized body parts is not always constant and can be influenced by abiotic and biotic factors such as temperature and food. In addition, apparent contradictions to Dyar's Law occur when two requisite conditions are not met: (1) the number of instars is constant and (2) head capsule growth occurs only at ecdysis. Despite these constraints, approximately 80% of the entomological studies published from 1980 to 2000 that have examined the validity of Dyar's observations provided support for his law.

There has been some disagreement as to whether Dyar should be credited with the findings of geometric progression in growth. In a paper published 4 years prior to Dyar's article, Brooks reported that total larval length of a species of crustacean stomatopod (Stomatopoda: Squillidae) sequentially increased in size by a factor of 1.25 at each molt. Brooks also noted, as did Dyar, that this relationship could be used to determine whether larval stages were missing from the series. Thus, Dyar's Law may occasionally also be referred to as Brooks' Law (or Rule) in the literature. It is likely that entomologists were unaware of Brooks' observations because they were documented in a specialized publication on stomatopods.

II III IV V VI VII VIII IX X Instar

FIGURE 2 Conformity of head capsule width to Dyar's Law for giant swallowtail (Pa. cresphontes, Lepidoptera: Papilionidae) and banded woollybear (Py. isabella, Lepidoptera: Arctiidae) (Data from Dyar, 1890.)

II III IV V VI VII VIII IX X Instar

FIGURE 2 Conformity of head capsule width to Dyar's Law for giant swallowtail (Pa. cresphontes, Lepidoptera: Papilionidae) and banded woollybear (Py. isabella, Lepidoptera: Arctiidae) (Data from Dyar, 1890.)

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