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As shown below, positive correlations between forceps length and body size are common in earwigs. Intraspecific (or static) allometry is usually described in terms of an allometric slope (b), based on Etoposide the equation Y?=?aXb, or log(Y)?=?b log(X)?+?log(a), where Y and X are indices of trait size and body size, respectively. Isometry (b?=?1) occurs when the ratio of a trait to body size remains constant across the range of body size. Positive (hyper-) allometry occurs when larger individuals have relatively larger traits (b?>?1). To avoid underestimation of the true allometric slope (b), use of geometric (Model?II) regression such as the reduced major axis (RMA) is recommended instead of ordinary least-squares (OLS) regression (Green 1999). Sexually selected characters used in precopulatory processes, such as exaggerated and bizarre structures used in combat or display, are usually characterized by positive allometry (Petrie 1988, 1992; Green 1992; Emlen & Nijhout 2000). However, sexual selection itself does not necessarily cause positive allometry in the selected trait, and positive allometry can evolve for reasons other than sexual selection (Bonduriansky & Day 2003; Bonduriansky 2007; Bertin & Fairbairn 2007). Using a simple allocation trade-off model, Bonduriansky and Day (2003) demonstrated that various types of allometric relationships, from negative ones (b? of selection that are working on body and trait sizes. Their analysis specified the condition for evolution of positive allometry as a marginal fitness gain from an increase in relative trait size that is greater for larger than smaller individuals. For 42 earwig species belonging to Apachyidae, Labiduridae, Spongiphoridae, Chelisochidae and Forficulidae, Simmons and Tomkins (1996) examined allometry in male forceps and elytra (or tegmina; forewings), which are considered to be sexually and a naturally selected traits, respectively, on body size (pronotum width). For species with dimorphic male forceps, only brachylabic morphs (males with shorter forceps) were measured and analyzed. Their analysis revealed that the slope was usually larger for forceps allometry than for elytra. In addition, species with more exaggerated male forceps tended to have a larger allometric slope for the forceps. This finding remained true after considering the phylogenetic relationships among the species (based on Sakai's 1982 system). Although the slope showed high variation, among the 42 species examined, only four species had negative slopes (b?