Exoskeletal chitin scales isometrically with body size in terrestrial insects

Abstract

The skeletal system of animals provides the support for a variety of activities and functions. For animals such as mammals, which have endoskeletons, research has shown that skeletal investment (mass) scales with body mass to the 1.1 power. In this study, we ask how exoskeletal investment in insects scales with body mass. We measured the body mass and mass of exoskeletal chitin of 551 adult terrestrial insects of 245 species, with dry masses ranging from 0.0001 to 2.41 g (0.0002-6.13 g wet mass) to assess the allometry of exoskeletal investment. Our results showed that exoskeletal chitin mass scales isometrically with dry body mass across the Insecta as M(chitin) = a M(dry) (b), where b = 1.03 +/- 0.04, indicating that both large and small terrestrial insects allocate a similar fraction of their body mass to chitin. This isometric chitin-scaling relationship was also evident at the taxonomic level of order, for all insect orders except Coleoptera. We additionally found that the relative exoskeletal chitin investment, indexed by the coefficient, a, varies with insect life history and phylogeny. Exoskeletal chitin mass tends to be proportionally less and to increase at a lower rate with mass in flying than in nonflying insects (M(flying insect chitin) = -0.56 x M(dry) (0.97); M(nonflying insect chitin) = -0.55 x M(dry) (1.03)), and to vary with insect order. Isometric scaling (b = 1) of insect exoskeletal chitin suggests that the exoskeleton in insects scales differently than support structures of most other organisms, which have a positive allometry (b > 1) (e.g., vertebrate endoskeleton, tree secondary tissue). The isometric pattern that we document here additionally suggests that exoskeletal investment may not be the primary limit on insect body size.

Body size is not critical for criticalPO2 in scarabaeid and tenebrionid beetles

“…Investment in structural support may also contribute to an upper size limit for vertebrates, because endoskeleton scales hypermetrically with body mass (Prange et al, 1979), although it probably does not generally do so for insects, because exoskeletal chitin increases isometrically with body mass (M 1 ) (Lease and Wolf, 2010). Two exceptions, interestingly, are Coleoptera and Orthoptera, where exoskeletal chitin scaling is hypermetric (M 1.1 ; statistically distinguishable from M 1 for Coleoptera) (Lease and Wolf, 2010). Regardless of the exoskeleton's potential role in limiting beetle maximal body size, there is a large and growing body of evidence that insect body size is constrained by tracheal oxygen delivery .…”

Rediscovering and Reviving Old Observations and Explanations of Metabolic Scaling in Living Systems

“…For example, SC models, including DEB theory [85,156] are supported by the positive allometry of fat mass in mammals [17], and of 'dead wood' in plants [28,29], but they are contradicted by the isometry of fat and skeletal tissues in insects [157,197] (see also Section 2.2.1). Furthermore, a study designed to test SC effects showed that hypometric metabolic scaling is not caused by a positive allometry of either fat or exoskeletal materials in 15 species of carabid beetles [198].…”

Mutation of a Cuticular Protein,BmorCPR2, Alters Larval Body Shape and Adaptability in Silkworm,Bombyx mori

“…The chitin content in the new cuticle of dsBmorCPR2 individuals (with RNAi phenotype) was significantly lower than that of the control group (dsRed as a control) ( Figure 4G). The process of cuticle extension and increasing of body size require chitin as crude material (Merzendorfer and Zimoch 2003;Lease and Wolf 2010), and therefore simultaneously, larval molting is a critical period for the formation and growth of new cuticles. Accordingly, we propose that suppression of BmorCPR2 decreases the chitin content and level of BmorCPR2, resulting in limitation of remodeling and extension of the larval cuticle and weakened body cavity capacity.…”

“…In procuticles, chitin fibers are arranged in laminae in an antiparallel manner and superimpose each other, forming sheets of fibrils that are stacked in a helicoidal fashion, maintaining cuticle structure, elasticity, and stability (Bouligand 1965;Neville and Luke 1969;Moussian 2010). In terrestrial insects, the chitin content is positively correlated with body size, suggesting a close relationship with cuticle extension and expansion (Merzendorfer and Zimoch 2003;Lease and Wolf 2010).Cuticular proteins (CPs), the principal structural constituents of cuticle, are encoded by more than 100 genes in known insect genomes (Andersen et al 1995 The soft, flexible cuticle of Lepidoptera larvae not only bears pressure from the internal contents on the integument to stabilize the long cylindrical body, but is also conducive to movement and expanding feeding and survival areas (Carter and Locke 1993;Brackenbury 1997;Lin et al 2009). However, the mechanisms by which cuticular proteins participate in body-shape stability and adaptability of Lepidoptera is presently unclear.…”

“…In procuticles, chitin fibers are arranged in laminae in an antiparallel manner and superimpose each other, forming sheets of fibrils that are stacked in a helicoidal fashion, maintaining cuticle structure, elasticity, and stability (Bouligand 1965;Neville and Luke 1969;Moussian 2010). In terrestrial insects, the chitin content is positively correlated with body size, suggesting a close relationship with cuticle extension and expansion (Merzendorfer and Zimoch 2003;Lease and Wolf 2010).…”