| Arthropods are incredibly successful, and represent most of the diversity on earth. They are one of the most abundant and most species-rich components of food webs, they inhabit land, sea, and air, and they range in distribution from tropical forests to extremely and deserts, and from deep ocean vents to high mountain peaks. Arthropods are a critical component of ecosystems, yet we know relatively little about their ecology and physiology. Understanding general patterns of arthropod structure and function is thus exceedingly valuable because it allows us to extend limited knowledge about individual taxonomic groups across an extremely speciose group. This dissertation documents the investigation of three different types of investment by arthropods: investment in support structures, investment in energy reserves, and investment in respiratory structures. I looked at each of these types of investment in the context of body size.;Differences in body size affect both processes and structures. The slope of the allometric equation can vary, so that Y can be an increasing (b>1), decreasing (b<1), or constant (b=1) function of body mass. That is, you can have greater, smaller, or the same investment in a trait, as body size increases. Most of our ideas of the patterns of how traits vary with body size are based on vertebrates (Calder 1984, Brown and West 2000, Peters 1983, Schmidt Nielson 1984), and there is comparatively very little data on invertebrates. In this dissertation, I strive to correct this discrepancy. I used an allometric construct to determine how body size affects arthropod investment in support structures (exoskeletal mass), energy reserves (lipid content), and respiratory volume (tracheal volume) in arthropods.;Several taxonomic levels were used to analyze how arthropod investment in these anatomical traits changes with respect to body size. Tracheal volume was analyzed at the species level (1 species from Class Insecta), exoskeletal investment was analyzed at the class level (15 orders, 91 families and 245 species from Class Insecta), and lipid content was analyzed at the phylum level (5 classes, 25 orders, 113 families and 324 species from Phylum Arthropoda).;Two major themes encompassed my approach to looking at the relationships between arthropod physical traits and body size. (1) How Do Traits Scale? Is there a single relationship that describes what happens to a trait as body size increases that can be generalized across many orders of magnitude of body size and many taxonomic groups? (2) How Do Traits Vary? What physiological and ecological characteristics of animals cause them to deviate from general arthropod scaling patterns ? An approach like this is valuable, because it extracts patterns---baselines for understanding how arthropods invest in structures and functions across a wide range of body sizes---and it highlights variation in these patterns.;Many anatomical and physiological characteristics of animals are directly related to body size. Measuring the variation in an animal trait over a wide range of body sizes produces allometric predictions, which have the mathematical form: Y = a Mb (where Y is a dependent variable, a is a normalization constant, M is body mass, and b is the scaling exponent). B, or the slope, is a particularly important variable because it indicates how investment in a trait changes as a function of body size.;In this dissertation, clear scaling patterns were indeed found that robustly describe the relationships between body size and arthropod investment in exoskeletal mass, lipid content, and tracheal volume. Physiological and ecological variation was also documented for each of these traits. Factors such as phylogeny, locomotion, gender, reproductive condition and developmental stage influenced the wide range of variation in the scaling patterns that were derived. I describe these scaling patterns and their variance in the subsequent chapters.;Abstract references. Brown, J.H. & West, G.B. (2000) Scaling in biology. Oxford University Press, New York. Calder, W.A. III (1984) Size, function, and life history. Harvard University Press, Cambridge. Peters, R.H. (1983) The ecological implications of body size. Cambridge University Press, Cambridge. Schmidt-Nielsen, K. (1984) Scaling: why is animal size so important? Cambridge University Press, New York. |
