Intraspecific body size variation in insects

Gouws, Emeline Jeanne (2007-03)

Thesis (MSc)-- University of Stellenbosch, 2007.

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ENGLISH ABSTRACT: This thesis investigates several aspects of intraspecific variation in the body sizes of insects. More specifically, it concerns how body size is distributed within populations of a species and how body size within populations varies over space and time. The motivation for this study is the relative paucity of information in the literature pertaining to how body size varies at the intraspecific level for insects, and what mechanisms might underlie this variation. In particular, it is shown that although a lognormal frequency distribution in body size is expected at all taxanomic levels, there is great variation in these body size frequency distribution patterns at the intraspecific level for insects. This study also highlights the need to consider all possible factors that might influence the pattern of body size frequency distributions, including sexual size dimorphism of a species, how many size classes or bins are used in the distributions and the sample size. Furthermore, if a better understanding of these patterns is sought, especially with regard to the mechanisms underlying how body size of a population is distributed, factors affecting the life history, physiological and ecological responses of individuals in a population need to be considered. This is also the case for geographical variation in body size of insects. Altitudinal variation in insect body size therefore, is of particular interest, and here it was used as the basis for an investigation of the possible mechanisms underlying clinal patterns in body size. Variation was found in the patterns observed for the beetle species considered in this study. Although one species (Sternocara dentata) did ' not vary significantly in mean size along the altitudinal gradient, Thermophilum decemguttatum and Zophosis gracilicornis both showed a decrease in size with altititude, contrary to what is expected from the temperature-size rule for ectotherms. The responses in the body sizes of the latter two species to several environmental variables along the altitudinal gradient, including mean annual temperature and some vegetation variables indicated that the size variation is subject to the combined effects of temperature, resource availability and resource acquisition. This finding provides support for the resource allocation switching curve mechanism (one of several alternative mechanisms) thought to underlie clinal size variation.

AFRIKAANSE OPSOMMING: Hierdie tesis ondersoek 'n verskeidenheid eienskappe van intra-spesifieke variasie in liggaams groottes van insekte. Die ondersoek is spesifiek gemik op hoe liggaams grootte versprei is binne populasies van 'n spesie en hoe liggaams grootte sodoende varieer oor tyd en ruimte. Motivering vir hierdie studie gaan gepaard met die onvoldoende informasie beskikbaar oor hoe liggaams grootte varieer tussen individue van 'n spesie en ook watter meganismes hierdie variasies tot grondslag kan he. Alhoewel dit die verwagting is dat die liggaams grootte van alle taksonomiese vlakke 'n log-normale frekwensie verspreiding sat aanneem, is daar huidig 'n groat verskeidenheid patrone in die frekwensie verspreidings binne insek spesies. Hierdie studie beklemtoon dat daar 'n groat behoefte is daaraan om alle faktore wat moontlik die patrone van liggaams grootte frekwensie verspreidings in ag te neem. Hierdie faktore sluit in die seksuele dimorfisme in grootte, die hoeveelheid grootte klasse wat gebruik word, asook die aantal monsters wat gebruik word. Om hierdie patrone beter te verstaan, veral met betrekking tot die meganismes wat die liggaams grootte verspreiding van 'n spesie populasie veroorsaak, is dit nodig om faktore wat die lewens geskiedenis, fisiologie en ekologiese reaksies van individue in ag te neem. Dit is ook die geval vir geografiese variasie in die liggaam grootte van insekte. Daarom is die variasie in insek liggaams grootte met hoogte bo seespieel veral van belang en was hier gebruik in 'n ondersoek tot die meganismes wat geografiese variasie in liggaams grootte moontlik maak. Daar was weereens variasie teenwoordig tussen die patrone in liggaams grootte van die drie kewer spesies wat ondersoek is. Alhoewel een spesie glad nie betekenisvol gevarieer het met hoogte bo seespieel nie (Stenocara dentata) , het Thermophilum decemguttatum en Zophosis gracilicornis albei verklein in liggaams grootte met hoogte bo seespieel. Die reaksies in liggaams grootte in die laasgenoemde twee spesies tot 'n verskeidenheid omgewings veranderlikes tangs die hoogte gradient, soos byvoorbeeld gemiddelde jaarlikse temperature en sommige vegetasie veranderlikes, het daarop gedui dat die variasie in liggaams grootte moontlik onderworpe was aan die gekombineerde effekte van temperatuur, hulpbron beskikbaarheid en hulpbron verwerfing.

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