Whilst the geographical range of species is a fundamental unit of macroecology and a leading predictor of extinction risk, the evolutionary dynamics of species’ ranges remain poorly understood. In some aquatic beetles, thermal niche has been shown to be related to both the relative range size and position of congeners but whether other physiological niche parameters play a role is unknown. Here, immunocompetence and metabolic plasticity were related to biogeography in these insects. Immunocompetence was first compared within a rare-common pair of Hydroporus species, finding species-specific immunity, which was affected by sex and acclimation time in the laboratory, with no clear relationship with rarity. This relationship was explored further in Deronectes species, whilst controlling for sex and acclimation time. Southern, narrow-ranging species had higher phenoloxidase (PO) activity, lower parasite load and antimicrobial peptide (AMP) activity that was stronger against Gram-negative Bacteria but weaker against Gram-positive Bacteria than their wider-ranging counterparts. As both of these studies found that PO activity did not positively correlate with encapsulation or AMP activity as reported in the literature, the pathway was investigated further in Tribolium castaneum. The data showed that the assumptions of the widely-used PO assay were violated, with differential activity between PO-driven reactions in melanin synthesis and different substrates used by larvae and adults. Future work should be wary of using the PO assay as a marker of potential melanin production and take into account the developmental requirements for melanin at different life stages. The relationship between metabolic plasticity and range was then assessed in Deronectes, finding that southerly species had more marked changes in glucose and protein content under elevated temperature stress than their northerly counterparts. Glucose content was also significantly positively correlated to lipid content, indicating that the energetics of species exhibiting differing range sizes may warrant future study. As the results from Hydroporus suggested that there may be trade-offs between immune defence and metabolism, the data on metabolic plasticity and immunocompetence in a sub-sample of Deronectes species were combined with thermal physiology, dispersal ability, body size and phylogenetic relatedness to assess which of these best explained variation in range size and position. Whilst variation in range extent and position were explained in part by thermal physiology, aspects of metabolic plasticity and immunocompetence also appeared to be important. This thesis provides one of the first indications that immunocompetence and metabolic plasticity may be related to geographical range and suggests parameters that may be worthy of exploration in other taxa.