NSF Award
0344963
Collaborative Research: Discontinuous Gas Exchange in Insects<br/>Michael C. Quinlan and Allen G. Gibbs<br/>Midwestern University (Glendale) and the University of Arizona<br/><br/> This project examines the respiratory physiology of insects in an effort to better understand the enormous success of these organisms in terrestrial environments. A major challenge for any terrestrial organism is the retention of body water, and thus it is not surprising that insects and other terrestrial arthropods have a evolved a suite of adaptations that serve to reduce evaporative water loss. Behavioral strategies to minimize dehydration include nocturnal activity, exploitation of moist microhabitats and seasonal activity synchronized with periods of high moisture availability. Equally important are physiological adaptations that reduce water losses through the integument and respiratory system. Water loss through the arthropod integument (cuticle) is generally very low due to the presence of complex lipid mixtures that permeate and coat the chitinous cuticle. Respiratory water loss is less well understood in insects, in part because it is difficult to measure, and in part because it is highly variable, both within and among individuals. Recent research suggests that respiratory water loss can be an important determinant of survival in many insects, particularly those from xeric habitats. It is the intent of this project to study the interaction of respiratory behavior and respiratory water loss in a wide range of insects. <br/> Many terrestrial arthropods perform cyclic, discontinuous release of carbon dioxide through precise regulation of the valves (spiracles) that control access to their air-filled respiratory structures (tracheae). For over 50 years our understanding of gas exchange in these organisms has been dominated by the view that water balance was the primary selective force behind this discontinous gas exchange (DGE). Numerous recent studies, however, have questioned the applicability of the water conservation model to terrestrial arthropods in general, and several alternative hypotheses have been proposed. The chthonic hypothesis proposes that DGE arose as a means to increase the gradients for carbon dioxide release and oxygen uptake in burrows and other hypercapnic/hypoxic habitats. Another hypothesis holds that DGE acts to reduce internal oxygen levels and thereby minimize cellular damage from free radicals. A non-adaptive hypothesis views DGE as emerging from the simple regulatory mechanisms of insect ventilation. None of these hypotheses have been subjected to systematic investigation, and no experiments have been performed to distinguish between models in a rigorous manner. <br/>The present project will test these hypotheses by thoroughly analyzing ventilatory patterns in closely related insects from arid and mesic habitats, from burrowing, arboreal and surface habitats, and from different altitudes. This work will be performed in a rigorous phylogenetic context, using families of beetles from the New and Old World (Tenebrionidae, Carabidae, Scarabaeidae) and fruit flies (Drosophila), for which phylogenetic information is available. In addition to testing specific predictions of each model, critical tests will be performed to distinguish between models. For example, if the water conservation hypothesis is true, then DGE should be more evident in xeric-adapted organisms and DGE cycles should change in other predictable ways. If the chthonic hypothesis is true, then DGE should be more prevalent in fossorial taxa, as compared with arboreal forms. Under the oxidative damage hypothesis, high-altitude species should exhibit DGE less often than low-altitude species. In biochemical tests of these hypotheses, buffering capacities, carbonic anhydrase levels, and oxidative defense enzymes will be assessed.
