Bimodal breathing in jumping spiders

double respiratory system consisting of book lungs and tube tracheae. The book lungs are the most important for gas exchange in many species and cope alone with the oxygen demands. The tracheae are often only poorly developed and are presumed to be of little importance in overall gas exchange (Anderson, 1970; Anderson and Prestwich, 1982; Strazny and Perry, 1984). In contrast, several species, e.g. the diurnal jumping spiders, have both book lungs and tracheae that are well developed (Schmitz and Perry, 2000), but nothing is known about the gas-exchange potential of the tracheae in comparison with that of the lungs. In Salticus scenicus, the tracheal system consists of four thick tube tracheae that originate from the atrium of the spiracle and that branch into secondary tracheae. These run parallel, without further branching, tapering slightly as they go. Some of the secondary tracheae end in the opisthosoma while others extend as bundles into the prosoma. Most tracheae end in the haemolymph; only the gut and the nervous system are penetrated in both the proand opisthosoma (Schmitz and Perry, 2000). One way of describing the gas-exchange potential of a respiratory organ is to evaluate its morphological diffusing capacity. Quantitative morphological data on spider lungs have been obtained by counting the number of lung lamellae or interlamellar spaces and multiplying by the area of an average lamella (Anderson and Prestwich, 1980). Morphological diffusing capacities cannot, however, be calculated using this method. Only in Tegenaria spp. has the morphological diffusing capacity of the lungs been evaluated using stereological methods (Strazny and Perry, 1984). In addition, there is a lack of quantitative data on the surface areas, diffusion barriers and diffusing capacities of tracheal systems in spiders. Thus, evaluation of the functional morphology of tracheae, which initially involves identifying where gas exchange occurs, has been restricted to descriptive and speculative accounts. It is 4321 The Journal of Experimental Biology 204, 4321–4334 (2001) Printed in Great Britain © The Company of Biologists Limited 2001 JEB3630