Functional morphology of the respiratory organs in the cellar spider Pholcus phalangioides (Arachnida, Araneae, Pholcidae)
Morphometric evaluation of the lungs of male and female cellar spiders (Pholcus phalangioideus) was carried out in 2 test groups with different body masses (mean value 10.8, males, and 26.6 mg, females). Males have significant higher lung volume to body mass ratios (2.49 vs. 2.13 × 10−3 cm3 g−1), which might result from the differences in body mass between sexes. Moreover, males have slightly more respiratory surface area per body mass (8.2 vs. 7.7 cm2 g−1) and a little bit larger morphological diffusing capacities for oxygen (9.3 vs. 8.2 nmol s−1 g−1 kPa−1) than females, but both values were not significant. Metabolic rates were measured using flow through respirometry under video tracking: the CO2 release of male and female spiders was measured. Resting rates were 1.7 (males) and 1.5 nmol s−1 g−1 (females). Gluing of one spiracle did not influence the resting metabolic rate. Factorial scopes during stimulation to maximum metabolic rates were about 12 in intact animals, while elimination of one spiracle reduced the factorial scope to 5.2. Comparison with other araneomorph spiders strengthens the hypothesis that tracheae in spiders increase the metabolic rates of the tracheated species and do not only replace reduced lung capacity.