Morphometric diffusing capacity and functional anatomy of the book lungs in the spider Tegenaria spp. (Agelenidae)

The presence of both book lungs and a tracheal system in many spiders raises the question of the functional significance of this double respiratory system. The present physiological and morphometric study of the house spider [Tegenaria atrica, T. picta and T. domestica] reveals that the diffusing capacity (DtO2) of the lungs alone suffices during rest and following exercise to meet measured rates of O2 consumption (.ovrhdot.VO2) at driving pressures .**GRAPHIC**. similar to those calculated for vertebrate lungs. During molting .**GRAPHIC**. may rise to more than double the vertebrate values, implying the possible insufficiency of book lungs during this critical life phase. Resting .ovrhdot.VO2 is greatest (92 mm3/h.cntdot.g) during the early morning and lowest (66 mm3/h.cntdot.g) near midday: during molting .ovrhdot.VO2 rises to 278.7 mm3/.cntdot.g. In spiders recovering from exercise .ovrhdot.VO2 is consistently greater than during rest: neither value is significantly reduced by blockage of the tracheal stigmas. Regression calculations of morphometric values for a hypothetical 100-mg Tegenaria yield a total lung volume of 0.578 mm3, a pulmonary surface area of 69.8 mm2, and a surface-to-volume ratio of 120.89 mm2/mm3. In spite of the similar thickness of the chitinous and hypodermal components of the air-hemolymph barrier (eadch .apprx. 0.2 .mu.m in nonmolting animals), the low permeability of chitin for O2 makes this layer the greater barrier to diffusion. For a 100-mg specimen DtO2 is 3.5 mm3/h.cntdot.torr, similar to that of a turtle (Pseudemys) on a gram-body weight basis.