The silk cocoon of the silkworm, Bombyx mori: macro structure and its influence on transmural diffusion of oxygen and water vapor.

The cocoon of insect larvae is thought to help conserve water while affording mechanical protection. If the cocoon is a barrier to water loss, then it must also impose a barrier to inward oxygen diffusion. We tested this hypothesis in pupae of the silkworm, Bombyx mori. The rate of water loss and oxygen uptake (VO(2)) at 25 degrees C was measured in control pupae in their naturally spun cocoon and in exposed pupae experimentally removed from their cocoon. Additional measurements included the oxygen diffusion coefficient, DO(2), of the cocoon wall and dimensions and density of the cocoon fibers. Water loss (as % body mass loss) in both control and exposed pupae was approximately 1% x day(-1), and was not significantly different between populations. Similarly, VO(2) was statistically identical in both control and exposed pupae, at 0.22+/-0.01 and 0.21+/-0.02 mL g(-1) x h(-1), respectively. The silk fiber diameter was significantly different in the outer fibers, 26+/-1 microm, compared with 16+/-1 microm for the inner fibers lining the cocoon. Inner fibers were also spun significantly more densely (20.8+/-1.2 mm(-1) transect) than outer fibers (8.3+/-0.2). Mean DO(2) at 25 degrees C was 0.298+/-0.002 cm(2) x s(-1), approximately the same as unstirred air. These data indicate that the cocoon, while creating a tough barrier offering mechanical protection to the pupa, imposes no barrier to the diffusion of oxygen or water vapor.