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It is generally accepted that the exchange of pulmonary gases between "alveolar air" and pulmonary blood takes place by simple diffusion across the "pulmonary membrane." 2 Two properties of this membrane, namely, its permeability to gases and its surface area, are of considerable interest since a decrease in either could limit the rate of gaseous diffusion. There is no known physiological method for measuring specifically the permeability as contrasted to the area of the pulmonary diffusing surface in man. In 1915, M. Krogh (1), using carbon monoxide, made the first determinations of the pulmonary "diffusion constant" in human beings. This constant was considered to be proportional to both permeability and area of the diffusing surface and was expressed in cc. of CO diffusing per mm. Hg of CO partial pressure difference on either side of the pulmonary membrane. Roughton (2) has viewed the diffusion constant of normal persons as a measure of the number and size of patent pulmonary capillaries. Several investigators (3), in considering the diffusion constant for oxygen, have recommended that it be referred to as the "diffusing capacity" because its magnitude is greater during exercise than at rest.Carbon monoxide is peculiarly suitable for studies of gas diffusion across the pulmonary membrane because of the tenacity with which it is bound by hemoglobin after it has diffused into the red cells. For this reason, CO is taken up from respired gas for considerable periods of time (4) without there being a significant change in the Pco, or "escaping tendency," in the pulmonary capillary blood. The driving force responsible for CO transport across 1 These studies were aided by a contract between the Office of Naval Research, Department of the Navy, and the Trudeau Foundation

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Baseline ecological studies of pristine mangroves are important for monitoring, management and conservation of mangrove ecosystems. Diversity, density, biomass and community structure of crab and molluscan macrofauna were studied in a near-pristine mangrove forest in Sematan, Sarawak. In each of the nine 100-m2 plots, six 15-min catching periods were used to collect crabs and three 1-m2 quadrats were sampled for molluscan epifauna. Environmental variables and vegetation were also recorded in each 100-m2 plot. Thirty-one crab species and 44 molluscan species were recorded, including a few species not previously reported from Sarawak. Crab community structure was correlated with topographical height and surface water pH and salinity. Gastropod community structure was correlated with redox potential of water at depth, topographical height, surface water pH and leaf litter. Crab community structure and species number were positively correlated with tree and seedling community structure and diversity, suggesting that the mangrove vegetation is important to the crab fauna as a habitat and food supply. Molluscan abundance was positively correlated with sapling diversity and negatively correlated with the numbers of mangrove associates and tree species. The young leaves on mangrove saplings may provide a better food source than those on mature trees or mangrove associates. The data from this study provide a valuable baseline for future use at this site and for comparison with more degraded mangrove habitats elsewhere in South-East Asia.

It is well documented that ants can protect plants against insect herbivores, but the underlying mechanisms remain almost undocumented. We propose and test the pheromone avoidance hypothesis-an indirect mechanism where insect herbivores are repelled not only by ants but also by ant pheromones. Herbivores subjected to ant predation will experience a selective advantage if they evolve mechanisms enabling them to avoid feeding within ant territories. Such a mechanism could be based on the ability to detect and evade ant pheromones. Field observations and data from the literature showed that the ant Oecophylla smaragdina distributes persistent pheromones throughout its territory. In addition, a laboratory test showed that the beetle Rhyparida wallacei, which this ant preys on, was reluctant to feed on leaves sampled within ant territories compared with leaves sampled outside territories. Thus, this study provides an example of an ant-herbivore system conforming to the pheromone avoidance hypothesis.

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Ants of the genus Oecophylkz are predators of other insects and are able to protect a variety of terrestrial plants against pest insects; however, observations on the ecology of these ants in mangrove forests are lacking. General observations on the ecology of Oecophylkz smaragdina were carried out in a Thai mangrove forest to determine if these ants can protect their host plants in less favorable mangrove habitats. Leaf herbivory and the density of 0. smaragdina ants were measured on Rhizophora mucronata trees at two sites. The results showed a negative correlation between ant density and herbivory. At both sites, the mean percent damaged leaf area was more than four times higher on trees without ants compared to "ant-trees." A significant negative correlation was found between tree mean percent leaf damage and the density of ants on the tree. Furthermore, on trees with ants, there was less herbivory on leaves close to ant nests compared to other leaves on the tree. Most damage was caused by chrysomelid beetles (62%) and sesarmid crabs (25%) and both types of herbivory were significantly reduced on ant-trees.

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