Thrips damage?

August 2000. An orchid grower in the USA has plants with white cloudy areas under the epidermis of some leaves. The cloudy areas begin as small circles (about 1-2mm). If left unchecked, the spots become more numerous and enlarge. They are not readily noticed unless the leaf is viewed with light from behind. There is a very tiny dark area in the center of these spots. Of the 50 different orchids in the collection only a few Phaleonopsis have been affected and one in the genera Oncidium. There is no fungal growth or any insect activity in the spots.

In response, it was suggested that the symptoms were remininscent of thrips damage. If that was the case, it might be expected that some flecks of excrement would be present.