Electron microscopy of symbiotic bacteria in developing oocytes of the American cockroach, Periplaneta americana.

The symbiotic microorganisms found in some groups of insects present many stimulating problems from both the evolutionary and the physiological standpoint. Of particular interest are the symbiotes, usually referred to as bacteroids, of the Blattaria, which were first reported by Blochmann in 1887. These microorganisms have been found in every species of cockroach thus far investigated and also in the primitive termite, Mastotermes darwinensis Frogg, which apparently evolved from a Blattaria-like ancestor (Holmgren, 1909-13, cited by Brooks, 1954). It is now generally agreed that these symbiotic organisms are bacteria, although their systematic classification is in doubt. Gier (1936), after a detailed study, concluded that they were closely related to both the diptheroids and the Rickettsiae. Richards and Brooks (1958), and Roth and WVillis (1960) have presented a rather complete summary of our present knowledge of these microorganisms. Thus, the bacteroids have been reported to be about 0.9 u in diameter and are from 1.5 to 9 ,u in length. They are grampositive or gram-negative, depending on their physiological state. They give a positive, although diffuse, Feulgen reaction and stain after Giemsa and hematoxylin. They are not stained after Sudan dyes, the Baker acid hematin test for phospholipides, nor the periodic acidSchiff reaction for polysaccharides, and they are not detectably fluorescent or birefringent. They give an intense reduction of neotetrazolium. Using shadowed material, de Haller (1955) concluded after electron microscopic