Quarterly Journal of Microscopical Science, Vol s3-100, 433-444, Copyright © 1959 by Company of Biologists

The Association of a Slime Bacterium with the Inner Envelope of the Egg of Dytiscus marginalis (Coleoptera), and the less common Occurrence of a similar Bacterium on the Egg of D. semisulcatus

¹ North Cliff, St. Andrews, Fife

The egg of Dytiscus marginalis L. is laid under water in the mesophyll of the leaf of various aquatic plants. The egg increases in size during development and this results in the splitting of the chorion, the outer envelope of the egg, usually into two halves. The inner envelope of the egg, the vitelline membrane, then forms the only complete shell of the egg in its later stages.

During the development of the embryo of D. marginalis the cysts of a slime bacterium (myxobacteriale) appear in great numbers on the vitelline membrane along the dorsal surface of the embryo and around each end of the egg. They are at first small and scattered, but by the time the larva hatches they are present in dense colonies.

These bacterial cysts occur on eggs collected from water plants in ponds; but they develop just as readily on eggs obtained in the laboratory by placing the female D. marginalis in jars of tap-water with leaves that have never before been immersed in water.

Slime bacteria in the vegetative stage are carried in great numbers by the female D. marginalis, as has been shown by trailing the apex of the abdomen of a living beetle over an agar culture, when swarms of the characteristic rod-like cells have been obtained. Some of these bacteria will be transferred from the beetle to the egg during oviposition

The bacterial cysts do not occur on unhealthy eggs and they rarely show normal development on eggs parasitized by the mymarid, Caraphractus cinctus Walker. This suggests that their development is dependent upon the healthy growth of the Dytiscus embryo.

In the egg of D. semisulcatus Müll. the chorion remains in close contact with the vitelline membrane throughout embryonic development and only occasionally shows small irregular cracks before hatching. On some eggs of this species a few scattered cysts of slime bacteria occur on the vitelline membrane. On occasional eggs they were more numerous, but they were never observed to be present in sufficient numbers to form the even covering which is so striking a feature of certain parts of the egg of D. marginalis.