Diseases of insects are important factors reducing the number of insects in natural populations. They are density dependent and they act especially under conditions of outbreaks and close contact of susceptible hosts. They are im­ portant allies of man in his effort to replace toxic chemicals with non-toxic, biological means in the conservation of the environment of mankind. The importance of diseases as reducing factors has often been disregarded in studies on insect ecology and population dynamics. Most cases of mortality in carefully watched experiments have simply been characterized as "natural mortality", neglecting the fact that there is no "natural" mortality before the insects have finished their life assignment, the deposition of their eggs. The environmental factors including the pathogens are responsible for any other than natural mortality of the larvae, pupae and young adults of the pests. It is rather difficult to distinguish the different symptoms of a disease in insects. Therefore they are not noticed and their influence on populations is overlooked. We feel that a series of photographs of different stages of a collection of insect diseases could help such workers in a general orientation and we are sure from own experience that each specialist needs some broader oriented guide for the fields bordering his own homeland.