Nuclear polyhedrosis viruses. Coloured transmiss- ion electron micrograph (TEM) of nuclear polyhedrosis insect viruses. The viruses have an envelope (blue) which may contain several virus particles. After being eaten by an insect, they infect the cells lining the insect's gut, before spreading to the rest of its body and killing it. Inside the nuclei of the insect's cells, they form structures which are eaten by other insects, continuing their life-cycle. These viruses have been used as a means of biologically controlling insect pests, as they are not pathogenic in other organisms. Magnification unknown.