There are many species of scale insects that feed on a wide range of host plants. They can be hard or soft shelled, brown or white, glossy, fuzzy or matte, and vary in size. It is often difficult to identify them as an insect because their features are hidden under a protective shell. 

One type of scale, brown soft scale (Coccus hesperidum), are adult females that are smooth, oval, brown, flattened, and up to 1/5 inch long. Nymphs are similar, but smaller and almost transparent. Both can be difficult to see on leaf veins and twigs. Adult males are tiny, two-winged insects.

Many plants are hosts to this insect. Indoor plants such as citrus, and other tropical and subtropical fruit; also a wide range of foliage plants and greenhouse ornamental plants can be attacked by scale insect as well as outdoor hardy plants. The University of Wisconsin has some great images of scale insects for you to refer to.

Courtesy of the NYBG Plant Information Service