Many insects get nutrients for growth and development from plant leaves. Plant leaves consist of cells mostly devoted to photosynthesis, but some dedicated to structure, transport or defense. Plant cell contents are surrounded by a cell wall. The cell wall contains cellulose and other factors that are refractory to digestion by the majority of insect herbivores. Cell wall components cannot be assimilated by most insects and are discarded in the frass.
The cell contents are more…
Source: Living With Insects Blog