DROP is a free application written in R language. Basically, DROP is designed for imaging densitometry of ninhydrin-detected excreta of plant sap-feeding insects. DROP enables successive densitometric analyses of multiple images, and automatically create a result table and a scatter or a box plot. DROP requires an installation of R with the EBImage package in Bioconductor (URL https://www.bioconductor.org/packages/release/bioc/html/EBImage.html) installed.