There are multiple metric units to choose from! This tutorial goes through the process of choosing the best metric unit to use for measuring the mass of a given object.
The most common aphids on houseplants are the light green ones (pear aphids), but aphids can also be found colored pink, white, grey and black. Additionally, winged aphids can appear when colonies are established and fly to infect new plants. Juvenile aphids (nymphs) look like smaller versions of the adults.
It appears that you might have aphids based on the distortion and color of your lime leaves. The brown undersides could be a collection of brown aphids or sooty mold caused by the honeydew that aphids secrete. In order to be sure, use our tips to identify the small bugs.