Utah State University Extension
The oystershell scale belongs to a group of insects called the armored scales. The adult insect is enclosed in a shell made up of its own shed skins and waxy secretions. Under this covering is a sac-like animal that is quite different in appearance from most insects. It has no legs, eyes, or antennae.
Karren, Jay B.; Roe, Alan H.; and Davis, Ryan, "Oystershell (Mussel) Scale (Lepidosaphes ulmi)" (2019). All Current Publications. Paper 1359.