what is that substance of which the covers are made?
They secrete a waxy coating for defence, making them resemble reptilian or fish scales, and giving them their common name.
Scale insect
They secrete a waxy coating for defence, making them resemble reptilian or fish scales, and giving them their common name.
what is the scale insect? They are small insects of the order Hemiptera, suborder Sternorrhyncha. are they pests? Yes,  crop pests. what are the various stages of their life? They develop from the egg through a first instar (crawler) stage and a second instar stage before becoming adult. what do they look like? They  vary dramatically in appearance, from very small organisms (1–2 mm) that grow beneath wax covers (some shaped like oysters, others like mussel shells), to shiny pearl-like objects (about 5 mm), to animals covered with mealy wax.
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Scale insect [SEP] Description
Scale insects vary dramatically in appearance, from very small organisms (1–2 mm) that grow beneath wax covers (some shaped like oysters, others like mussel shells), to shiny pearl-like objects (about 5 mm), to animals covered with mealy wax. Adult females are almost always immobile (apart from mealybugs) and permanently attached to the plant on which they are feeding. They secrete a waxy coating for defence, making them resemble reptilian or fish scales, and giving them their common name. The group is extremely sexually dimorphic; female scale insects, unusually for Hemiptera, retain the immature external morphology even when sexually mature, a condition known as neoteny.