are insects the only source of this substance, or are there other sources as well?
No, they can be obtained from plants and animals as well. The best known animal wax is beeswax. Lanolin is a wax obtained from wool
Wax
The best known animal wax is beeswax used in constructing the honeycombs of honeybees, but other insects also secrete waxes. A major component of the beeswax is myricyl palmitate which is an ester of triacontanol and palmitic acid. Its melting point is 62-65 °C. Spermaceti occurs in large amounts in the head oil of the sperm whale. One of its main constituents is cetyl palmitate, another ester of a fatty acid and a fatty alcohol. Lanolin is a wax obtained from wool, consisting of esters of sterols.
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. 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.
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Wax [SEP] Animal waxes
The best known animal wax is beeswax used in constructing the honeycombs of honeybees, but other insects also secrete waxes. A major component of the beeswax is myricyl palmitate which is an ester of triacontanol and palmitic acid. Its melting point is 62-65 °C. Spermaceti occurs in large amounts in the head oil of the sperm whale. One of its main constituents is cetyl palmitate, another ester of a fatty acid and a fatty alcohol. Lanolin is a wax obtained from wool, consisting of esters of sterols.