Complex sentence: The procuticle in most land Arthropoda, particularly insects, is covered externally with a thin, waxy, water-resistant outer layer containing no chitin: this is the "epicuticle", and it is much thinner than the procuticle.
Split into simpler sentences: The procuticle in most land Arthropoda, particularly insects, is covered externally with a thin, waxy, water-resistant outer layer containing no chitin. That outer layer is the "epicuticle", and it is much thinner than the procuticle.