Sentence:  Even closely related species may have <WSD>chorions</WSD> differing in thickness. In the silkmoths, for example, the <WSD>chorion</WSD> of "Bombyx" is about 25 μm thick while that of "Hyalophora" is 55 μm thick. 
Meaning of the word marked with <WSD> tags:  The outer case of an insect egg.