15.The fact that half of the known speciesare thought to inhabit the world's rain forests does not seem surprising，considering the huge numbers ofinsects that comprise the bulk of the species．

(同位语从句fact that…；定语从句that comprise the bulk…)

考虑到组成大部分物种的昆虫的巨大数量，认为半数的已知物种栖息在世界的雨林中这一事实看起来并不令人吃惊。

分句1：The fact does not seem surprising

分句2：that half of the known species arethought to inhabit the world's rain forests

分句3：considering the huge numbers ofinsects

分句4：that comprise the bulk of thespecies

分句1嵌套分句2，分句2解释说明分句1的主语the fact,是同位语从句，分句3作为分句1的原因状语，分句4是定语从句，修饰分句3的the huge numbers of insects