less so(指的是less impressed印象愈发淡漠) with the structural differences (定语从句)that seem,at first glance,（插入语）to constitute such an immense gulf between them.
虽然这些结构上的差异（重复了前文the structural differences ）增加了表递进结构的量词“虽然”根据逻辑关系补充的
to constitute such an immense gulf between them.构成了二者间一条无法愈越的鸿沟；
二者间指的是昆虫和脊椎动物群落之间