As my own studies have advanced，I have been increasingly impressed with the functional similarities between insect and vertebrate societies and less so with the structural differences that seem，at first glance，to constitute such an immense gulf between them.

and less so with the structure differences：

so 是一个替代词，替代前文的 impressed（有印象的），这个 with 与前文的 with 一致。less so 表示印象反而差了，不深了。
意为：而对结构上的差异，印象却越来越淡漠。
so 作为替代词，替代前文的某些内容，而且经常跟 more 或 less 连用，以简化表达。

下面是我以前的一个解答，可参考：
Wins and losses matter everywhere, but perhaps nowhere more so than in South Korea.
【翻译】赢与输在任何地方都很重要，但在韩国或许尤为如此。（= 字面：但也许没有地方比得上韩国）
so 是个“替代词”，它可以替代一个词、一个词组，甚至表示并替代修饰前句谓语动词的一个程度副词。这句的用法便是。然后被 more 修饰。
详解so 被much 或 more 修饰：
Are you enjoying your holiday?   — "Very much so. 你的假期过得愉快吗？ —— 非常愉快。《柯林斯高阶英汉双解词典》
——这个so 替代 I am enjoying my holiday 的, very much 表示程度，修饰动词 enjoying。等于说：
I am enjoying my holiday very much. 
The band is popular and likely to become more so. 这支乐队很受欢迎，而且可能会越来越受欢迎。《朗文当代英语词典》
——这个 so 替代 popular。所以，more so = more popular。等于说：
The band is popular and likely to become more popular.
He's quite bright.  — well, certainly more so than his brother. 他相当聪明——嗯，肯定比他哥哥聪明得多。《剑桥高阶英语词典》
——这个 so 替代 bright。所以，more so = more bright。等于说：
He's quite bright.  — well, certainly more bright than his brother.
She had always been silent and thoughtful, and now she was more so than ever. 
她一直就是一个沉静和深思的女孩子，现在她变得更是这样了。