英语语法问题We owe a lot to the birds and beasts who eat insects b

We owe a lot to the birds and beasts who eat insects but all of them put together kill only a fraction of the number destroyed by spiders.这句话最后number后面的destroyed by spiders被动语态destroyed为什么不用was destroyed这样的形式
destroyed by spiders=过去分词短语作后置定语,修饰前面的名词

首先你得明白desroyed不是谓语动词 它是是个过去分词做定语的。。。。相当于一个定语从句。。。number后面省略了which was 而已。。。。。。去看看定语从句那部分。。。。。
不懂请追问 满意望采纳。。。。。。

destroyed by spiders实际是that is destroyed by spiders，这是定语从句，只不过可以省略成那样罢了