spiders have eight legs while insects have six l

　　这句子贼熟……托福题或者哪本语法书上的……

　　是省略的情况,and前后连接两个句式和语法结构完全并置的语句时相同部分可省略

　　你分析一下,and后面能省的部分全部都省掉了,包括定冠词,谓语,宾语

　　写完整：

　　a spider always has eight legs and an insect never has more than six
legs

　　a和an虽然拼写不同,但是都是冠词,表意也一样,故此省略,并不是共用什么的.

　　关于所谓的“总称”,给一个对比(牛津)

　　insect

　　noun

　　any small creature with six legs and a body divided into three
parts.Insects usually also have wings.ANTS,bees and flies are all insects:insect
pests / larvae / species ◆ insect repellent (= a chemical that keeps insects
away) ◆ an insect bite ◆ The pesticide is lethal to all insect life.

　　stationery

　　noun [U]

　　materials for writing and for using in an office,for example paper,pens and
envelopes

　　special paper for writing letters on

　　首先总称会在义项内标出[U],其次释义里面,insect用的是any开头,而stationery上来用的就是复数的materials,所以不存在当总称这一说法,要泛指请加冠词