對一般人來說:
bug是比較口語的詞，可以泛指任何小蟲子，
特別是會爬、惹人厭的蟲蟲，所以bug引申有很多負面類的意思。
insect指昆蟲綱內的昆蟲(Insecta=昆蟲綱)，是比較專業的詞。

對於生物學家來說:
bug如果是當作比較專業的詞，其實是特別指昆蟲綱下面的「半翅目昆蟲」(「真蟲」)喔。
(特徵:有吸管狀口氣來吸食植物汁液的昆蟲)

可以參考這裡的說明:
The word bug is often used to refer to tiny creatures that crawl along, such
as insects and even small animals that are not insects, such as spiders and
millipedes.

But for scientists the word has a much narrower meaning. In the
strictest terms bugs are those insects that have mouthparts adapted for
piercing and sucking. The mouthparts of these bugs are contained in a
beak-shaped structure. Thus scientists would classify a louse but not a
beetle or a cockroach as a bug. In fact, scientists often call lice and their
relatives true bugs to distinguish them better from what everyone else calls
"bugs."

http://tinyurl.com/2a5foqg


※ 引述《meteore (meteore♂台北)》之銘言：
: ※ [本文轉錄自 Eng-Class 看板 #1CEsFdSv ]
: 這兩個字都可以當小蟲子
: 不知道有什麼差別
: 是大部份通用嗎？
: 還是bug比較常用
: 我在網站上查到
: 【词义辨析】
: bug, insect, pest, worm
: 这些名词均含“虫，昆虫”之意。
: bug: 指小昆虫，尤指吸血的臭虫，也可指细菌。
: insect: 含义广泛，多指蚂蚁、黄蜂、蝉等等。
: pest: 指有害的昆虫或有害的小动物。
: worm: 侧重指蚯蚓、蚕、毛虫等小而无骨的虫。
: http://dict.cn/insect
: 不知道版上前輩們有什麼其它見解:)

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