Why 独 〔獨〕 dú is alone? I am learning the word 独 dú alone.
Dog plus Insect become alone? Any ancient history or story behind this?
There is no 「虫」 (venomous snake; insect) in the character, that is an abbreviation used in Simplified Chinese. The orthodox character is 「獨」.

「獨」 is made up of semantic 「犬・犭」 (dog &gt; beast) and phonetic 「蜀」 (Mandarin Pinyin: shǔ, Li Fang-Kuei OC: /*djuk/), originally representing the name of a mythical beast. From the Classic of Mountains and Seas: Classic of the Mountains: North:

《山海經・北山經・北次二經》
……曰北嚻之山，無石，其陽多碧，其陰多玉。有獸焉，其狀如虎，而白身犬首，馬尾彘鬛，名曰獨。……
...It is called the Mountain of 北嚻. This mountain is without boulders; its southern side is abundant with green jade, and its northern side is abundant with nephrite. There is a beast in this mountain, shaped like a tiger with a white body; it has the head of a dog, tail of a horse, and neck-bristles of a boar. Its name is 「獨」...


「獨」 were two characters created specifically to represent the name of this beast (Mandarin Pinyin: dú gǔ, Li Fang-Kuei OC: /*duk kuk/), which consists of two reduplicated syllables.

The meaning alone, solitary for the character 「獨」 is a rebus borrowing for this word; there were other borrowed characters for this word in the pre-Qín era, most prominently

「蜀」 itself;
「鹿」


References:

《王力古漢語字典》
先秦甲骨金文簡牘詞彙資料庫
小學堂上古音