wow (and bill (and all you rabble)),

many of these collective nouns are not just fanciful, but rather have some etymological basis. to wit, the first sense of rabble:

†1. A pack, string, swarm (of animals). Obs.
13+ Gaw. & Gr. Knt. 1703 [The dogs] runnen forth in a rabel. Ibid. 1899 Renaud com+& alle þe rabel+ryŠt at his helez. 1513 Douglas Æneis xi. ix. 29 The rawk vocit swannis in a rabyll. a1529 Skelton P. Sparowe 1313 He brought out a rable Of coursers and rounses. 1591 Sylvester Du Bartas i. v. 909 Flies, Butterflies, Gnats, Bees, and all the rabbles Of other Insects.

-joe (i am a member of the rabble in good standing) friday

2. a. A tumultuous crowd or array of people, a disorderly assemblage, a mob.

...and an array of butterflies is certainly a disorderly assemblage.