Not something I really like to use for wild butterflies but a necessary evil for the very small subjects, especially eggs and caterpillars.
A macro-flash, which is a variant on a ring flash is one possibility. It can lead to rather shadow-less, flat looking images. Ones that only fire one half generally produce better relief on the subject. Also they are quite low powered and so only really work from 1:2 (i.e. half size) upwards.
Flash gun(s). I have only ever used a top mounted flash for a bit of fill-in flash. However I know many people swear by flash guns mounted on the front of the lens, especially a pair either side.
Tripod. Not much use for butterflies, but for stationary subjects an essential part of the wildlife photographers kit list. As heavy as you can carry/afford I have heard is a good rule. Also the head needs to be easily adjustable for the angle you need. An ability to get low down is essential for flowers and other low lying subjects.
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