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Yep, Tony is quite right.
This could be Copestylum isabellina, which is known from decaying cacti. There is a picture of a female depositing her eggs on a cactus on Bugguide.
Btw, there more you go south in the Americas, the more species of Copestylum you will find. There are a few hundred species in South America, of which a lot of them are until now undescribed by science!
greetings,