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One thing to consider is the ease of identification / availibility of keys (or expertise) with the groups you choose.

Also, how much previous work has been carried out on them, if you catch a particular species can you give references as to its level of rarity or conservtion status for the country / region where the sample comes from. You may well be able to identify species but if there is no information as to the conservation status of the species then knowing you have "Species X" on site may not help you with an assesment of your site.

Personaly, I would not use Tachinidae as here in the UK many species would have to be classed as "Data Deficient", there is simply not enough information about their distribution to to qualify a list of Tachind species. Some we know about, others we do not.

Matt