What identification criterion, used for distinguishing families like Hydrophilidae and Spercheidae, is often difficult to observe in field collections?

The tarsal formula (the number of segments on the foot)

While external features like antennal structure and body color are useful field indicators, formal scientific identification keys often rely on minute details that require careful examination of a preserved specimen. When researchers attempt to definitively separate families that are closely related, such as Hydrophilidae from Spercheidae, they often must resort to microscopic examination of structures like the tarsal formula, which details the exact number of segments comprising the foot structure. Additionally, examining the precise structure of the underside of the beetle might be necessary, which is information often unavailable when specimens are collected and briefly observed in a standard field setting.