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genitalia is the safiest way to ID the specimens. They are unique (there are exceptions, sometimes they could be like a hell to decide... it happens with some Xysticus females spiders, for example), they are the "fingertip" of the species. The most reliable way would be a DNA analysis.. but it is not very pratical for obvious reasons. Genes are apparently the only natural thing... all these stuff we talk are artificial.. humans like to organize the organisms, and of course, Nature is very much complex than we can tought. There are many greys (hybrids, for example) that difficult this organization. There is no 100% satisfatory organization... hence it is normal to have many reorganizations... like Canacidae/Tethinidae (polemic subject!)...