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Missing bristles shouldn't be a big problem - the socket will remain. Bristles are distinguished from hairs by the fact that they are mounted in articulating/sensory sockets, unlike hairs that emerge directly from the exoskeleton.

Don't worry - Siphona are probably the most frustrating genus... they are very common and 95% will turn out to be geniculata... but you should key them all (especially really orange ones) because the other species do crop up regularly. :D