Tens of billions of insects are set to appear across the US over the next few weeks in an ultra-rare synchronised ’emergence’ – the first of its kind in over 200 years. Two broods of infamously loud cicadas are due to start popping up from underground across the American Midwest and Southeast from the beginning of May to undertake a mass mating ritual. Some experts have warned that over one trillion of the insects could cover areas where the two broods coexist – but, fortunately, the species is harmless to humans. For insect experts, the emergence of both sets of cicadas is truly a once-in-a-l…