What is the significance of the fact that now we are told that these locusts have tails and stings like scorpions?

An advancing army typically holds their weapons visibly ready for attack. In this case, those who are attacked see threatening faces and teeth and hear terrifying sounds, but they do not see any weapons. Those weapons are hidden behind the locusts, in their tails. Scorpion‑like, these tails can suddenly flip forward over the body of the locust to sting the surprised victim with venomous poison. That sting will hurt badly. Attacking with your tail is a characteristically hellish maneuver.