General rull of thumb:
1) If the major color of the bug is black or brown, go to the doctor.
2) If the major color of the bug is not black or brown, wait and see.
1) If you are prone to rashes or have many alergies, go to doctor at first sign of any symptoms.
2) If you have asthma and are prone to frequent attacks, dont wait, just go.
3) If bitten in australia, and not a native to australia (as in born and raised there), just go. They have a variety of bugs that tend to trigger dermal necrosis in those that may be susceptable. The hospital cant stop your skin from rotting off but they have a variety of drugs that make the process less painful.
Good luck. Sleep well. Hope you live.