Possible Clues:

• Antelope can't stand crossing river, most advantageous to traverse end of June
• Horny African's garbled threats when ringing buzzer
• Energetic-sounding insects primarily tormenting large antelope
• S African native — worker in Francis Bret’s time?
• Animal feeding insects to mad hatter
• When trained he's a better animal
• Terrible threat overwhelming insects and antelope
• Strong, healthy-sounding insects start to torment wild animal
• Creature bares teeth viciously
• With heart pounding, worker's beginning to tame wild animal