Rhiannon Kearns, eight, had been through the drive-thru at the fast-food restaurant with her mum Jemma, 28, and her partner Tom Cartridge, 27, on Sunday (2/8).
The family pulled in and the schoolgirl tucked into her cheese melt dippers before opening her fries.
But she was horrified when what appeared to be “greasy chip” suddenly started wriggling in her food.
Rhiannon threw the fries across the car and started screaming and shouting as the three-inch white bug crawled on the seat.
Stomach-churning video footage taken by the family shows the creepy-crawlie wriggling violently and attaching itself to a handbag strap.
Mum Jemma then marched into the McDonald’s restaurant in Chesterton, Staffs, to get the manager who apologised and offered the family a free meal.
The bug has now been sent off for analysis but the family are convinced it’s a tomato hornworm, a major garden pest more commonly found on defoliated tomato plants.