Mantids are enormous, insect-like demons.
Mantids resemble giant praying mantises. They have small triangular heads and wide mouths with rows of teeth. They have "pallid, flat, and milky" eyes; green, slimy, elongated bodies; jagged, razored forelegs; and massive arms with sharp, bony protrusions. They make insect sounds, such as chittering noises, high shrieks, and a range of clicking and hissing sounds.
The demons attack using their forelegs to grab and crush, or to tear apart, their victims. Mantids are shown to be quite strong and have some degree of intelligence. Mantids can be killed using seraph blades and normal runed knives to cut through the places on their bodies where the chitin is thinnest, severing limbs and heads. Like all demons, Mantids have corrosive green-black ichor.
In August 2012, while investigating the Guardian's lair on a ley line convergence located on a bluff near Los Angeles, Emma Carstairs and Mark Blackthorn were attacked by a group consisting of dozens of Mantids, placed there by the Guardian to guard the entrance to the cave at night. The Mantids destroyed Mark's faerie steed to prevent their escape. Though they began to seemingly outnumber and overpower the pair, reinforcement arrived in the form of Julian Blackthorn and Cristina Rosales, and the group of Shadowhunters were able to fight them off long enough to regroup and escape.
Shortly after, when Emma stabbed the Guardian, the wards he had put on Johnny Rook's house shattered, allowing a group of Mantid demons to burrow up through the floor. Johnny managed to get his son, Kit, to hide while he stepped into a protective circle—the circle, however, crumbled away along with the floor, and the demons went on to tear Johnny in half and eat him. Kit was rescued before the demons could kill him as well.