Crawlers, who live in the Underland are giant cockroaches, they are neutral and as Gregor explains: "Just wants everyone to leave them alone" but have especially good relations with the Hissers and Singers as they can understand one another. They reside on flat land and have a large population as to protect the whole area, as one Crawler on their own are very weak, Crawlers have wings but rarely use them and worship Boots (Gregor's sister) as their princess.
Notable Crawlers include:

Temp

-Boots' protector throughout the series

Tick

-Boots' protector in the first book, died to save Boots

Humans and fliers commonly mock the Crawlers for their stupidity and cowardice throughout the books, Gregor sticks up for them as he is not as accustommed to their ways. They often speak in a odd way where the last part of the speach is repeated at the start eg: "Follow us, you will Follow us"

Crawlers	Information
Overland equivilant	Cockroaches
Features	Wings Antennas Shell
All named characters	Temp Tick Pend Min
Geographical location	Flat land near the humans