this is straight from the stomatopd forum
The answer to this question lies soley in what species your mantis is, how big your mantis is, and how big your tank is. Generally the bigger the tank, the thicker the glass. Thicker glass is better.
A peacock mantis in the 7 inch range is for sure capable of cracking some glass. Capable meaning it can if provoked or not provided enough sand to "seat" it's hard shelled prey when smashing.
Typically speaking, the largest of Peacock and Chiragra are species most capable of glass damage. If possible it is recommended to use an acrylic tank for this size of specimen. If this is not an option then a 20gal or larger glass tank should be only slightly risky with a nice 2"+ bed of sand. In addition, the bottom could be lined with acrylic sheet. Be sure not to taunt your large mantis!