No they weren't. Did you watch too many Godzilla movies?
Sea scorpions were aquatic arthropods, using gills. There is no size restriction for gill-breathing aquatic animals, except maybe statical constraints.
Insects are primarily land insects breathing with tracheal tubes, and are size restricted due to this point. They may reach sizes a little larger than seen today, but the only time when insects and other arthropods were considerably larger was the Carboniferous (due to a higher oxygen content of the atmosphere). Mantids are not as old. Other info is outdated.
So, I do not have to be there to know there
IS no mantid as large as 29 cm. There is enough serious literature on this issue. The internet contains a lot of wrong information on mantids. Once I wanted to contribute to the Wikipedia article, but ceased it after more and more bull**** accumulated. I did not find one site on mantids without wrong info yet.
So, sometimes the good old (or even new!) paper literature is by far better than any site on the net. Journals edit and review the information they get. On the net, everyone can post everything, regardless whether he knows about it or not.
To make a long story short: I
know that there
is no mantid known to science (
science, not the www!) which reaches that size. Maybe that Chinese thing was a stick insect or something.
regards,
Christian