When it comes to calculating the minimum enclosure size, it is based to give enough room for molting, correct? I was planning on keeping a Rainforest mantis in a 8x8x12 terrarium. Once it has substrate this will cut it down to less than 12 high. I was told I should have taller because they can grow up to 4" long. So I bought the next size up which was much bigger at 18" high.
It seems you hit the important points yourself, just talking out loud about it.
Yes, the minimum sizes is to ensure proper molting. That is the adult mantid length size - the habitat needs to be at least twice the mantid length for habitat width and length, and three times that for the height.
Indeed the current instar has a bearing on habitat size too. For example a L2 nymph is often lost in such a large tank when it comes feeding time. In that aspect many keepers will have multiple habitat sizes for nymphs as they grow to ensure they have molting room, but still easily find their prey. While mantids in the wild are only limited by the Earth, they have an entire ecosystem and can easily move to where their is prey (but are not options in a habitat).
For my Carolina nymphs (Stagmomantis carolina) I start them off in
9oz cups, move them in 32oz deli cups, and lastly tall 1quart containers.
If you do use smaller habitats and switch them as needed, ensure that the habitat fits them for at least one molt. Using the mantids current size the habitat should be at least three times their length for height, but I error on the side of caution and use four times their current length.
I'm just thinking shouldn't minimum final enclosure size be calculated at subadult?
If you are putting in a mantid that is not an adult yet the answer is no - if it is an adult see below response.
The subadult is a stage that needs the most molting room as it will be molting to a large adult. Besides molting from the exuviae (shed exoskeleton) and sitting on it and stretching to dry, they also will uncurl their wings seemingly inflating them. I like to give my mantids extra room especially at this final molt to ensure they do not have deformed wings or other common issues.
But then I started thinking, adults don't molt, so a 4" at adult would not require molting room, therefore wouldn't the 8x8x12 be large enough to house an adult?
Adults with wings of course no longer molt, and can be housed in smaller habitats. I've heard keepers sometimes use a tank size that is twice the mantids length for all measurements. Personally when possible I give my adults the opposite, massive amounts of room. Such as my 2.5" Carolina female I put into a 32" x 12" x 13" habitat I made that filled a entire shelf.
My adults that were housed in large habitats were more active exploring their tanks, seemed more content (no longer pacing the same few inches, or never moving), able to hunt their prey over longer distances (and is a show from active mantid hunters), were typically more prolific breeders (easier to mate, more and bigger ooths, etc), and picky females had much more areas to lay their ooths.
If space is a premium, keep males in smaller tanks and females in larger ones for the reasons listed. Also one trick that helps keep size down is to have a large tank that is normally stored away, but used for breeding and/or laying ooths.
