i started out with smaller home made enclosures
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and im slowly upgrading as they grow. i really dont give them more room than is really needed but i do giv ethem enough so that if they were to choose to take a stroll they have that option.. even the ghosts, which yes, barely move. but because they hardly choose to move im able to keep them together in a larger enclosure.
i alwasy add color for them. as i was telling some other people that had come to me with the topic, its like living in a single room with a biege chair in the center. ya, you get to stand-up, sit down move around if you want ... but how boring is that?! that would make any creature unhappy.
im learning from experience where the furniture should go on the inside based on the personalities of each mantis. my Stagmomantis Limbata prefers to to run around upside down (and crickets like to hide under flowers placed on the bottom of the enclosure) so i glued a large red flower on one side and branch on the other side. there is plenty of room directly in the middle where she always chooses to molt. i adjust everything to each individual personality. you cant generalize and keep everything in the exact same conditions... just my opinion. ide usually keep to myself but i was asked to share by several other members
hope this helps