Your mantis is probably coming up for a moult; make sure your tank is nice n humid, remove all food items.
Double-check that your temperatures are on point. A mantis too cold may go off its food.
If she’s a female with a very swollen abdomen it might be that she’s going to lay soon. Female mantis whether they have been mated or not can sometimes go off their food for a couple of days prior to laying.
If you mantis is old, it might be because he/she has reached the end of his/her life. Be proud that you got them to adulthood and gave them the best life you could. You could hold a little mantis funeral and when you are ready... treat yourself to another one.
Mantis vomit when they have over eaten or eaten a dirty food item. Always aim to keep live food on clean substrate and gut loaded. Like us...when a mantis vomits they can get dehydrated so offer them some water via a dropper. Give them a day off food and maybe offer a speck of honey off the end of a toothpick, a cotton bud with the bud pulled off or end of a chopstick. The honey acts as a nice treat and will hopefully give them a bit of a pick me up. If you don’t have any honey, I have used pure maple syrup or a sugar water mix instead. Make sure your temperatures are on point and put the mantis in a clean enclosure. Ensure you clean up all of the vomit and disinfect the enclosure and any decor.
Wildcaught mantis and ootheca can carry parasites. These can be in the form of worms, parasitic larvae and pupae. For this reason I choose not to have wildcaught mantis in my collection. Look up parasite and mantis on youtube. Some of the footage is like out of an alien movie. The link below features a mantis with a parasite. Its quite an uncomfortable watch!
Some crickets can carry parasites like pin worm. Something to consider when thinking about livefood options. The carriers tend to be large black or brown crickets, banded crickets and locusts don’t tend to be affected as much.
Adult female mantis will lay egg cases called oothecas. She can lay as little as 2 to 15 in her lifetime depending on how well fed she is and her species. I find the smaller the species the more they tend to lay. Creobroter Phyllocrania species are prolific breeders and just lay and lay and lay. A female mantis will lay oothecas' whether she has been mated or not.
This can happen from time to time. I don’t know why but I have had it with my adults in cages with more than enough room and ample humidity. As long as the rest of the mantids body moulted fine, I wouldn’t worry!
As long as they can feed and get around their enclosures fairly well... they still have a quality of life.
Act quickly. Grab your mantis by the tips of the exoskeletons' feet and hold it in your fingertips as if it was still attached to the perch. Hold and hold and wait as still as you can until your mantis pulls itself out and begins to harden. Depending on the side of the nymph this can take anything from a few minutes to an hour. Once it’s sturdy enough to want to detach from its shed you can hang it upside down on its mesh and leave to dry. If you can get to your mantis quick enough they can be saved. Any imperfections can be moulted out if you can get it to the next moult. If all 4 legs are in tact then that's fab! Their chances are very good to make it to the next moult. If the fore arms are affected and they can't catch prey you will need to hand-feed.
Mantis have an incredible ability to regenerate lost or damaged limbs. As long as she/she can hang upside down, there is every chance they will moult out of slight deformities and grow back legs and arms moult by moult. If you mantids forearms were affected then simply hand feed until they regenerate. I do this by removing or crushing the head of a prey item to get juices flowing and gently hold up to the mantids mouth with tweezers, the mantis will most of the time eat straight away whilst others will be a bit more stubborn. Be persistant, it won’t be long until the mantis moults and corrects itself...hopefully. In the case of adult mantis the prognosis isn’t so good. Once a mantis has its wings it is an adult and will not moult anymore. As long as your adult mantis can stand and walk and hunt it still has a quality of life and simply enjoy him/her for the time you have left. If your mantis cannot hunt, stand and walk then you maybe want to think about euthanasia. If your mantis simply cannot hunt but can walk and get about its cage then you can always hand feed.
Unfortunately a mantis laying on the floor is never a good sign, unless it’s a ground dwelling species of course as that would be natural but they don’t tend to be very common in the pet trade. It’s possible your mantis has fallen whilst moulting and if it has hardened on the floor its chances of being able to hunt again will be very slim. If they harden in an unnatural position they won’t be able to hunt, or walk or moult out of it. The kindest thing would be to euthanize your mantis.
Euthanasia is best carried out by wrapping your mantis in tissue and giving a swift hard impact from a heavy book. Some people opt to put their poorly mantis in the freezer but I feel this is too slow of a death.
Sadly this is is a part of animal keeping and sometimes euthanasia is necessary to end suffering. If in doubt, drop me a message with photos and I'll confirm if your mantis has a chance of pulling through a bad moult. Sometimes they can be saved and some pull through to the next moult. Its not always curtains.