@AltarMantis Yes it is recommended to feed crickets well for a few days before offering to your nymphs. Most cricket arrive to pet stores hungry from shipping, and typical not given food while there, forced to cannibalize or eat the dead. In most cases the cricket is very hungry and if fed to a nymph too small can turn the tables and eat the nymph.
Feeding the cricket for a few days will load the digestive trait with good nutrients for your mantid nymphs, makes the crickets more docile (not in staving mode), and will be easier in handling/moving/feeding.
The ideal choice of cricket food is cut up potatoes as it provides food and water to them.
Carrots are infamously said to cause deaths in mantids that eat crickets who digested them (true or not is debatable as it has not be scientifically studied).
I highly recommend avoiding the Fluke brand (or any) cricket products as they are nothing more than clear jello/gelatin with usually orange dye added. Besides the ridiculous markup/costs for the buyer for the water, there has been linked issues in the past with materials finding their way into the product at the factories in the case of the food (which is loaded with things for a reptile and can harm mantids).
Fish flakes is a easy food source as Tyler mentioned as well, but if you do not keep fish then a much cheaper solution is to buy a small bag of any brand dry cat food. Grind the cat food up in a food processor/grinder/etc until powder - crickets devour it. If you want to get fancy with feeding them you can make your own cricket food blend,
see here, but is not necessary.
Also besides potatoes for water you can make a
easy cricket waterer, use wet cotton balls, or use the small 1 quart chicken waterers (with material added to the bowl to keep crickets from drowning).
