House crab spider Thanatus vulgaris
This spider is among the least "crabby" of the crab spiders. That is to say, its normal posture is less crab-like than other, wider-stanced crab spiders like the inconspicuous running crab spider or the goldenrod crab spider. They all similarly have longer front pairs of legs than rear pairs, with which they sieze insect prey. In the photo above, the spider adopted the crab spider pose after I manipulated her into place to get a head-on image.
The house crab spider is thought to be native to the central part of North America, in flat open areas, but is now found wild in California and Florida and other places. It can be found in houses, as the common name suggests, but the habitat this specimen was found in--a shipment of captive-bred crickets--was my clue to its identification. A search of online sources shows the frequent appearance of Thanatus vulgaris in pet forums, as a hitchhiker in shipments of crickets purchased as food for lizards and other creatures. The spider is not dangerous to pets or pet owners, and is probably regularly consumed with impunity by animals happy to have a switch from crickets (owned by people who didn't notice something different mixed with the usual chow). The spider pictured here has become a pet at the Franklin Park Zoo's Children's Zoo, for the time being. It has crafted an egg sac, but no spiderlings have been seen yet.
Are the spiders entering the cricket stream singly, as opportunists that drop in one at a time, or is there a permanent breeding colony of T. vulgaris that lives only within the microlivestock facilities, in amongst their limitless prey?
I found people trying to identify this spider from shipments of food insects here, here, here, here, and here.
The backyard arthropod project was very helpful in my quest, as was The Kansas Crab Spider Checklist.