It is a black-tailed red sheetweaver, _Florinda coccinea_.
Identified by its red body colour , location (USA) and black caudal tubercle.
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> It is a species of web-building spider belonging to the family Linyphiidae. It is the only species in the genus Florinda. It is sometimes known as the red grass spider. This species is common in the southeastern United States, inhabiting grasslands, lawns, and agricultural fields.
Reference: Wikipedia