Hundreds of Red Spiders in Individual Webs
It is a black-tailed red sheetweaver, Florinda coccinea. 

Identified by its red body colour , location (USA) and black caudal 
tubercle.



More images to be found on bugguide.net


  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