Plecia nearctica (Lovebug).
Size: approx. 10-15 mm. Photo: 09/01/11, 10 am.

There are two species of lovebugs in North America. This one is Plecia nearctica. Adults feed on nectar. Copulating adults often form swarms. Females live approximately one week; males 2-3 days. This species has two generation in Florida that peak in April/May and August/September. P. americana appears only in Spring approximately 2-3 weeks before P. nearctica

bug guide (these photos):