Medium-sized larvae. Head capsule yellowish, rounded-oval; cephalic index 0.95. Dorsally, DP posterior to, S5 anterior to, S7 and S8. Ventrally SSm posteromedian to longitudinally-aligned S9 and S10, far anterior to VP which is lateralmost.
About 1.25x as long as mandible. Antennal ratio c 9.0. Basal segment about 8x as long as basal width, with ring organ at 0.75; segment 2 3-4 x as long as wide; 3rd segment 1.5 times longer than wide, subequal to apical segment, 4th shortest. Style extending approximately to mid-segment 4, withdrawn from immediate apex of 2nd segment as are short peg sensilla. Blade slightly shorter than flagellum. Accessory blade slightly shorter than blade.
Uniformly curved, slender. Apical tooth narrow, without distinct inner tooth; mola projecting, bearing 1-2 small, apically-directed accessory teeth at base of seta subdentalis. Ventrolateral setae simple (1) or bifid (2 and 3).
Basal segment of palp about 3x as long as wide, with ring organ at 0.3.
With fringe of swim-setae, otherwise undescribed except posterior parapods with normal and 3-4 wide claws.
Bethbilbeckia, erected for B. floridensis, belongs to the tribe Macropelopiini and resembles Macropelopia in all life stages. The location of the ring organ of maxillary palp in the basal third, the yellow-brown ligula with inner teeth directed forward, dorsomental plates with 6 (5 large, 1 small) teeth each and with medial extension that nearly reaches the pseudoradula, may allow distinction (Epler, 2000).
Larvae of B. floridensis occur in cool seeps, springs and small streams over a wide range of elevations and geographic regions in eastern North America from Ohio (Bolton 1992) to Florida. An undescribed prepupal larva from eastern North Carolina differs from B. floridensis in the pupal respiratory organ structure (C. Watson pers. comm). Because of their restricted habitat, larvae may be overlooked in routine biomonitoring of larger streams, and because of larval similarity to other Macropelopiini.