The characters used in the systematic study of the frogs in this group
are those usually employed in anuran systematics. Of the various
measurements and proportions, the snout-vent length and the relative
size of the tympanum to the eye apparently are of more taxonomic
importance than the others (Table 1). In all of the species the
tympanum is at least partially covered by a heavy, dermal
supratympanic fold, and in some specimens of _H. pachyderma_ the
tympanum is completely obscured. In two species (_H. bistincta_ and
_H. charadricola_) the snout is square, whereas in the other species
it is round.
The fingers are long and slender in _H. crassa_, _pachyderma_, and
_robertsorum_ and somewhat shorter with more webbing in _H. bistincta_
and _charadricola_. Breeding males of _Hyla pachyderma_ have
moderately large nuptial spines; the other species have small spines,
except _H. charadricola_ in which spines apparently are absent. A
well-defined thoracic fold is present in _H. pachyderma_, and a weak
fold is present in _H. robertsorum_; the other species lack folds. In
all species there is an anal sheath; this sheath is longest in _H.
bistincta_, in which the anal opening is directed ventrally at the
level of the lower edge of the thighs.
TABLE 1.--COMPARISON OF CERTAIN MEASUREMENTS AND PROPORTIONS
IN THE SPECIES OF THE HYLA BISTINCTA GROUP. (DATA FROM ADULT
MALES; MEANS ARE GIVEN IN PARENTHESES BELOW THE RANGES.)
A: Snout-vent length
B: Tibia length / Snout-vent length
C: Head width / Snout-vent length
D: Tympanum / Eye
====================================================================
SPECIES | N | [A] | [B] | [C] | [D]
---------------+----+-----------+-----------+-----------+-----------
_H. bistincta_ | 38 | 43.0-53.8 | 0.47-0.52 | 0.32-0.37 | 0.35-0.48
| | (46.3) | (0.49) | (0.34) | (0.42)
| | | | |
_H. | 10 | 35.3-44.4 | 0.50-0.54 | 0.31-0.33 | 0.30-0.37
charadricola_ | | (40.4) | (0.52) | (0.32) | (0.34)
| | | | |
_H. | 26 | 39.9-47.9 | 0.48-0.51 | 0.30-0.36 | 0.36-0.47
robertsorum_ | | (43.1) | (0.49) | (0.32) | (0.41)
| | | | |
_H. pachyderma_| 1 | 39.9 | 0.53 | 0.32 | ...
| | | | |
_H. crassa_ | 1 | 53.7 | 0.50 | 0.33 | 0.28
---------------+----+-----------+-----------+-----------+-----------
Frogs in this group are rather drab in appearance. The dorsal color
varies from dull green to various shades of brown. The most distinct
aspect of the coloration is the different color patterns on the flanks
and posterior surfaces of the thighs. The flanks in all species are
marked with spots or reticulations.
_Hyla bistincta_ differs from other members of the group in having
vocal slits and a distensible vocal sac. Only this species has been
reported to call (Shannon, 1951:473). Insofar as is known, the other
species are mute. Examination of skeletal preparations of _H.
bistincta_, _charadricola_, and _robertsorum_ and X-rays of the other
species shows no notable specific differences in the osteology. Since
the tadpoles of only _H. bistincta_ (Duellman, 1961:47) and _H.
robertsorum_ (Rabb and Mosimann, 1955) are known, larval characters
are of limited use in intra-group systematics.