56 
Gary L. Paukstis and Lauren E. Brown 
in other genera dig backwards by scraping soil with their hind feet 
(Emerson 1976). The most conspicuous morphological adaptations for 
this type of burrowing are well-developed metatarsal tubercles on the 
undersides of the hind feet. Thus, P. ornata and P. streckeri are quite 
different from their relatives, the hylid treefrogs, and also from other 
burrowing salientians in more distantly related groups. 
Brown et al. (1972) showed experimentally that P. streckeri had a 
strong preference for digging in sand and that the species was unable to 
burrow in more resistant black prairie sod because of its unusual 
method of digging. This helps explain the restriction of P. streckeri in 
the Midwest to areas with sandy soils (Smith 1966, Brown and Brown 
1973, Axtell and Haskell 1977). Brown and Means (1984) also found 
that P. ornata is associated with easily penetrated sandy soils. An exper- 
iment conducted by Brown (1978) demonstrated that P. streckeri can 
feed while underground, and it is probable that this is one of the main 
reasons why forward burrowing evolved in P. streckeri instead of back- 
ward burrowing. No other species of salientian is known to feed under- 
ground. Pseudacris ornata and P. streckeri have rarely been found on 
the surface except during their breeding seasons, and both species emit 
vocalizations underground that may be of an aversive nature (Brown 
1978 and unpubl. data, Brown and Means 1984, Smith 1966). 
The other species of Pseudacris — P. brachyphona, P. brimleyi, P. 
clarki, P. nigrita, and P. triseriata — live on the surface of the ground 
and occasionally climb on low vegetation. These species, which have not 
been observed to burrow, have narrow limbs and thin fingers with small 
toe pads. Thus, the subjects of our research are three groups of frogs in 
the family Hylidae that differ considerably in behavior, habitat, and 
gross morphology. 
An extra element termed the intercalary cartilage occurs between 
the terminal and subterminal phalanges of the digits of members of the 
family Hylidae. Although the intercalary cartilage is probably of con- 
siderable importance for arborealism, relatively little has been published 
on its structure. Publications of greatest significance that deal with this 
subject include Howes and Davies (1888), Noble and Jaeckle (1928), 
Noble (1931), Savage and De Carvalho (1953), and Tyler and Davies 
(1978). These studies examined a number of species of frogs in the fam- 
ily Hylidae, but none considered the intercalary cartilage of Pseudacris . 
The objective of our study was to compare the intercalary cartilages 
of the digits of the hylid treefrogs, the more terrestrial species of Pseu- 
dacris that are not known to burrow, and the forward-burrowing spe- 
cies of Pseudacris. Primary emphasis was placed on determining how 
the specialized intercalary cartilage in the family Hylidae was modified 
through evolution to facilitate forward burrowing. 
MATERIALS AND METHODS 
Histological preparations were made of the digits of 122 alcohol- 
preserved specimens of Pseudacris: 16 P. brachyphona, 8 P. brimleyi, 8 
