Both species are highly variable and very difficult to distiguish without capturing them. There is a difference in the relative length of the hind limb, but this is impossible to determine without capturing the frog in areas where the ranges overlap.
There are two subspecies of Southern Cricket Frogs (Florida and Southern) and possibly two subspecies of Northern Cricket Frog (Northern and Blanchard's). Some authorities regard the Blanchard's Cricket Frog as a separate species.
|Northern Cricket Frog (Acris crepitans blanchardi)|
|Southern Cricket Frog (Acris gryllus)|
Although these two species are almost impossible to distinguish physically, there is a subtle but consistent difference in their calls. Both species make a "click-click-click" call that has been likened to two pebbles being hit together. At first, those two calls also sound very similar. However, by listening carefully, you can learn to distinguish the two species.
Here is a recording of a "Blanchard's" Cricket Frog (Acris crepitans) from Texas. Notice how "broad" the clicks are. It sounds like someone hitting two stones together at a slight angle so that they are actually scraping against each other for a fraction of a second.
Contrast that call with the shorter, sharper clicks of the Southern Cricket Frog (A. gryllus), this time from Pasco County, Florida. Once again, it sounds like two stones or marbles striking each other, but this time they are hitting straight on so the contact is shorter in duration.
The following recording contains a single typical "click" recording from a Northern and Southern Cricket Frog. Each recording has been repeated 5 times so you can compare the length of each click. The first 5 clicks are the Blanchard's Cricket Frog (A. crepitans) and the last five clicks are the Florida Cricket Frog (A. gryllus).
Notice that the call of the Northern/Blanchard's Cricket Frog is longer and more complex than the sharp click of the Southern Cricket Frog.
Here is a spectrogram comparing the two individual calls. The first call is a single click from a Northern Cricket Frog (Acris crepitans/blanchardi) and the second is a single click from a Florida Cricket Frog (Acris gryllus dorsalis). Both segments of the spectrogram represent the same time period (0.1 secoonds). What you can see from the comparison is that although the clicks are superficially similar, the click of the Northern Cricket Frog is actually longer in duration. The click of the Southern Cricket Frog is much shorter and sharper.
In some Northern Cricket Frogs, the clicks appear to be two separate sounds a few hundredths of a second apart.