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Upper Midwest Environmental Sciences Center

Farm ponds as critical habitats for native amphibians
A Field Guide to Amphibian Larvae and Eggs of Minnesota, Wisconsin, and Iowa
Field guide contents

How to use the keys

Most amphibians lay their eggs in clusters or strings. Four general types of amphibian egg masses are illustrated (Figure 1); globular clusters are large and easier to see than eggs in long strings, whereas eggs laid singly or in small clusters are difficult to find, especially in dense

To use the circular keys (Figures 2, 3, and 4), start in the center and choose among the available options, ending in the outer circle with the identification of a species or group of species. In some instances, dashed lines are used if a species exhibits multiple traits. These traits are described to the left and right of the section enclosed by the dashed line. After making an identification, use the species description to confirm the identification. If the identification seems in error (e.g., the collection location is geographically distant from the range of that species, the specimen looks nothing like the illustration, or the description doesn’t match the specimen), return to the key and work backwards to find another species that more closely fits your specimen.

The jellies of most amphibian eggs are very small when first laid, but quickly swell with water. All amphibian eggs in this key are pigmented and have one or several gelatinous envelopes surrounding them (Figure 2). The gelatinous capsules are difficult to see, but it helps to use a magnifying lens and adjust the lighting. The egg key groups most ranids together, and mole salamanders also have similar eggs.

Salamanders (Figure 3) have four limbs and feathery external gills, whereas tadpoles (Figure 4) have four limbs only when they are close to metamorphosis and their gills are internal. The location of the eyes on the tadpole is an important character separating the treefrogs (hylids) from the true frogs (ranids). Eyes dorsal means the eyes do not interrupt the lateral margin of the head when viewed from above (superior view; Figure 4a). Another early characteristic in the key is whether the vent is medial or dextral (Figure 4b). This is difficult to see on a squirming live tadpole; however, hold it with the bottom (ventral) side up, straighten the tail fin, and look carefully at the location of the opening of the vent. A magnifying lens will help.

The scale bar by each drawing in the species description indicates 1 cm. Drawings of typical individuals are presented, but all specimens will not look exactly like the drawings because of natural variation. The size ranges (TL = total length from tip of snout to tip of tail, SVL = snout to vent length) may help rule out some larvae. The range maps were produced from information at Minnesota and Wisconsin DNR web sites (http://www.dnr.state.mn.us; http://www.dnr.state.wi.us) as well as from Christoffel et al. (2001) and Casper (1996) for Wisconsin, Oldfield and Moriarty (1994) and J. Moriarity (personal communication) for Minnesota, and Christiansen and Bailey (1991) and J. L. Christiansen (personal communication) for Iowa. The range maps are color-coded as follows: dark green indicates the current range of the species and red indicates the former range of the species (range contraction). Common and scientific names follow Crother (2000).

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URL: http://www.umesc.usgs.gov/terrestrial/amphibians/field_guide/use the keys.html
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Page Last Modified: December 29, 2010