A Field Guide to Amphibian Larvae and Eggs 

of Minnesota, Wisconsin, and Iowa

U.S. Department of the Interior

U.S. Geological Survey

Information and Technology Report

USGS/BRD/ITR–2002-0004
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Suggested citation: Parmelee, J. R., M. G. Knutson, and J. E. Lyon. 2002. A field guide to amphibian larvae and eggs 

of Minnesota, Wisconsin, and Iowa. U.S. Geological Survey, Biological Resources Division, Information and 

Technology Report USGS/BRD/ITR-2002-0004, Washington, D.C. iv + 38 pp.

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Front cover photo of Tiger Salamander eggs by Shawn Weick.

Back cover photo of Wood Frog by Allen Sheldon, Wood Frog eggs by Melinda Knutson.

Mention of trade names or commercial products does not constitute endorsement or recommendation for use by the 

U.S. Department of the Interior, U.S. Geological Survey.
A Field Guide to Amphibian Larvae and Eggs 

of Minnesota, Wisconsin, and Iowa

By Jeffrey R. Parmelee, Melinda G. Knutson, and James E. Lyon

Illustrations by Pearl Podgorniak

Information and Technology Report 2002-0004

June 2002

U.S. Department of the Interior

U.S. Geological Survey

Washington, D. C. 20240
About the authors

Jeff Parmelee is an assistant professor of biology at Simpson 

College in Indianola, Iowa, where he teaches herpetology, anatomy, 

and general biology. He obtained his Ph.D. from the University of 

Kansas in 1998 and his research interests include tropical herpetology 

and rattlesnake ecology (parmelee@simpson.edu). Melinda Knutson 

is a wildlife biologist at the USGS in La Crosse, Wisconsin, where 

she studies amphibians and birds and their habitats. She obtained her 

Ph.D. from Iowa State University in 1995 and her research interests 

include conservation biology and landscape ecology (melinda_

knutson@usgs.gov ; http://www.umesc.usgs.gov/). James Lyon is a 

biologist and naturalist with special interests in amphibians, reptiles, 

birds, and native plants. Pearl Podgorniak created the drawings of larvae 

and eggs for this publication as an undergraduate research assistant with 

Dr. Parmelee, supported by Simpson College. 
Contents

Title ............................................................................................................................................ 1

How to use the keys................................................................................................................... 3

Preserving amphibian eggs and larvae....................................................................................... 4

Raising larvae............................................................................................................................. 4

Field key to Amphibian Eggs .................................................................................................... 8

Field Key to Larval Salamanders............................................................................................. 10

Field Key to Larval Anurans.................................................................................................... 12

Species descriptions................................................................................................................. 14

Blue-spotted Salamander (Ambystoma laterale)............................................................... 14

Spotted Salamander (Ambystoma maculatum).................................................................. 15

Small-mouthed Salamander (Ambystoma texanum).......................................................... 16

Tiger Salamander (Ambystoma tigrinum) ......................................................................... 17

Four-toed Salamander (Hemidactylium scutatum)............................................................ 18

Mudpuppy (Necturus maculosus) ..................................................................................... 19

Eastern Newt (Notopthalmus viridescens) ........................................................................ 20

Eastern Red-backed Salamander (Plethodon cinereus) .................................................... 21

Northern Cricket Frog (Acris crepitans) ........................................................................... 22

American, Canadian, Great Plains, Woodhouse’s, and Fowler’s Toads 

 (Bufo americanus/B. hemiophrys/B. cognatus/B. woodhousii/B. fowleri) ................. 23

Gray and Cope’s Gray Treefrog (Hyla versicolor/H. chrysoscelis).................................. 25

Spring Peeper (Pseudacris crucifer) ................................................................................. 26

Western Chorus and Boreal Chorus Frog (Pseudacris triseriata/P. maculata)................. 27

Leopard, Pickerel, and Crawfish Frogs 

 (Rana pipiens/R. sphenocephala/R. blairi/R. palustris/R. areolata) .......................... 28

American Bullfrog (Rana catesbeiana) ............................................................................ 30

Green and Mink Frogs (Rana clamitans/R. septentrionalis)............................................. 32

Wood Frog (Rana sylvatica).............................................................................................. 34

Plains Spadefoot (Spea bombifrons) ................................................................................. 35

Acknowledgments.................................................................................................................... 36

References................................................................................................................................ 36

1

 Apparent worldwide declines in amphibian 

populations (Pechmann and Wake 1997) have 

stimulated interest in amphibians as bioindicators 

of the health of ecosystems. Because we have 

little information on the population status 

of many species, there is interest by public 

and private land management agencies in 

monitoring amphibian populations. Amphibian 

egg and larval surveys are established methods 

of surveying pond-breeding amphibians. Adults 

may be widely dispersed across the landscape, 

but eggs and larvae are confined to the breeding 

site during a specific season of the year. Also, 

observations of late-stage larvae or metamorphs 

are evidence of successful reproduction, which 

is an important indicator of the viability of the 

population. The goal of this guide is to help 

students, natural resources personnel, and 

biologists identify eggs and larval stages of 

amphibians in the field without the aid of a 

microscope. 

 Anyone undertaking field identification 

of amphibians has a responsibility to avoid 

harming the amphibians or their habitats. 

Persons planning to sample amphibians should 

work in cooperation with state or federal wildlife 

professionals. Lack of knowledge about sensitive 

habitats or populations could result in the spread 

of diseases, damage to breeding habitats, or local 

reproductive failure of amphibian populations. 

State and federal laws protect amphibians from 

exploitation. Collection permits are required 

from the appropriate state or federal authorities 

before capturing, handling, or collecting 

amphibians. Permission for sampling should 

also be obtained from the landowner. 

A Field Guide to Amphibian Larvae and Eggs 

of Minnesota, Wisconsin, and Iowa

By

 Jeffrey R. Parmelee

Simpson College

Department of Biology

701 North C Street

Indianola, Iowa 50125

and 

Melinda G. Knutson and James E. Lyon

U.S. Geological Survey

Upper Midwest Environmental Sciences Center

2630 Fanta Reed Road

La Crosse, Wisconsin 54603
2

 Research for this publication began with a 

study of farm ponds in southeastern Minnesota. 

We quickly recognized the need for a way to 

identify eggs and larvae of the amphibians 

encountered. We used our field knowledge of 

most of the species included here to produce 

this field guide. Additional information came 

from published keys for Wisconsin amphibians 

(Vogt 1981; Watermolen 1995; Watermolen and 

Gilbertson 1996) and descriptions from a variety 

of sources (e.g., Johnson 2000; Petranka 1998; 

Harding 1997; Russell and Bauer 1993; Stebbins 

1985; Wright and Wright 1949).

 There is no simple way to morphologically 

distinguish between many amphibian species 

during the egg and tadpole stage. This might 

be expected in closely related species such as 

the gray treefrogs Hyla chrysoscelis and Hyla 

versicolor, which vary in chromosome number 

and in subtle morphological traits. It is surprising 

that, to our knowledge, no one has reported a 

way to morphologically distinguish between 

larvae of several common frog species such as 

the Crawfish Frog and Southern Leopard Frog, 

which bear little resemblance to one another as 

adults. Even with the technical keys available to 

herpetologists, amphibian eggs and larvae are 

often difficult to identify to species. An excellent 

key to all species in the United States (Altig et 

al. 1998) is available on the internet (http:

//www.pwrc.usgs.gov/tadpole/). The Altig key 

(along with earlier versions such as Altig [1970] 

for tadpoles and Altig and Ireland [1984] for 

salamander larvae) requires detailed knowledge 

of the anatomy of amphibian larvae. Altig et 

al. (1998) requires examination of tadpole 

mouthparts, which are only observable with dead 

specimens and the aid of a microscope. Tadpole 

biology is summarized by McDiarmid and Altig 

(1999). Additional research is needed to better 

describe and understand larval amphibians.

 Three major factors make amphibian larvae, 

especially tadpoles, difficult to identify. First, 

many species are conservative in their anatomy 

and members of the true frog (Ranidae) and toad 

(Bufonidae) families are very similar in larval 

form. Second, morphology among tadpoles of 

a single species can vary geographically and 

with developmental stages. Third, larvae reveal 

differing anatomies depending on the physical 

and biotic environment where they are found. 

The proportions, coloration, and anatomy of 

larvae change as they grow from hatchlings to 

metamorphs. Larvae in early stages frequently 

do not show the characteristics necessary for 

their identification.

 In one description, we summarize closelyrelated species that cannot be distinguished 

without the aid of a microscope. We believe 

that the weakness of specific identification 

is balanced by ease of use in the field. In 

many instances, identification to species is 

not necessary to meet land managers’ goals. 

However, professional herpetologists working 

for universities, museums, and state and federal 

agencies can assist with identifying larvae to 

species. 

 No key by itself can help identify every 

specimen; there will always be variant 

individuals. In identifying anurans, for example, 

knowledge of which species were calling at the 

pond may help sort out identification questions. 

It also is helpful to note the geographic 

distribution of species; you can reasonably rule 

out Canadian Toads if the site is in southern 

Iowa. Ruling out a species by geography is not 

easy or reliable, however, if your survey site is 

near the edge of a species’ range. You could 

also return several times to the breeding site to 

observe late-stage metamorphs. Amphibians 

often are easier to identify once they develop 

mature traits. If a specimen cannot be identified 
3

using this key, preserve the specimen and attempt 

identification using a more detailed key such as 

Altig et al. (1998). If still not confident in the 

identification, contact the senior author of this 

key or another herpetologist in your area for 

assistance. Another approach, if adequate time 

and facilities are available, is to raise a few 

eggs or larvae to metamorphosis and identify 

the late-stage metamorphs or adults from their 

mature characteristics. We anticipate that rapid 

molecular DNA techniques suitable for field 

identification will be available within the next 

few decades. Until then, keys will be needed. 

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 

vegetation. 

 To use the circular keys (Figures 2–4), 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) 
4

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).

Preserving amphibian eggs and larvae

 Samples of amphibian eggs and larvae 

can easily be preserved to make a voucher or 

reference collection or to send to a specialist 

for positive identification. Most states require 

collection permits issued by the state Department 

of Natural Resources or other similar agency. 

Remember to observe all wildlife laws and 

only collect where it is legal and where the 

collection of a few individuals will not affect 

the population. 

 Larvae should be anesthetized according to 

the procedure recommended by Green (2001). 

There is no perfect preservative and techniques 

for preserving specimens are still debated 

(McDiarmid and Altig 1999). We recommend 

preserving amphibian eggs and larvae by 

placing them in a small vial filled with a 10% 

formalin solution. Alcohol is more pleasant 

to work with and safer than formaldehyde, 

but tends to dehydrate specimens. Whatever 

preservative you use, read the relevant Material 

Safety Data Sheets (MSDS) to learn how to 

safely handle and store that chemical. Larvae 

can be placed individually, or as a lot of 5-20 

individuals, in screw top vials. Do not place too 

many individuals in one container. Immediate 

labeling is a must; use pencil or indelible ink on 

all submerged tags. Field tags should be linked 

to corresponding field notes; labels with detailed 

information must be kept with the specimens. 

Do not rely on memory as a record of locality, 

date, and habitat information. The minimum 

information includes date, locality (kilometers 

from a crossroad or other landmark, or GPS 

coordinates), habitat description, and name of 

the collector. We recommend maintaining a 

numbered log that links to tags on the vials. 

Other important information includes notes on 

live coloration (specimens quickly lose color in 

preservative). Specimens should be deposited in 

a museum or university collection where they 

can be appropriately cataloged, maintained, and 

available for researchers worldwide.

Raising Larvae

 If a specific identification cannot be 

accomplished in the field, raising amphibian 

eggs or larvae to metamorphosis is one 

alternative. To appropriately provide for the 

needs of captive larvae, consult Mattison (1993). 

We provide a general description of larval care 

here to familiarize the reader with the procedures 

involved. 

 Raising amphibian larvae in an aquarium is 

not difficult, but avoid overcrowding (raise only 

one or two animals per ~4 liters [~1 gallon] of 

water). Water quality is the most important factor. 

Treat the water as you would for aquarium fish 

or collect natural pond water or rain water. Tap 

water contains chlorine, which will kill larvae 

as well as many fish. Water can be effectively 

de-chlorinated by letting it stand uncovered for 

several days. Change the water every few days 

or as needed. An aquarium or large, shallow 

container is preferred; the larger the surface area 

the more oxygen will be available. An airstone 

is usually not necessary. Do not clean the tank 

with soap or other cleansers because residues 

may kill the animals.

 Salamander larvae are carnivorous and must 

be fed an animal diet. Smaller salamanders can 

be fed “zooplankton” (small invertebrates) netted 

out of streams or ponds. Larger salamander 
5

larvae can be fed small worms (earthworms or 

tubifex), tadpoles, feeder fish, and even pieces of 

liver if you wriggle it in front of their mouths. If 

you feed them liver, remember to not leave any in 

the container, as it will quickly foul the water.

 Frog and toad tadpoles can be raised on a 

diet of fish flakes or rabbit pellets. Boiled lettuce 

or spinach, naturally collected algae (be careful 

not to introduce predaceous invertebrates such 

as dragonfly larvae or diving beetles), and small 

amounts of cooked egg yolk can also be fed to 

tadpoles. Offer these foods daily, but be sure to 

replace the aquarium water frequently.

 The length of time required for metamorphosis 

varies from several years for American Bullfrogs 

to a few weeks in the Plains Spadefoot. Some 

Tiger Salamanders never metamorphose and 

remain aquatic. Most treefrogs and toads take 

about two months to develop from eggs to 

metamorphs. Salamander metamorphosis is less 

dramatic than frog metamorphosis. Salamanders 

absorb their gills and any tail fins when they 

metamorphose. Because amphibians are 

ectothermic, the temperature of the environment 

affects their development time. Raise larvae at 

room temperature or slightly warmer. Light will 

help maintain the algae that tadpoles feed on; 

direct sunlight may cause overheating.

 If your tadpoles are healthy and developing 

as expected, you will notice the appearance of 

hind limbs, then a reshaping of the body, and 

finally, front limb emergence. At this stage, the 

tadpole will begin to absorb its tail and many 

changes will occur both internally and externally 

as it develops into a juvenile frog. Feeding will 

cease at some point; the metamorphosing 

frog is quite vulnerable at this stage. It can no 

longer swim well, but is not yet able to survive 

on land. Provide a solid surface area for the 

metamorphosing animals to climb onto and 

cover the container to prevent escape. Tilting 

the container is an easy way to offer both land 

and water. You may now be able to identify the 

frog from the developing adult characteristics. 

If you need to continue to raise your larvae for 

identification or other purposes, you should feed 

the froglet or newly-terrestrial salamander tiny 

insects such as pinhead crickets, wingless fruit 

flies, or bloodworms, which can be purchased 

in pet stores. 

 To prevent the spread of disease to native 

populations, any frogs or salamanders you 

raise should not be released back into the 

environment. Lab-raised amphibians can be 

anesthetized and euthanized with benzocaine 

or tricaine methanesulfonate (MS 222, Green 

2001). If you anticipate difficulty complying 

with this guidance, you should not undertake 

raising larvae in captivity.
6

a

b

Figure 1. Egg masses of amphibians; (a) a thin raft of eggs (e.g., Green Frog), (b) a globular cluster 

of eggs (e.g., Leopard Frog), (c) eggs laid singly or in small clusters (e.g., Chorus Frog), (d) eggs laid 
7

c

d

in strings (e.g., toads). 
8

Figure 2. Field key to amphibian eggs. 
9

Figure 2a. Frog and toad eggs. Dark lines represent 

distinct egg envelope borders, gray lines represent 

indistinct borders.
10

Figure 3. Field key to larval salamanders. 
11

Figure 3a. Tail fin and costal grooves.

Figure 3b. Mole salamander digits (front feet shown).

Figure 3c. Tail fin extending onto body (Four-toed Salamander).
12

Figure 4. Field key to larval anurans. 
13

Figure 4a. Eye location (dorsal view).

Figure 4b. Vent location (ventral view).

Figure 4c. Tadpole anatomy (lateral view).
14

Species Descriptions

Blue–spotted Salamander Ambystoma laterale

Status: Wisconsin – Common

 Minnesota – Common

 Iowa – Endangered

Size at hatching, 8 – 10 mm total length; at metamorphosis, ~ 34 mm snout–vent length 

1 cm

 The Blue-spotted Salamander 

is one of four mole salamanders in 

our region. Other mole salamanders 

include the Spotted, Small-mouthed, 

and Tiger Salamanders. As adults, mole 

salamanders live under cover objects such 

as rotting logs or in burrows in the forest 

floor (Parmelee 1993). In early spring 

(March and April), adults migrate to 

temporary ponds to breed. Larvae develop 

during spring and summer and usually 

metamorphose in the fall. 

 The Blue-spotted Salamander is a woodland species of northern 

North America. The eggs are laid in small clumps (7–40 eggs) attached 

to vegetation or debris at the bottom of ponds. The larvae are similar to 

Spotted Salamanders but are more darkly colored with the fins mottled 

with black, and dark blotches on the dorsum. Blue-spotted Salamanders 

metamorphose at about the same size as Spotted Salamanders but are 

darker, sometimes with flecks of blue. 
15

Spotted Salamander Ambystoma maculatum

Status: Wisconsin – Locally abundant

 Minnesota – Status to be determined 

Size at hatching, 12 – 17 mm; at metamorphosis, 49 – 60 mm total length

1 cm

 The Spotted Salamander is present only 

in the northeastern part of our range (where 

it is sympatric with up to two other mole 

salamanders). Females deposit eggs in a 

firm oval mass (60–100 mm in diameter) 

attached to vegetation near the surface 

of the water. The eggs (1–250 per mass) 

are black, but the egg mass may be clear 

or milky, with a greenish hue because of 

symbiotic algae. The larvae have a light 

chin and throat with a dull dorsum and tail 

fin lacking blotches. Spotted Salamanders gain their spots shortly after 

metamorphosis.
16

Small-mouthed Salamander Ambystoma texanum

Status: Iowa – Declining 

Size at hatching, 7 -14 mm; at metamorphosis, 48 – 61 mm total length

1 cm

 The Small-mouthed Salamander is found 

in the southern third of Iowa. The adult of this 

species resembles its common name in having 

a relatively smaller head and mouth compared 

with other mole salamanders. The eggs are laid 

singly or in small loose clusters on vegetation 

or debris in ponds (rarely in streams). Larger 

masses may be sausage-shaped. The larvae 

are similar to other mole salamanders but have 

pigmentation on their chin. They are a uniform 

brown after metamorphosis.
17

Tiger Salamander Ambystoma tigrinum

Status: Wisconsin – Common

 Minnesota – Common

 Iowa – Common

 Size at hatching, 13 – 17 mm; at metamorphosis, 

 7.5 – 12.5 cm, sometimes reaching 24 cm total length

1 cm

 The Tiger Salamander has the widest 

distribution of any salamander in the United 

States (Conant and Collins 1991). It tolerates 

human disturbance and survives in agricultural 

regions. Adults migrate to breeding ponds 

very early in the spring (March), as soon as 

ponds are free of ice at northern latitudes. 

Eggs are laid singly or in masses of 18–110 

eggs. Tiger Salamanders use any fish-free 

body of water from ponds to cattle tanks. 

Fresh egg masses are firm (6–7 cm diameter) 

and older egg masses are flimsy. In older egg masses, you can identify 

the developing embryos as salamanders, not frogs, because they have 

external gills and an elongated shape. The eggs of Tiger Salamanders 

are large, 2–3 mm in diameter, with three gelatinous envelopes.

 The digits of larval Tiger Salamanders are quite different than the 

other three species of mole salamanders. They are flattened and pointed 

from base to tip (Figure 3b). Late-stage Tiger Salamander larvae reach 

> 100 mm total length versus < 61 mm for the other mole salamanders.
18

Four–toed Salamander Hemidactylium scutatum

Status: Wisconsin – Special Concern

 Minnesota – Endangered

 

Size at hatching: 11 – 15 mm. Size at metamorphosis: 17 – 25 mm total length 

1 cm

 As the name suggests, this salamander 

has only four toes on the hind feet instead 

of five as in other salamanders (except the 

Mudpuppy). This species is in the large family 

of lungless salamanders, many of which lay 

their eggs on land and do not have a freeswimming larval stage. Four-toed Salamanders 

usually lay their eggs in moss near the edges 

of bogs. The female will stay with the eggs, 

guarding them. After the eggs hatch, the 

larvae drop into the water and spend the next 

several months as free-swimming larvae before 

metamorphosing into terrestrial adults. The larvae also have four toes, 

but are much smaller than Mudpuppy larvae and have a dorsal fin that 

extends onto their body (Figure 3c).
19

Mudpuppy Necturus maculosus

Status: Wisconsin – Locally abundant, but possibly in decline

 Minnesota – Uncommon 

 Iowa – Uncommon 

Size at hatching, 21 – 25 mm; adult features appear at 13 – 15 cm total length

1 cm

 The Mudpuppy is a paedomorphic 

species; it retains larval characteristics in its 

adult form. Mudpuppies are only found in 

large lakes and rivers. The eggs are easily 

identified, laid in nests constructed by 

females on the underside of cover objects 

such as rocks, boards, and other sunken 

debris. The eggs are suspended individually 

in an area 15–30 cm in diameter; each egg 

is approximately 5–6.5 mm in diameter. The 

larvae are striped, with large external gills, 

four toes on the hind feet, and a dorsal fin extending only onto the tail, 

not onto the body (Figure 3a). Mudpuppies remain in the larval stage for 

several years and never acquire a terrestrial form.
20

Eastern Newt Notopthalmus viridescens

Status: Wisconsin – Common

 Minnesota – Common

 Iowa – Endangered

Size at hatching, 7 – 9 mm; at metamorphosis, 21 – 38 mm total length 

1 cm

 The Eastern Newt is widely distributed 

over the eastern United States and is found 

in ponds in its aquatic form. The female lays 

the eggs singly, attaching them to sticks, 

leaves, and stems in the pond. The eggs 

have a tough, rubbery jelly, are often oval, 

and have three envelopes with a sticky outer 

envelope that attaches to vegetation. The 

larvae are unique among our salamanders in 

that they lack costal grooves (Figure 3a) and 

have a dark stripe through the eye. They also 

have yellow spots along the dorsum.
21

Eastern Red-backed Salamander Plethodon cinereus

Status: Wisconsin – Locally common

 Minnesota – Locally common

| 

Hatchling size, 2 cm total length; adults, 6-10 cm total length

(Drawing of a female attending her eggs)

_____ _____ 1 cm | 

 This species is our only amphibian that 

foregoes the aquatic larval stage, laying 

terrestrial eggs that develop directly into 

juvenile salamanders. The eggs can be 

distinguished from the Four-Toed Salamander 

because there will be an attending female 

Eastern Redback Salamander with her eggs. 

Three to 17 eggs are laid in rotten logs, 

or in damp soil under rocks or logs. The 

eggs are laid individually but in a cluster 

in subterranean cavities, usually naturally 

occurring cracks and crevices. The female 

remains coiled around the egg cluster until they hatch. The young hatch 

in about two months and are approximately 2 cm long.
22

Northern Cricket Frog Acris crepitans

Status: Wisconsin – Endangered

 Minnesota – Endangered

 Iowa – Declining

2.7 – 4.4 cm total length

1 cm

 Northern Cricket Frogs lay eggs singly or 

in loose clusters of 7–40 eggs near the surface 

of streams and other bodies of water. Cricket 

Frogs have distinctive tadpoles when their 

tail tips are black. No other tadpole will have 

such an obvious black tail in contrast to the 

rest of the tail and body. This trait may or may 

not be present depending on conditions. For 

example, tadpoles that developed in ponds with 

predaceous dragonfly larvae had black tails 

(directs attacks away from the body), whereas 

those developing in lakes or streams with fish 

had clear tails (less visible; Caldwell 1982). Tadpoles of this species can 

also be identified by their eyes, which are located between dorsal and 

lateral, and the bands of pigment that lie along the dorsal edge of the tail 

musculature.

 Northern Cricket Frogs are the least arboreal of the tree frog family 

in our area. The adults are found along muddy banks of streams, ponds, 

and lakes. There is considerable concern over the disappearance of this 

species in the northern parts of its range (indicated in red). A range 

contraction has occurred in the last few decades and the cause is under 

investigation (Lannoo 1998; Hay 1998). There is some evidence that this 

species is more susceptible to UV radiation than the leopard frogs, toads, 

and other tree frogs (Van Gorp 2002).
23

American, Canadian, Great Plains, Woodhouse's, and 

 Fowler's Toads 

 American Toad Bufo americanus

 Canadian Toad Bufo hemiophrys

 Great Plains Toad Bufo cognatus

 Woodhouse's Toad Bufo woodhousii 

 Fowler's Toad Bufo fowleri

 Status: B. americanus – Common to locally abundant in Wisconsin, 

 Minnesota, and Iowa.

 B. hemiophrys – Locally abundant in Minnesota

 B. cognatus – Locally abundant in Minnesota and Iowa

 B. woodhousii – Locally abundant in Iowa

 B. fowleri – Locally abundant in Iowa

(American toad tadpole) 1.8 – 2.4 cm total length

1 cm

 Toad eggs and tadpoles can be found in almost any aquatic situation, 

from muddy farm ponds to clear swampy areas, and are distinctively 

different from those of other frogs. Toads lay their eggs in long strings 

that may be benthic or entwined in (but not attached to) vegetation. 

You should be able to distinguish the eggs of any species of toad based 

on site location and characteristics of the envelope surrounding the 

eggs. Canadian Toads are found in northwestern Minnesota and have a 

single tubular envelope enclosing the eggs. The American Toad has a 

double envelope surrounding the eggs, with eggs separated by partitions. 

Woodhouse’s and Fowler’s Toads, are found in western and southeastern 

Iowa and have a single tubular membrane not separated by partitions. 

Great Plains Toads are found in western Iowa and Minnesota and have a 

scalloped egg string. 

 The larvae of different toad species are very similar. Toad tadpoles 

are small and black or dark brown, often with some bright metallic spots. 

(continued next page)
24

Toad Range Maps

B.americanus B. hemiophrys

B. cognatus B. woodhousii

 (western Iowa)

B. fowleri

 (eastern Iowa)

They have a clear fin that is rounded at the tip. The eyes are dorsal and the 

vent is medial. Metamorphosing toads quickly acquire their unique warty 

skin and squatty shape. 
25

Gray and Cope's Gray Treefrogs

 Gray Treefrog Hyla versicolor

 Cope's Gray Treefrog Hyla chrysoscelis 

 Status: Wisconsin – Common

 Minnesota – Common

 Iowa – Common

 

3.2 – 3.8 cm total length

1 cm

 Gray Treefrogs and Cope’s Gray Treefrogs 

cannot be distinguished based on morphology; 

herpetologists use calling rate or chromosomes for 

identification (Oberfoell and Christiansen 2001). 

Differences in eggs and tadpoles are unknown. 

The adults spend most of the year in trees and 

vegetation and descend to small ponds to breed 

in May–June. They lay clumps of 30–40 light 

brown eggs attached to floating vegetation near 

the surface. The outer jelly layer is weak and 

indistinct. Tree frog tadpoles are striking, with 

high (1.5 times depth of tail musculature) tail fins that often have a red-orange 

tinge (this color quickly disappears in preservative). Their tails usually end 

in a distinct flagellum. Eyes are lateral, and older larvae have very heavily 

pigmented tail fins with the pigment covering both the fins and musculature. 

Metamorphs have smooth skin and large toe pads.
26

Spring Peeper Pseudacris crucifer

Status: Wisconsin – Common

 Minnesota – Common

 Iowa – Common adjacent to Mississippi River

~ 3.4 cm total length

1 cm

 Spring Peeper adults are associated with 

forested or brushy areas. Peepers lay eggs 

in shallow water singly or in small clusters 

(2-3 eggs) attached to dead leaves, sticks, and 

grasses. Spring Peeper tadpoles are highly 

variable. They have lateral eyes and the 

pigmentation of the tail can be heavy or almost 

non-existent. When there is pigmentation on 

the tail it is concentrated along the outer edge 

and there is a clear area near the musculature.
27

Western Chorus and Boreal Chorus Frogs

 Western Chorus Frog Pseudacris triseriata

 Boreal Chorus Frog Pseudacris maculata 

 Status: Wisconsin – Common

 Minnesota – Common

 Iowa – Common

2.6 – 3.7 cm total length

1 cm

 The Western Chorus Frog and the Boreal 

Chorus Frog are difficult to distinguish and 

have overlapping ranges (Platz 1989). The 

exact range of the Boreal Chorus Frog has 

not been determined. Differences in eggs 

and larvae are unknown and we treat them 

together in this account. The Chorus Frog 

is often one of the first frogs to call and lay 

eggs in the spring. They breed in ponds and 

smaller bodies of water and lay eggs in loose, 

irregular clumps of 5–20 eggs (occasionally 

over 100 eggs) attached to vegetation or debris in shallow water. The 

tadpoles can be confused with other tree frogs such as Spring Peepers, 

which also have lateral eyes and a high tail fin. Chorus Frog tadpoles are 

brownish, black, or gray above and bronze or silvery below, and usually 

have clear fins with a bicolored tail musculature (dark on the dorsal half, 

light on the ventral half).
28

Leopard, Pickerel, and Crawfish Frogs 

 Northern Leopard Frog Rana pipiens

 Southern Leopard Frog Rana sphenocephala

 Plains Leopard Frog Rana blairi

 Pickerel Frog Rana palustris

 Crawfish Frog Rana areolata 

 Status: R. pipiens – Common in Wisconsin, Minnesota and Iowa

 R. sphenocephala – Uncommon and declining in Iowa

 R. blairi – Locally abundant but declining in Iowa

 R. palustris – Special Concern in Wisconsin, 

 uncommon in Iowa and Minnesota

 R. areolata – Extirpated in Iowa (?)

(Northern Leopard Frog tadpole) 4.5 – 8.5 cm total length

1 cm

 We group the eggs and larvae of the ranid species above together 

in this account. Geography and observation of adults may be necessary 

to help with identification. Southern Leopard Frogs are incidental in 

extreme southeastern Iowa and the Crawfish Frog has not been seen in 

extreme southern Iowa in over 60 years (Christiansen 1998). The eggs 

of these species are laid in globular masses attached to vegetation or on 

the bottom in shallow water. The Northern Leopard Frog and Pickerel 

Frog can be distinguished from each other because the Northern 

Leopard Frog has eggs that are black on one side and white on the 

other, whereas the Pickerel Frog has eggs that are brown on one side 

and yellowish on the other. The tadpoles are greenish or brownish with 

dorsal eyes and mottling on the tail. The intestinal coil is at least partly 

visible through the skin on the belly. Pickerel Frog tadpoles have been 

described as green or olive green with large dark blotches on their 

tail musculature and fins. Leopard Frogs are green or yellowish green 

without the large dark blotches.
29

Leopard, Pickerel, and Crawfish Frog Range Maps

R. pipiens R.blairi

R. palustris R. areolata

R. sphenocephala
30

American Bullfrog Rana catesbeiana

Status: Wisconsin – Common

 Minnesota – Range expanding through introductions

 Iowa – Increasingly common

1 cm ~16.2 cm total length

 American Bullfrogs are the largest frogs 

in our area, produce the greatest number of 

eggs, and have very large larvae that usually 

overwinter at the bottom of ponds and 

mature after a couple of years in the tadpole 

stage. They are now found statewide in 

Iowa because of introductions with stocked 

fish. Introductions of American Bullfrogs in 

the western United States have caused the 

decline of native species (Hayes and Jennings 

1986) because adult bullfrogs will eat other 

frogs, as well as other large prey. American 

Bullfrogs are at home along permanent bodies of water and are able to 

sustain populations in the presence of fish predators. American Bullfrogs 

and Green Frogs have either skin toxins or behavioral defenses against 

larval predation (Kats et al. 1988; Werner and McPeek 1994).

 American Bullfrogs lay their eggs in large (may exceed 1 m 

diameter), flat (about one egg deep) floating rafts in June or July. The 

eggs are small (0.12–0.17 mm in diameter) with up to 20,000 eggs in a 

mass. The only other frog with a large floating mass is the Green Frog, 

but this species has smaller eggs (0.10–0.15 mm diameter) and fewer 

of them (< 5,000 eggs) in the mass. American Bullfrogs have only 

(continued next page)
31

one weak jelly layer; Green Frog eggs have two jelly layers per egg. 

American Bullfrog tadpoles get quite large (16 cm or larger) and are 

greenish brown with dorsal eyes. Intestines are visible in small tadpoles, 

but the belly becomes opaque in larger larvae. American Bullfrogs have 

distinct black spots on the dorsum of the body and on the tail, these spots 

are concentrated on the dorsal half of the tail. The spots are round and 

distinct, whereas the Green Frog has fuzzier, less distinct spotting.
32

Green and Mink Frogs 

 Green Frog Rana clamitans

 Mink Frog Rana septentrionalis

 Status: R. clamitans – Common in Wisconsin, Minnesota and Iowa

 R. septentrionalis – Locally abundant Wisconsin and Minnesota

1 cm

 Green Frogs are found near springs, streams, and ponds and Mink 

Frogs are often found among lily pads in ponds, swamps, and streams. 

Adult Green Frogs superficially resemble American Bullfrogs as adults 

but are smaller and have dorsolateral folds along the back. Mink Frog 

adults can be difficult to distinguish from Green Frogs, but mature 

Green Frogs have bands, “tiger stripes,” on their hind legs instead of the 

indefinite patterns of spots and blotches found in Mink Frogs. Also, Mink 

Frogs emit an odor similar to rotten onions when their skin is rubbed 

roughly. Mink Frogs are found only in northern Minnesota and Wisconsin 

within our range. Mink Frog and Green Frog tadpoles are similar, 

although Mink Frogs have been reported to have pink-buff colored spots 

on their tail.

 Mink Frogs lay eggs in loose globular masses (500–3,000 eggs in a 

mass), often in deep water (> 1 m depth). Mink Frogs need cold, welloxygenated water for egg development because the egg jelly is thick, 

and they are not laid on the surface of the water as are Green Frogs and 

American Bullfrogs. Green Frogs lay eggs in a large floating film with 

up to 5,000 eggs. Green Frog and Mink Frog tadpoles have a muscular 

tail with low fins. The belly is completely opaque and the greenish 

bodies have numerous dark blotches consisting of fuzzy dots, dashes, and 

splotches, but not distinct round dots.
33

Green and Mink Frog Range Maps

R.clamitans

R. septentrionalis
34

Wood Frog Rana sylvatica

Status: Wisconsin – Common

 Minnesota – Common

4.2 – 4.8 cm total length 1 cm

 Adult Wood Frogs have the same general 

shape as other members of their genus—but 

are brown instead of green—with a dark 

brown eye mask. They breed early in spring 

and their eggs are often the first ranid eggs 

laid in the year. The eggs (500–3,000) are 

laid in loose, round globular masses (5-12 cm 

diameter) attached to submerged vegetation, 

often near the surface. Many females lay 

their eggs in the same vicinity. Egg masses 

are rather flimsy (although less flimsy than 

Northern Leopard Frog egg masses) and lose their shape when lifted 

from the water. A large amount of clear jelly in the egg mass causes the 

eggs to be widely separated. Larval Wood Frogs have clear tail fins that 

end in a sharp point. The fins are higher than other ranid tadpoles. The 

musculature of the body and tail is colored uniformly brownish with a 

greenish sheen. The belly is usually cream-colored with a cream line 

along the edge of the mouth.
35

Plains Spadefoot Spea bombifrons

Status: Iowa – Locally abundant

4.0 – 4.8 cm total length

 Plains Spadefoots are present in extreme 

western Iowa. Adults spend most of the 

year underground and breed explosively (all 

within a few days or weeks) in small pools 

after heavy spring rains (even depressions in 

agricultural fields) and occasionally in more 

permanent ponds. The eggs and tadpoles 

develop quickly; eggs hatch in about 20 hours 

at 30oC (Justus et al. 1977), and a population 

in Oklahoma required only 13–14 days from 

egg to metamorphosis (King 1960). The eggs 

are laid in a cylindrical or elliptical mass of 10–250 eggs that is attached 

to submerged vegetation. Plains Spadefoot tadpoles have a medial vent, a 

characteristic they share only with toads in our region. The body is dark 

brown or bronze in color, and is large and bulbous (broadest just behind 

the eyes) with mostly clear tail fins. Pigmented “veins” are usually found 

on the clear fins. The spiracle is found lower on the body than our other 

frogs (Figure 4c). Tadpoles may also be rapidly developing “carnivorous 

morphs,” with larger heads and mouths than the typical omnivorous 

forms (Pfennig 1992). A black “spade” will be observable on the hind 

feet well before metamorphosis.
36

Acknowledgments

 Funding was provided through the 

Minnesota Environment and Natural 

Resources Trust Fund as recommended by 

the Legislative Commission on Minnesota 

Resources (LCMR), the USGS Upper 

Midwest Environmental Sciences Center, the 

USGS Amphibian Research and Monitoring 

Initiative (ARMI), and Simpson College. The 

drawings, with one exception, are by Pearl 

Podgorniak from specimens in the collections 

of the University of Kansas, Bell Museum of 

Natural History, or the personal collection of 

Jeff Parmelee. The illustration of the Eastern 

Red-backed Salamander is by Sian Marcone 

from a photo by Erica Crespi.

 Bill Duellman and John Simmons at the 

University of Kansas, and Andrew Simons at 

the Bell Museum lent us specimens in their 

care. We thank R. W. Van Devender for his 

excellent photographs of a larval Mudpuppy, 

Four-toed Salamander, Eastern Newt, and 

Small-mouthed Salamander, which served 

as models for the drawings. We thank Jim 

Christiansen and John Moriarty for helping 

us with the distribution maps of amphibians. 

We borrowed the idea of the circular key from 

Oldfield and Moriarty (1994) and the general 

format from Christoffel et al. (2001). We also 

thank Jerry Cox, Shawn Weick, Josh Kapfer, 

William Richardson, John Moriarty, Carl 

Korschgen, Sam Bourassa, Ben Campbell, 

Joel Jahimiack, and Andy Kimball for 

logistical and field assistance. We also thank 

the private landowners who participated in 

the farm pond research study that led to the 

development of this field guide. 

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39
40
Upper Midwest Environmental Sciences Center

CENTER DIRECTOR

Leslie E. Holland-Bartels

ACTING CHIEF, TERRESTRIAL SCIENCES BRANCH

Thomas W. Custer

REPORT EDITOR

Jerry D. Cox