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This article details the important external morphological features used in the identification of Anurans (frogs and toads).

Two of the captured anurans whose photos were used in this article were released shortly after capture and no animal was harmed. The first is an Asian common toad (Duttaphrynus melanostictus) the second is an Indian common bullfrog (Hoplobatrachus tigerinus) and the third is an Indian burrowing frog (Sphaerotheca breviceps). All photos were taken by Omkar Dhavale.

 

external morphology of a toad head

 

 

                                             

                                                                  Duttaphrynus melanostictus (Asian Common Toad)

 

Paratoid Glands -

 

These are large external glands running along from the back of the neck behind the eye, to an extent that could be below the shoulder in some species. Paratoid glands are found on toads and are characteristic of their family (Bufonidae) and are the distinguishing feature between toads and anurans of other families which are commonly called frogs. These contain an alkaloid neurotoxin called bufotoxin which the toad releases as a defense mechanism against predators. This can cause mild to severe irritation when in contact with human skin depending on the species.

 

Tympanum –

 

This is an external organ which transmits sound to the inner ear and protects it from the external environment.

 

Parotoid Glands -

 

Parotoid glands are external glands present on the skin of anurans belonging to family Bufonidae, the toads. It is a characteristic hallmark of this family. The gland produces an alkaloid neurotoxin called bufotoxin and can be mild to severely irritating when it comes into contact with human skin depending on the species.

 

Gular Sac -

 

The gular sac contain swells up with air when the anuran croaks and apart from assisting in vocalizing, is also used as a bright or coloured visual display in some anurans.

 

Nictitating Membrane-

 

The nictitating membrane is a transparent membrane that functions as a third eyelid that can cover the eye of the anuran, and serve to moisten it. It's advantage over the other two eyelids is that the animal can see through it to some extent.

 

Cranial Ridges -

 

Cranial ridges are generally found in toads rather than other anurans. These are slight elevations on the cranium (which is the term used to denote the bony cage that surrounds the and protects the brain). The supra-orbital (above eye), pre-orbital (before eye), and post-orbital (after eye) and the supra-tympanic ridge (above the tympanum) (see photo below) constitute the cranial ridges. There are also other ridges not seen in this photo such as parietal ridge which move inwards towards each other behind the orbits in a few species.

 

Cornification -

 

The hardening and darkening of tissue on the surface of the skin is called cornification. It is mostly seen in toads.  

 

 

external morphology of a toad body

 

                                                                              Duttaphrynus melanostictus (Asian Common Toad)

Bones of the limbs -

The femur, tibio-fibula, humerus, radio-ulna are all bones of the limbs. These terms are often used to describe the parts in taxonomic books.

Trunk -

The trunk is the part of the body that lies below the head and to which the limbs are attached.

Snout -

The length of the head lying beyond the eye to the tip of the canthus rostralis called the snout. The relative length of the snout is important with respect to species identification.

 

external morphology of the body of a frog

 

                      Hoplobatrachus tigerinus (Indian Bullfrog)

Folds of Skin -

Certain anurans will also have folds of skin on the body such as the supra-tympanic fold and interrupted (short broken up) and uninterrupted longitudinal folds.

Canthus rostralis -

This is the tip of the snout region.

Inter-orbital space-

The eye balls are often called the orbits and the space between these is a characteristic that is useful in identification.

Body Colouration -

Body colouration does help with identification to an extent but should never be solely relied upon as there are man species that share similar colouration and many colour variations (called colour morphs) within the same species.

 

morphology of the hind foot of a frog

 

 

Underside of the hind foot of Sphaerotheca breviceps (Indian burrowing frog)



Tubercles on the underside of the hand and foot -

The relative size and shape as well as the relative distance between the tubercles under the hand and foot are also important characters used to identify anurans.

The tubercle farthest away from the beginning of the digit and closest to the tip of the digit is the distal tubercle, prior to that is the penultimate tubercle and prior to that is the antepenultimate tubercle (only found on the 4th digit).

The inner metatarsal tubercle in this photo is highly enlarged as it is used for burrowing and this is a burrowing frog.

Digits -

The relative length of the digits as well as the tips of the digits are important for identification. Many arboreal (living in trees) species have elongated digits with the tips usually being circular/discoid in shape. Presence or absence and the degree of webbing between digits is also important.

 

 

 

external morphology of the forefoot of a frog

 

 

Underside of the manus (or hand) of Sphaerotheca breviceps (Indian burrowing frog)

 

Nuptial pads -

Nuptial pads help the male hang onto the female during mating, they develop during the mating season and are not developed in the individual in the photo.

 

About the Author of this Article

References 

Dutta, S and K, Manamendra-Aracchi . The Amphibian Fauna of Sri Lanka.