All of us are familiar with them. They are easily available in the ponds, streams, lakes etc. during the rainy season their croaking sounds can be easily heard which disturbs our peace. We are talking about the frogs. Frogs belong to the phylum Chordata and class Amphibia. They have the potential to live on water as well as on land. They are anuran which means that they are tailless and belong to the order Anura. Majority of the frogs possess long hind limbs, webbed digits, short body, protruding eyes and no tail. Frogs have great powers of jumping from here and there which is governed by their short body and long hind legs which help in performing jumping. They have a permeable skin and are semi-aquatic, prefer to live in the humid areas but can easily walk on land. They are oviparous and lay their eggs in ponds and lakes. The larvae that hatch out of the eggs are called tadpoles which are characterized by the presence of gills and they complete their development in water. Adult frogs are carnivorous and feed on gastropods, annelids and arthropods. They are easily noticed during their mating season by their croaking sound.
Frogs are easily available in the tropic and subarctic regions but majority of the species are found in the tropical rain forests. They form the diverse group of vertebrates and about 5,000 species are known till present. Some species are however on the verge of extinction. Frogs are different from the toads as the toads prefer to live in dry habitats and this distinction is made on the basis of convergent evolution. The word frog has been derived from an Old English word frogga which means ‘to jump’.
The order Anura encompasses about 4,810 species under 33 families out of which Leptodactylidae includes 1100 species, Hylidae includes 800 species and Ranidae includes 750 species. 88% of the amphibian species are the frogs. All the members of Anura are frogs but those belonging to the family Bufonidae are considered as true toads. The skin of the frogs is moist and smooth while that of the toads is dry and warty except the fire bellied toad Bombina bombina whose skin is slightly warty. Frogs and toads are broadly classified under three suborders. The first one is Archaeobatrachia, which comprises 4 families of primitive frogs. https://www.marshfasteners.com/ Second is Mesobatrachia, which includes 5 families of more evolutionary intermediate frogs and the third one is Neobatrachia, which is the largest group and contains the remaining 24 families of “modern” frogs,found throughout the world. Neobatrachia is further divided into the Hyloidea and Ranoidea. Some species of frogs can be hybridized easily for example the common edible frog Rana esculenta is the hybrid of Pool frog R.lessonae and the Marsh frog R.ridibunda.
Morphology and Physiology
Frogs are quite peculiar among the amphibians. Among the two groups of amphibians, the salamanders and the caecilians they are unusual as the adults lack tail and their hind legs are adapted more to jumping as compared to walking. Their physiology is similar to other amphibians as oxygen can easily penetrate through their moist skin. This feature enables them to breathe through their skins. The oxygen is easily dissolved in the moist layer of the skin in the form of a film and is passed to the blood from where it is transported later on. The frogs always keep their skin moist. This mode of respiration also makes frogs susceptible to the toxins dissolved in water and is responsible for decline of population of certain frog species. Frogs have long hind legs with elongated ankle bones. Their vertebral column is also short comprising of about ten vertebrae followed by a fused tail bone called as coccyx.
The body size of frogs range from 10 mm to 300 mm. the skin loosely covers the body as there is no connective tissue. The texture of the skin may be smooth, warty or folded. They have three eyelids. One is transparent which protects the eyes underwater and the two are translucent to opaque. They have tympanum on either side of the head which helps in hearing. They have pedicellate teeth. Most frogs have teeth only on the upper jaw and vomerine teeth on the roof of their mouth. The teeth are basically for holding the prey in place from where it is swallowed.
Feet and Legs
The legs and feet vary in structure depending upon the habitat of the frogs whether they live on trees, ground, water and in burrows. The frogs are supposed to be very quick to capture their prey and escape from their predators and must be suitably adapted to their environment. Frogs those living in water have webbed toes but the degree of toes to be webbed is directly proportional to the time spent in water by the frog. The African dwarf frog has completely webbed toes as it is completely aquatic whereas White’s tree frog has half webbed toes. The frogs adapted to arboreal mode of life have toe pads located on the ends of the toes for gripping on the vertical surfaces. The pads are made up of interlocking cells that have very small gap between the adjacent surfaces. When the frog applies pressure on the pads the interlocking cells help in gripping the irregular surface. The small gaps between the cells help in gripping by capillary action and this also allows the frog for maintain grip on the smooth surface also. The ground dwelling frogs lack all the adaptations that are present in the aquatic ad arboreal frogs. They have very small toe pads and very little or no webbing at all. Some burrowing frogs have a toe extension called the metatarsal tubercle which helps in burrowing.