15.2: The Life Cycle of Amphibians - Biology

Metamorphosis is a biological process by which an animal physically develops after birth or hatching, involving a conspicuous and relatively abrupt change in the animal’s body structure through cell growth and differentiation (Figure 1). Metamorphosis is iodothyronine-induced and an ancestral feature of all chordates.[1] Some insects, fishes, amphibians, mollusks, crustaceans, cnidarians, echinoderms and tunicates undergo metamorphosis, which is often accompanied by a change of nutrition source or behavior. Animals that goes through metamorphosis are called metamorphoses. Very few vertebrates undergo metamorphosis, but all the amphibians do to some extent.


In typical amphibian development, eggs are laid in water and larvae are adapted to an aquatic lifestyle. Frogs, toads, and newts all hatch from the eggs as larvae with external gills but it will take some time for the amphibians to interact outside with pulmonary respiration. Afterwards, newt larvae start a predatory lifestyle, while tadpoles mostly scrape food off surfaces with their horny tooth ridges.

Metamorphosis in amphibians is regulated by thyroxin concentration in the blood, which stimulates metamorphosis, and prolactin, which counteracts its effect. Specific events are dependent on threshold values for different tissues. Because most embryonic development is outside the parental body, development is subject to many adaptations due to specific ecological circumstances. For this reason tadpoles can have horny ridges for teeth, whiskers, and fins. They also make use of the lateral line organ. After metamorphosis, these organs become redundant and will be resorbed by controlled cell death, called apoptosis. The amount of adaptation to specific ecological circumstances is remarkable, with many discoveries still being made.

Frogs and toads

With frogs and toads, the external gills of the newly hatched tadpole are covered with a gill sac after a few days, and lungs are quickly formed. Front legs are formed under the gill sac, and hindlegs are visible a few days later. Following that there is usually a longer stage during which the tadpole lives off a vegetarian diet. Tadpoles use a relatively long, spiral‐shaped gut to digest that diet.

Rapid changes in the body can then be observed as the lifestyle of the frog changes completely. The spiral‐shaped mouth with horny tooth ridges is resorbed together with the spiral gut. The animal develops a big jaw, and its gills disappear along with its gill sac. Eyes and legs grow quickly, a tongue is formed, and all this is accompanied by associated changes in the neural networks (development of stereoscopic vision, loss of the lateral line system, etc.) All this can happen in about a day, so it is truly a metamorphosis (Figure 2). It is not until a few days later that the tail is reabsorbed, due to the higher thyroxin concentrations required for tail resorption.


Salamander development is highly diverse; some species go through a dramatic reorganization when transitioning from aquatic larvae to terrestrial adults, while others, such as the Axolotl, display paedomorphosis and never develop into terrestrial adults. Within the genus Ambystoma, species have evolved to be paedomorphic several times, and paedomorphosis and complete development can both occur in some species.[2]


In newts, there is no true metamorphosis because newt larvae already feed as predators and continue doing so as adults. Newts’ gills are never covered by a gill sac (Figure 3) and will be resorbed only just before the animal leaves the water. Just as in tadpoles, their lungs are functional early, but newts use them less frequently than tadpoles. Newts often have an aquatic phase in spring and summer, and a land phase in winter. For adaptation to a water phase, prolactin is the required hormone, and for adaptation to the land phase, thyroxin. External gills do not return in subsequent aquatic phases because these are completely absorbed upon leaving the water for the first time.


Basal caecilians such as Ichthyophis go through a metamorphosis in which aquatic larva transition into fossorial adults, which involves a loss of the lateral line.[3] More recently diverged caecilians (the Teresomata) do not undergo an ontogenetic niche shift of this sort and are in general fossorial throughout their lives. Thus, most caecilians do not undergo an anuran-like metamorphosis.[4]


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Salamander, (order Caudata), any member of a group of about 740 species of amphibians that have tails and that constitute the order Caudata. The order comprises 10 families, among which are newts and salamanders proper (family Salamandridae) as well as hellbenders, mud puppies, and lungless salamanders. They most commonly occur in freshwater and damp woodlands, principally in temperate regions of the Northern Hemisphere.

A brief treatment of salamanders follows. For more complete treatments, see caudata and amphibian.

Salamanders are generally short-bodied, four-legged, moist-skinned animals, about 10 to 15 cm (4 to 6 inches) long. Many are camouflaged, whereas others are boldly patterned or brightly coloured. The largest members of the order are the Chinese giant salamanders—Andrias sligoi can grow to 2 metres (6.6 feet), and A. davidianus can grow to 1.8 metres (5.9 feet) in length—and the Japanese giant salamander (A. japonicus), which can grow up to 1.7 metres (5.6 feet) in length.

Typical salamanders undergo a larval stage that lasts for a period of a few days to several years. Larval forms have external gills and teeth in both jaws and lack eyelids. These and other larval features may persist into sexual maturity—a condition known as heterochrony. A mud puppy (Necturus maculosus) of eastern North America and the axolotl (Ambystoma mexicanum) of central Mexico are common species that exhibit this phenomenon.

Salamanders feed on insects, worms, snails, and other small animals, including members of their own species. Like other amphibians, they absorb water through their skin, and they require a moist habitat. In regions where the temperature goes below freezing, they often hibernate.

Most adult salamanders hide by day and feed by night. Some remain hidden underground until the breeding season, or they may emerge only when levels of moisture and temperature are appropriate. Many species, especially in the family Plethodontidae, are strictly terrestrial and avoid ponds and streams.

Fertilization in the suborder Cryptobranchoidea is external. In all other salamanders, fertilization is usually internal males of such forms often produce a spermatophore, or sperm case, which the female takes into her body through the cloacal opening. Breeding often occurs in the water, but certain members of the Salamandridae and most species of the Plethodontidae families breed on land.

Life History of Mosquito (With Diagram)

In the life history of mosquito there are four stages namely—egg, larva, pupa and imago or adult.

After sexual union the female mosquito lays about 200-400 fertilised eggs in shallow stagnant water. The eggs of Anopheles and Aedes float separately in the water, but the eggs of Culex remain together and float as a single unit.

The individual egg of Anopheles is provided with two central extensions known as air floats which are attached laterally. The eggs float horizontally on the water surface separately. After 2 or 3 days the eggs hatch into larvae.

The larvae are very much active and they feed on algae, micro-organisms etc. The body of a larva is elongated which is divisible into head, thorax and abdomen. The head bears a pair of compound eyes, a pair of antennae and a pair of feeding brush.

The un-segmented thorax bears clusters of bristles and the abdomen is provided with respira­tory siphons. The larva moves to the surface of water for respiration. After about 7-10 days the larva is metamorphosed into pupa.

The pupal stage of mosquito is not stationary like other insects. The pupa moves in water but they do not feed on anything as they have no mouth aperture. It is comma (,) like structure and the head region is compara­tively larger. The dorsal siphon remains above the surface of water for respiration. The pupal stage continues for 2 days.

After two days there is a metamorphosis in the pupa and the imago or adult is formed. The adult comes out by breaking of the pupal shell and the adult mosquito remains sometime over the shell and flies away when its wings harden.

The life cycle of a mosquito is usually com­pleted in 15 days and the adult usually lives for about one month. (Table 15.2 and Fig. 15.3).

Life-History of Toad (With Diagram) | Zoology

Toads breed during the rainy season. Males come by the side of a suitable pond or ditch and begin to croak loudly by inflating their vocal sac. Invited by the song, the females come to the water in a large number. This is followed by mating or sexual re­production.

A male clasps a female, his grip on her being tight­ened by the thumb pads which are particularly well-formed during the breeding season. This state may continue for two or three days. The female how lays her eggs or ova in shallow water and the male pours his milt or spermatozoa over the eggs as they are extruded.

The eggs look like small black spots placed upon white strings of jelly, thus forming a convoluted ribbon which floats freely on the surface of the water. This is the toad spawn. The spermatozoa swim actively by lashing their tail and a swarm of them encircle every egg on the spawn.

Fertilization of Egg in Toad:

Each egg is fertilized by a sperm, the process taking place externally, that is outside the body, in water. After the entry of one sperm, the outer membrane of the egg becomes impervious to other sperms. The sperm which has succeeded to enter the egg loses its tail its head is now transformed into the male pro-nucleus.

In the meantime, the nucleus of the egg changes into the female pro-nucleus. In fertilization the male and female pro-nuclei fuse to form a single nucleus. The fertilized egg is known as the oosperm or zygote. It is a single cell which is destined to develop into a toad. Under abnormal circumstances, unfertilized eggs may develop by parthenogenesis and behave like fertilized eggs.

Embryonic Development in Toad:

Zygotes are left in water without and parental care. Each zygote undergoes a period of rest and then begins to develop by repeated cell division. The first cleavage results in two cells or blastomeres. A large number of blastomeres are rapidly formed, all enclosed within the outer mem­brane of the egg.

The blastomeres arrange themselves into groups in a complicated way and eventually produce a young embryo with three germinal layers:

(3) A mesoderm in between the two.

The embryonic development goes on for about two weeks. At the end of this period a small embryo is seen to wriggle within the egg. An embryo may be defined as the developing young of an animal inside the egg. During the course of its development the embryo draws its nourishment from the yolk, which is stored in the egg for this purpose. Eventually the embryo hatches out by rupturing the egg membrane.

Larval Stage of Toad:

The freshly hatched young tadpole bears no resemblance to its parent. It has an ovoid head, a short trunk, and a slender compressed tail with a small vent near its root. There is no limb, but there is an adhesive sucker on the ventral side, by which it attached to some submerged object.

As there is no mouth it cannot ingest any food and depends on the residue of yolk. Three pairs of branched processes, called external gills, develop on the sides of the head. These are highly vascular folds of skin which serve as the first set of respiratory organs.

After resting for a few days a mouth is formed near the sucker and a pair of horny jaws encircle the mouth. The tail grows longer and develops a dorsal and a ventral fold. It is thus converted into a fin. Moreover, V-shaped bands of muscles appear on both sides of the tail which are now used for active swimming.

The free-swimming young scrapes algae with its jaws and ingests small bits of water-weeds through its mouth. Its alimentary canal grows enormously long, and to accommodate the same within the short trunk, the intestine is coiled spirally like the spring of a watch.

Evidently the newly-hatched young, though self-supporting does not resemble the toad either in form or in habits. It is known as the tadpole larva. A larva is defined as the self-supporting immature young of an animal which bears no resemblance to the adult.

At a later stage, the pharynx becomes perforated at the sides by gill-slits. Internal gills are now formed in between the gill-slits. The internal gills are vascular outgrowths from the pharyngeal wall. A fold of skin called operculum appears and covers up the gills and gill-slits. The operculum or gill-cover fuses with the trunk ventrally and on the right side, leaving a small opening, called spiracle, on the left side.

Water enters through the mouth and while passing out through the pharyngeal gill-slits washes the internal gills and finally escapes through the spiracle. The internal gills are the second set of respiratory organs of the tadpole and with their formation the external gills wither away. At this stage, the tadpole is not only fish-like in appearance but it also resembles a fish in the manner of locomotion and respiration.

When the internal gills have been functioning for a time, a pair of lungs appear as outgrowths from the ventral surface of the pharynx. The lungs are the third set of respiratory organs of the tadpole. Meanwhile, the limbs have been developing as buds. The hind limbs appear, one on either side of the root of the tail. The forelimbs remain under cover of the operculum for a time and then emerge by bursting through it.

The tadpole now comes to live in shallow water. It can still breathe through the internal gills and comes to the surface occasionally for taking in a gulp of air into its newly-formed lungs. When the lungs are fully formed the internal gills rapidly disappear and the tadpole now looks like a miniature toad with limbs and everything but with a tail.

As the limbs grow in size the animal enters into a period of fasting and the tail is gradually absorbed into its body, the material of the tail being used as the source of nutrition. The young toad now leaves its watery home and hops on the land.

Finally the mouth widens and the horny jaws are replaced by true bony jaws. The animal changes its food and becomes a carnivore consequently its gut becomes short and more or less straight. The development is completed in about twelve weeks.

Metamorphosis of Toad:

The young tadpole larva which hatches out from the toad’s egg hardly resembles its parents. It is more like a fish than like a toad and lives a free-swimming independent life. Though immature, it can take care of itself and procure its own food. The term metamorphosis is applied to the series of changes which a larva has to undergo for altering its structure and mode of life before it comes to resemble its parents.

During metamorphosis of the tadpole, fish-like characters are given up and toad-like charac­ters are taken on. The larva is thoroughly changed in its mode of respiration, locomotion, and nutrition. The metamorphosis occurs rapidly towards the end of the tadpole’s aquatic life. There is, how­ever no abrupt transition from one stage to another.

Metamorphosis is hastened if tadpoles are fed artificially with the substance of the thyroid gland. In the American bull-frog the normal larval period is about two years experimental feeding with thyroid reduces this long larval stage to a very short period.

On the other hand, removal of thyroid from a tadpole completely stops metamorphosis although the larva may grow and attain a huge size. Anterior lobe of the pituitary body plays an important part in the metamorphosis by stimulating the thyroid gland.


The name tadpole is from Middle English taddepol, made up of the elements tadde, 'toad', and pol, 'head' (modern English poll). Similarly, pollywog / polliwog is from Middle English polwygle, made up of the same pol, 'head', and wiglen, 'to wiggle'. [1]

The life cycle of all amphibians involves a larval stage that is intermediate between embryo and adult. Tadpoles of frogs are mostly herbivorous, while tadpoles of salamanders and caecilians are carnivorous.

Anura Edit

Tadpoles of frogs and toads are usually globular, with a laterally compressed tail with which they swim by lateral undulation. When first hatched, anuran tadpoles have external gills that are eventually covered by skin, forming an opercular chamber with internal gills vented by spiracles. Depending on the species, there can be two spiracles on both sides of the body, a single spiracle on the underside near the vent, or a single spiracle on the left side of the body. [2] Newly hatched tadpoles are also equipped with a cement gland which allows them to attach to objects. The tadpoles have a cartilaginous skeleton and a notochord which eventually develops into a proper spinal cord.

Anuran tadpoles are usually herbivorous, feeding on soft decaying plant matter. The gut of most tadpoles is long and spiral shaped to efficiently digest organic matter, and can be seen through the bellies of many species. Though many tadpoles will feed on dead animals if available to them, only a few species of frog have strictly carnivorous tadpoles, an example being the frogs of the family Ceratophryidae, their cannibalistic tadpoles having wide gaping mouths with which they devour other organisms, including other tadpoles. Another example is the tadpoles of the New Mexico spadefoot toad (Spea multiplicata) which will develop a carnivorous diet along with a broader head, larger jaw muscles, and a shorter gut if food is scarce, allowing them to consume fairy shrimp and their smaller herbivorous siblings. [3] A few genera such as Pipidae and Microhylidae have species whose tadpoles are filter feeders that swim through the water column feeding on plankton. Megophrys tadpoles feed at the water-surface using unusual funnel-shaped mouths. [4]

As a frog tadpole matures it gradually develops its limbs, with the back legs growing first and the front legs second. The tail is absorbed into the body using apoptosis. Lungs develop around the time as the legs start growing, and tadpoles at this stage will often swim to the surface and gulp air. During the final stages of metamorphosis, the tadpole's mouth changes from a small, enclosed mouth at the front of the head to a large mouth the same width as the head. The intestines shorten as they transition from a herbivorous diet to the carnivorous diet of adult frogs.

Tadpoles vary greatly in size, both during their development and between species. For example, in a single family, Megophryidae, length of late-stage tadpoles varies between 3.3 centimetres (1.3 in) and 10.6 centimetres (4.2 in). [5] The tadpoles of the paradoxical frog (Pseudis paradoxa) can reach up to 27 centimetres (11 in), [6] the longest of any frog, [7] before shrinking to a mere snout-to-vent length of 3.4–7.6 cm (1.3–3.0 in).

While most anuran tadpoles inhabit wetlands, ponds, vernal pools, and other small bodies of water with slow moving water, a few species are adapted to different environments. Some frogs have terrestrial tadpoles, such as the family Ranixalidae, whose tadpoles are found in wet crevices near streams. The tadpoles of Micrixalus herrei are adapted to a fossorial lifestyle, with a muscular body and tail, eyes covered by a layer of skin, and reduced pigment. [8] Several frogs have stream dwelling tadpoles equipped with a strong oral sucker that allows them to hold onto rocks in fast flowing water, two examples being the Indian purple frog (Nasikabatrachus sahyadrensis) and the tailed frogs (Ascaphus) of Western North America. Although there are no marine tadpoles, the tadpoles of the crab-eating frog can cope with brackish water. [9]

Some anurans will provide parental care towards their tadpoles. Frogs of the genus Afrixalus will lay their eggs on leaves above water, folding the leaves around the eggs for protection. Female Pipa frogs will embed the eggs into their backs where they get covered by a thin layer of skin. The eggs will hatch underneath her skin and grow, eventually leaving as either large tadpoles (such as in Pipa parva) or as fully formed froglets (Pipa pipa). Female marsupial frogs (Hemiphractidae) will carry eggs on her back for various amounts of time, with it going as far as letting the tadpoles develop into tiny froglets in a pouch. Male African bullfrogs (Pyxicephalus adspersus) will keep watch over their tadpoles, attacking anything that might be a potential threat, even though he may eat some of the tadpoles himself. [10]

Males of the Emei mustache toads (Leptobrachium boringii) will construct nests along riverbanks where they breed with females and keep watch over the eggs, losing as much as 7.3% of their body mass in the time they spend protecting the nest. [11] Male midwife toads (Alytes) will carry eggs between their legs to protect them from predators, eventually releasing them into a body of water when they are ready to hatch. Poison dart frogs (Dendrobatidae) will carry their tadpoles to various locations, usually phytotelma, where they remain until metamorphosis. Some female dart frogs such as the strawberry poison dart frog (Oophaga pumilio) will regularly lay unfertilized eggs for the developing tadpoles to feed on. [12]

Despite their soft-bodied nature and lack of mineralised hard parts, fossil tadpoles (around 10 cm in length) have been recovered from Upper Miocene strata. [13] They are preserved by virtue of biofilms, with more robust structures (the jaw and bones) preserved as a carbon film. [14] In Miocene fossils from Libros, Spain, the brain case is preserved in calcium carbonate, and the nerve cord in calcium phosphate. Other parts of the tadpoles' bodies exist as organic remains and bacterial biofilms, with sedimentary detritus present in the gut. [13] Tadpole remains with telltale external gills are also known from several labyrinthodont groups.

Some tadpoles are used as food. Tadpoles of the megophryid frog Oreolalax rhodostigmatus are particularly large, more than 10 cm (3.9 in) in length, [5] and are collected for human consumption in China. [15] In India, Clinotarsus curtipes are collected for food, [16] and in Peru Telmatobius mayoloi tadpoles are collected for food and medicine. [17]

According to Sir George Scott, in the origin myths of the Wa people in China and Myanmar, the first Wa originated from two female ancestors Ya Htawm and Ya Htai, who spent their early phase as tadpoles ("rairoh") in a lake in the Wa country known as Nawng Hkaeo. [18]

In the Ancient Egyptian numerals, a hieroglyphic representing a tadpole was used to denote the value of 100,000.

These are tadpoles of the Yellow-Bellied Toad. Of course these tadpoles are born in the water. You can see the beginning of the formation of the hind limbs.

Amphibians reproduce sexually with either external or internal fertilization. They attract mates in a variety of ways. For example, the loud croaking of frogs is their mating call. Each frog species has its own distinctive call that other members of the species recognize as their own. Most salamanders use their sense of smell to find a mate. The males produce a chemical odor that attracts females of the species.

Amphibian Eggs

Unlike other tetrapod vertebrates (reptiles, birds, and mammals), amphibians do not produce amniotic eggs. Therefore, they must lay their eggs in water so they won&rsquot dry out. Their eggs are usually covered in a jelly-like substance, like the frog eggs shown in Figure below. The &ldquojelly&rdquo helps keep the eggs moist and offers some protection from predators.

Frog Eggs. Frog eggs are surrounded by &ldquojelly.&rdquo What is its function?

Amphibians generally lay large number of eggs. Often, many adults lay eggs in the same place at the same time. This helps to ensure that eggs will be fertilized and at least some of the embryos will survive. Once eggs have been laid, most amphibians are done with their parenting.

Amphibian Larvae

The majority of amphibian species go through a larval stage that is very different from the adult form, as you can see from the frog in Figure below. The early larval, or tadpole, stage resembles a fish. It lacks legs and has a long tail, which it uses to swim. The tadpole also has gills to absorb oxygen from water. As the larva undergoes metamorphosis, it grows legs, loses its tail, and develops lungs. These changes prepare it for life on land as an adult frog.

Frog Development: From Tadpole to Adult. A frog larva (tadpole) goes through many changes by adulthood. Notice the visible changes that occur at each stage. How do these changes prepare it for life as an adult frog?

Chapter Summary

The characteristic features of Chordata are a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail. Chordata contains two clades of invertebrates: Urochordata (tunicates) and Cephalochordata (lancelets), together with the vertebrates in Vertebrata. Most tunicates live on the ocean floor and are suspension feeders. Lancelets are suspension feeders that feed on phytoplankton and other microorganisms. Vertebrata is named for the vertebral column, which is a feature of almost all members of this clade.

29.2 Fishes

The earliest vertebrates that diverged from the invertebrate chordates were the jawless fishes. Fishes with jaws (gnathostomes) evolved later. Jaws allowed early gnathostomes to exploit new food sources. Agnathans include the hagfishes and lampreys. Hagfishes are eel-like scavengers that feed on dead invertebrates and other fishes. Lampreys are characterized by a toothed, funnel-like sucking mouth, and most species are parasitic on other fishes. Gnathostomes include the cartilaginous fishes and the bony fishes, as well as all other tetrapods. Cartilaginous fishes include sharks, rays, skates, and ghost sharks. Most cartilaginous fishes live in marine habitats, with a few species living in fresh water for part or all of their lives. The vast majority of present-day fishes belong to the clade Osteichthyes, which consists of approximately 30,000 species. Bony fishes can be divided into two clades: Actinopterygii (ray-finned fishes, virtually all extant species) and Sarcopterygii (lobe-finned fishes, comprising fewer than 10 extant species but which are the ancestors of tetrapods).

29.3 Amphibians

As tetrapods, most amphibians are characterized by four well-developed limbs, although some species of salamanders and all caecilians are limbless. The most important characteristic of extant amphibians is a moist, permeable skin used for cutaneous respiration. The fossil record provides evidence of amphibian species, now extinct, that arose over 400 million years ago as the first tetrapods. Amphibia can be divided into three clades: salamanders (Urodela), frogs (Anura), and caecilians (Apoda). The life cycle of frogs, like the majority of amphibians, consists of two distinct stages: the larval stage and metamorphosis to an adult stage. Some species in all orders bypass a free-living larval stage.

29.4 Reptiles

The amniotes are distinguished from amphibians by the presence of a terrestrially adapted egg protected by amniotic membranes. The amniotes include reptiles, birds, and mammals. The early amniotes diverged into two main lines soon after the first amniotes arose. The initial split was into synapsids (mammals) and sauropsids. Sauropsids can be further divided into anapsids (turtles) and diapsids (birds and reptiles). Reptiles are tetrapods either having four limbs or descending from such. Limbless reptiles (snakes) are classified as tetrapods, as they are descended from four-limbed organisms. One of the key adaptations that permitted reptiles to live on land was the development of scaly skin containing the protein keratin, which prevented water loss from the skin. Reptilia includes four living clades: Crocodilia (crocodiles and alligators), Sphenodontia (tuataras), Squamata (lizards and snakes), and Testudines (turtles).

29.5 Birds

Birds are endothermic, meaning they produce their own body heat and regulate their internal temperature independently of the external temperature. Feathers not only act as insulation but also allow for flight, providing lift with secondary feathers and thrust with primary feathers. Pneumatic bones are bones that are hollow rather than filled with tissue, containing air spaces that are sometimes connected to air sacs. Airflow through bird lungs travels in one direction, creating a cross-current exchange with the blood. Birds are diapsids and belong to a group called the archosaurs. Birds are thought to have evolved from theropod dinosaurs. The oldest known fossil of a bird is that of Archaeopteryx, which is from the Jurassic period. Modern birds are now classified into two groups, Paleognathae and Neognathae.

29.6 Mammals

Mammals in general are vertebrates that possess hair and mammary glands. The mammalian integument includes various secretory glands, including sebaceous glands, eccrine glands, apocrine glands, and mammary glands. Mammals are synapsids, meaning that they have a single opening in the skull. A key characteristic of synapsids is endothermy rather than the ectothermy seen in other vertebrates. Mammals probably evolved from therapsids in the late Triassic period, as the earliest known mammal fossils are from the early Jurassic period. There are three groups of mammals living today: monotremes, marsupials, and eutherians. Monotremes are unique among mammals as they lay eggs, rather than giving birth to young. Eutherian mammals are sometimes called placental mammals, because all species possess a complex placenta that connects a fetus to the mother, allowing for gas, fluid, and nutrient exchange.

29.7 The Evolution of Primates

All primate species possess adaptations for climbing trees, as they all probably descended from tree-dwellers, although not all species are arboreal. Other characteristics of primates are brains that are larger than those of other mammals, claws that have been modified into flattened nails, typically only one young per pregnancy, stereoscopic vision, and a trend toward holding the body upright. Primates are divided into two groups: prosimians and anthropoids. Monkeys evolved from prosimians during the Oligocene Epoch. Apes evolved from catarrhines in Africa during the Miocene Epoch. Apes are divided into the lesser apes and the greater apes. Hominins include those groups that gave rise to our species, such as Australopithecus and H. erectus, and those groups that can be considered “cousins” of humans, such as Neanderthals. Fossil evidence shows that hominins at the time of Australopithecus were walking upright, the first evidence of bipedal hominins. A number of species, sometimes called archaic H. sapiens, evolved from H. erectus approximately 500,000 years ago. There is considerable debate about the origins of anatomically modern humans or H. sapiens sapiens.

Biology - Life Cycle of Capillaria hepatica

The nematode (roundworm) Capillaria hepatica (=Calodium hepaticum) causes hepatic capillariasis in humans. Nomenclature varies in use globally and by discipline Capillaria hepatica is most frequently used in medical literature. C. hepatica is a zoonotic parasite with a low host specificity it primarily exists in rodent and carnivore hosts. Both true and spurious infections occur in humans.

Life Cycle

Capillaria hepatica has a direct life cycle, with no intermediate host. It can develop with only one definitive host, but likely requires two hosts to complete the life cycle. Adult worms are located deep within the liver parenchyma of the host, and lay hundreds of eggs in the surrounding parenchymal tissue . The eggs trapped in the parenchyma can not be passed in the feces of the host, and remain in the liver until the animal dies , or more likely, is eaten by a predator or scavenger . Eggs ingested by scavengers are unembryonated (not infectious) and are passed in through the digestive tract into and out in feces, providing an efficient mechanism to release eggs into the environment this is ecologically the most likely primary route of transmission . Eggs embryonate in the environment , where they require air and damp soil to become infective. Under natural conditions, embryonation is slow and may take between 6 weeks and 5 months. The cycle continues when embryonated eggs are eaten by a suitable mammalian host . Infective eggs hatch in the intestine, releasing first stage larvae. The larvae penetrate the intestinal wall and migrate via the portal vein to the liver parenchyma within 3-4 days. Larvae take about 3-4 weeks to mature into adults and mate. Humans are usually infected after ingesting embryonated eggs in fecally-contaminated food, water, or soil .

Notably, the presence of C. hepatica eggs in human stool during routine ova-and-parasite (O&P) examinations indicates spurious passage of ingested eggs, and not a true infection. Diagnosis in humans is usually achieved by finding adults and eggs in biopsy or autopsy specimens.


C. hepatica has a low host specificity, but rodents such as rats are generally believed to be the most typical host. Infections have also been identified in wild and domestic carnivores (e.g. foxes, dogs, cats), lagomorphs, swine, primates, and humans.

Geographic Distribution

C. hepatica has a broad global distribution in wildlife. Human cases have originated from all inhabited continents except for Australia, although there it exists in wildlife.

Clinical Presentation

Hepatic capillariasis is rare in humans. It typically manifests as an acute or subacute hepatitis with peripheral leukocytosis and eosinophilia, hepatomegaly, and persistent fever (which may be as high as 40℃). The deposition of eggs in the liver parenchyma causes granuloma formation and liver necrosis, which in heavy infections can lead to potentially fatal liver dysfunction. The true incidence in humans may be underestimated due to the nonspecific clinical presentation and difficulty of diagnosis.


The mouthparts are specially adapted for a liquid diet the mandibles and maxillae are reduced and not functional, and the other mouthparts form a retractable, flexible proboscis with an enlarged, fleshy tip, the labellum. This is a sponge-like structure that is characterized by many grooves, called pseudotracheae, which suck up fluids by capillary action. [7] [8] It is also used to distribute saliva to soften solid foods or collect loose particles. [9] Houseflies have chemoreceptors, organs of taste, on the tarsi of their legs, so they can identify foods such as sugars by walking over them. [10] Houseflies are often seen cleaning their legs by rubbing them together, enabling the chemoreceptors to taste afresh whatever they walk on next. [11] At the end of each leg is a pair of claws, and below them are two adhesive pads, pulvilli, enabling the housefly to walk up smooth walls and ceilings using Van der Waals forces. The claws help the housefly to unstick the foot for the next step. Houseflies walk with a common gait on horizontal and vertical surfaces with three legs in contact with the surface and three in movement. On inverted surfaces, they alter the gait to keep four feet stuck to the surface. [12] Houseflies land on a ceiling by flying straight towards it just before landing, they make a half roll and point all six legs at the surface, absorbing the shock with the front legs and sticking a moment later with the other four. [13]

The thorax is a shade of gray, sometimes even black, with four dark, longitudinal bands of even width on the dorsal surface. The whole body is covered with short hairs. Like other Diptera, houseflies have only one pair of wings what would be the hind pair is reduced to small halteres that aid in flight stability. The wings are translucent with a yellowish tinge at their base. Characteristically, the medial vein (M1+2 or fourth long vein) shows a sharp upward bend. Each wing has a lobe at the back, the calypter, covering the haltere. The abdomen is gray or yellowish with a dark stripe and irregular dark markings at the side. It has 10 segments which bear spiracles for respiration. In males, the ninth segment bears a pair of claspers for copulation, and the 10th bears anal cerci in both sexes. [4] [14]

A variety of species around the world appear similar to the housefly, such as the lesser house fly, Fannia canicularis the stable fly, Stomoxys calcitrans [14] and other members of the genus Musca such as M. vetustissima, the Australian bush fly and several closely related taxa that include M. primitiva, M. shanghaiensis, M. violacea, and M. varensis. [15] : 161–167 The systematic identification of species may require the use of region-specific taxonomic keys and can require dissections of the male reproductive parts for confirmation. [16] [17]

The housefly is probably the insect with the widest distribution in the world it is largely associated with humans and has accompanied them around the globe. It is present in the Arctic, as well as in the tropics, where it is abundant. It is present in all populated parts of Europe, Asia, Africa, Australasia, and the Americas. [4]

Though the order of flies (Diptera) is much older, true houseflies are believed to have evolved in the beginning of the Cenozoic Era. [18] The housefly's superfamily, Muscoidea, is most closely related to the Oestroidea (blow flies, flesh flies and allies), and more distantly to the Hippoboscoidea (louse flies, bat flies and allies). They are thought to have originated in the southern Palearctic region, particularly the Middle East. Because of their close, commensal relationship with humans, they probably owe their worldwide dispersal to co-migration with humans. [19]

The housefly was first described as Musca domestica in 1758 based on the common European specimens by the Swedish botanist and zoologist Carl Linnaeus in his Systema naturae and continues to be classified under that name. [20] A more detailed description was given in 1776 by the Danish entomologist Johan Christian Fabricius in his Genera Insectorum. [4]

Other Nematocera (crane flies, mosquitoes, etc.)

Other Muscomorpha (robber flies, etc.)

Hippoboscoidea (louse flies, bat flies, etc.)

Oestroidea (blow flies, flesh flies, etc.)

When metamorphosis is complete, the adult housefly emerges from the pupa. To do this, it uses the ptilinum, an eversible pouch on its head, to tear open the end of the pupal case. The adult housefly lives from two weeks to one month in the wild, or longer in benign laboratory conditions. Having emerged from the pupa, it ceases to grow a small fly is not necessarily a young fly, but is instead the result of getting insufficient food during the larval stage. [14]

Male houseflies are sexually mature after 16 hours and females after 24. Females produce a pheromone, (Z)-9-tricosene (muscalure). This cuticular hydrocarbon is not released into the air and males sense it only on contact with females [13] it has found use as in pest control, for luring males to fly traps. [25] [26] The male initiates the mating by bumping into the female, in the air or on the ground, known as a "strike". He climbs on to her thorax, and if she is receptive, a courtship period follows, in which the female vibrates her wings and the male strokes her head. The male then reverses onto her abdomen and the female pushes her ovipositor into his genital opening copulation, with sperm transfer, lasts for several minutes. Females normally mate only once and then reject further advances from males, while males mate multiple times. [27] A volatile semiochemical that is deposited by females on their eggs attracts other gravid females and leads to clustered egg deposition. [28]

The larvae depend on warmth and sufficient moisture to develop generally, the warmer the temperature, the faster they grow. In general, fresh swine and chicken manures present the best conditions for the developing larvae, reducing the larval period and increasing the size of the pupae. Cattle, goat, and horse manures produce fewer, smaller pupae, while fully composted swine manure, with a water content under 40%, produces none at all. [29] Pupae can range from about 8–20 milligrams (0.00028–0.00071 oz) in weight under different conditions. [30]

The life cycle can be completed in seven to 10 days under optimal conditions, but may take up to two months in adverse circumstances. In temperate regions, 12 generations may occur per year, and in the tropics and subtropics, more than 20. [14]

Houseflies play an important ecological role in breaking down and recycling organic matter. Adults are mainly carnivorous their primary food is animal matter, carrion, and feces, but they also consume milk, sugary substances, and rotting fruit and vegetables. Solid foods are softened with saliva before being sucked up. [8] They can be opportunistic blood feeders. [15] : 189 Houseflies have a mutualistic relationship with the bacterium Klebsiella oxytoca, which can live on the surface of housefly eggs and deter fungi which compete with the housefly larvae for nutrients. [31]

Adult houseflies are diurnal and rest at night. If inside a building after dark, they tend to congregate on ceilings, beams, and overhead wires, while out of doors, they crawl into foliage or long grass, or rest in shrubs and trees or on wires. [14] In cooler climates, some houseflies hibernate in winter, choosing to do so in cracks and crevices, gaps in woodwork, and the folds of curtains. They arouse in the spring when the weather warms up, and search out a place to lay their eggs. [32]

Houseflies have many predators, including birds, reptiles, amphibians, various insects, and spiders. The eggs, larvae, and pupae have many species of stage-specific parasites and parasitoids. Some of the more important are the parasitic wasps Muscidifurax uniraptor and Spalangia cameroni these lay their eggs in the housefly larvae tissue and their offspring complete their development before the adult houseflies can emerge from the pupae. [14] Hister beetles feed on housefly larvae in manure heaps and the predatory mite Macrocheles muscae domesticae consumes housefly eggs, each mite eating 20 eggs per day. [33]

Houseflies sometimes carry phoretic (nonparasitic) passengers, including mites such as Macrocheles muscaedomesticae [34] and the pseudoscorpion Lamprochernes chyzeri. [35]

The pathogenic fungus Entomophthora muscae causes a fatal disease in houseflies. After infection, the fungal hyphae grow throughout the body, killing the housefly in about five days. Infected houseflies have been known to seek high temperatures that could suppress the growth of the fungus. Affected females tend to be more attractive to males, but the fungus-host interactions have not been fully understood. [36] The housefly also acts as the alternative host to the parasitic nematode Habronema muscae that attacks horses. [37] A virus that causes enlargement of the salivary glands, salivary gland hypertrophy virus (SGHV), is spread among houseflies through contact with food and infected female houseflies become sterile. [38]

Houseflies are a nuisance, disturbing people while at leisure and at work, but they are disliked principally because of their habits of contaminating foodstuffs. They alternate between breeding and feeding in dirty places with feeding on human foods, during which process they soften the food with saliva and deposit their feces, creating a health hazard. [39] However, housefly larvae are as nutritious as fish meal, and could be used to convert waste to insect-based animal feed for farmed fish and livestock. [40] Housefly larvae have been used in traditional cures since the Ming period in China (1386 AD) for a range of medical conditions and have been considered as a useful source of chitosan, with antioxidant properties, and possibly other proteins and polysaccharides of medical value. [41]

Houseflies have been used in art and artifacts in many cultures. In 16th- and 17th-century European vanitas paintings, houseflies sometimes occur as memento mori. They may also be used for other effects as in the Flemish painting, the Master of Frankfurt (1496). Housefly amulets were popular in ancient Egypt. [42] [43]

As a disease vector

Houseflies can fly for several kilometers from their breeding places, [44] carrying a wide variety of organisms on their hairs, mouthparts, vomitus, and feces. Parasites carried include cysts of protozoa, e.g. Entamoeba histolytica and Giardia lamblia and eggs of helminths e.g., Ascaris lumbricoides, Trichuris trichiura, Hymenolepis nana, and Enterobius vermicularis. [45] Houseflies do not serve as a secondary host or act as a reservoir of any bacteria of medical or veterinary importance, but they do serve as mechanical vectors to over 100 pathogens, such as those causing typhoid, cholera, salmonellosis, [46] bacillary dysentery, [47] tuberculosis, anthrax, ophthalmia, [48] and pyogenic cocci, making them especially problematic in hospitals and during outbreaks of certain diseases. [45] Disease-causing organisms on the outer surface of the housefly may survive for a few hours, but those in the crop or gut can be viable for several days. [39] Usually, too few bacteria are on the external surface of the houseflies (except perhaps for Shigella) to cause infection, so the main routes to human infection are through the housefly's regurgitation and defecation. [49]

In the early 20th century, Canadian public health workers believed that the control of houseflies was important in controlling the spread of tuberculosis. A "swat that fly" contest was held for children in Montreal in 1912. [50] Houseflies were targeted in 1916, when a polio epidemic broke out in the eastern United States. The belief that housefly control was the key to disease control continued, with extensive use of insecticidal spraying well until the mid-1950s, declining only after the introduction of Salk's vaccine. [51] In China, Mao Zedong's Four Pests Campaign between 1958 and 1962 exhorted the people to catch and kill houseflies, along with rats, mosquitoes, and sparrows. [52]

In warfare

During the Second World War, the Japanese worked on entomological warfare techniques under Shirō Ishii. Japanese Yagi bombs developed at Pingfan consisted of two compartments, one with houseflies and another with a bacterial slurry that coated the houseflies prior to release. Vibrio cholerae, which causes cholera, was the bacterium of choice, and was used in China in Baoshan in 1942, and in northern Shandong in 1943. Baoshan had been used by the Allies and bombing produced epidemics that killed 60,000 people in the initial stages, reaching a radius of 200 kilometres (120 mi) which finally took a toll of 200,000 victims. The Shandong attack killed 210,000 the occupying Japanese troops had been vaccinated in advance. [53]

In waste management

The ability of housefly larvae to feed and develop in a wide range of decaying organic matter is important for recycling of nutrients in nature. This could be exploited to combat ever-increasing amounts of waste. [54] Housefly larvae can be mass-reared in a controlled manner in animal manure, reducing the bulk of waste and minimizing environmental risks of its disposal. [55] [56] Harvested maggots may be used as feed for animal nutrition. [56] [57]


Houseflies can be controlled, at least to some extent, by physical, chemical, or biological means. Physical controls include screening with small mesh or the use of vertical strips of plastic or strings of beads in doorways to prevent entry of houseflies into buildings. Fans to create air movement or air barriers in doorways can deter houseflies from entering, and food premises often use fly-killing devices sticky fly papers hanging from the ceiling are effective, [49] but electric "bug zappers" should not be used directly above food-handling areas because of scattering of contaminated insect parts. [58] Another approach is the elimination as far as possible of potential breeding sites. Keeping garbage in lidded containers and collecting it regularly and frequently, prevents any eggs laid from developing into adults. Unhygienic rubbish tips are a prime housefly-breeding site, but if garbage is covered by a layer of soil, preferably daily, this can be avoided. [49]

Insecticides can be used. Larvicides kill the developing larvae, but large quantities may need to be used to reach areas below the surface. Aerosols can be used in buildings to "zap" houseflies, but outside applications are only temporarily effective. Residual sprays on walls or resting sites have a longer-lasting effect. [49] Many strains of housefly have become immune to the most commonly used insecticides. [59] [60] Resistance to carbamates and organophosphates is conferred by variation in acetylcholinesterase genes. [61]

Several means of biological pest control have been investigated. These include the introduction of another species, the black soldier fly (Hermetia illucens), whose larvae compete with those of the housefly for resources. [62] The introduction of dung beetles to churn up the surface of a manure heap and render it unsuitable for breeding is another approach. [62] Augmentative biological control by releasing parasitoids can be used, but houseflies breed so fast that the natural enemies are unable to keep up. [63]

In science

The ease of culturing houseflies, and the relative ease of handling them when compared to the fruit fly Drosophila, have made them useful as model organism for use in laboratories. The American entomologist Vincent Dethier, in his humorous To Know A Fly (1962), pointed out that as a laboratory animal, houseflies did not trouble anyone sensitive to animal cruelty. Houseflies have a small number of chromosomes, haploid 6 or diploid 12. [15] : 96 Because the somatic tissue of the housefly consists of long-lived postmitotic cells, it can be used as an informative model system for understanding cumulative age-related cellular alterations. Oxidative DNA damage 8-hydroxydeoxyguanosine in houseflies was found in one study to increase with age and reduce life expectancy supporting the hypothesis that oxidative molecular damage is a causal factor in senescence (aging). [64] [65] [66]

The housefly is an object of biological research, partly for its variable sex-determination mechanism. Although a wide variety of sex-determination mechanisms exists in nature (e.g. male and female heterogamy, haplodiploidy, environmental factors), the way sex is determined is usually fixed within a species. The housefly is, however, thought to exhibit multiple mechanisms for sex determination, such as male heterogamy (like most insects and mammals), female heterogamy (like birds), and maternal control over offspring sex. This is because a male-determining gene (Mdmd) can be found on most or all housefly chromosomes. [67] Sexual differentiation is controlled, as in other insects, by an ancient developmental switch, doublesex, which is regulated by the transformer protein in many different insects. [68] Mdmd causes male development by negatively regulating transformer. There is also a female-determining allele of transformer that is not sensitive to the negative regulation of Mdmd. [69]

The antimicrobial peptides produced by housefly maggots are of pharmacological interest. [70]

In the 1970s, the aircraft modeler Frank Ehling constructed miniature balsa-wood aircraft powered by live houseflies. [71] Studies of tethered houseflies have helped in the understanding of insect vision, sensory perception, and flight control. [72]

In literature

The Impertinent Insect is a group of five fables, sometimes ascribed to Aesop, concerning an insect, in one version a fly, which puffs itself up to seem important. In the Biblical fourth plague of Egypt, flies represent death and decay, while the Philistine god Beelzebub's name may mean "lord of the flies". [73] In Greek mythology, Myiagros was a god who chased away flies during the sacrifices to Zeus and Athena Zeus sent a fly to bite Pegasus, causing Bellerophon to fall back to Earth when he attempted to ride the winged steed to Mount Olympus. [74] In the traditional Navajo religion, Big Fly is an important spirit being. [75] [76] [77]

William Blake's 1794 poem "The Fly", part of his collection Songs of Experience, deals with the insect's mortality, subject to uncontrollable circumstances, just like humans. [78] Emily Dickinson's 1855 poem "I Heard a Fly Buzz When I Died" speaks of flies in the context of death. [79] In William Golding's 1954 novel Lord of the Flies, the fly is, however, a symbol of the children involved. [80]

Ogden Nash's humorous two-line 1942 poem "God in His wisdom made the fly/And then forgot to tell us why." indicates the debate about the value of biodiversity, given that even those considered by humans as pests have their place in the world's ecosystems. [81]


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Amphibian, (class Amphibia), any member of the group of vertebrate animals characterized by their ability to exploit both aquatic and terrestrial habitats. The name amphibian, derived from the Greek amphibios meaning “living a double life,” reflects this dual life strategy—though some species are permanent land dwellers, while other species have a completely aquatic mode of existence.

Approximately 8,100 species of living amphibians are known. First appearing about 340 million years ago during the Middle Mississippian Epoch, they were one of the earliest groups to diverge from ancestral fish-tetrapod stock during the evolution of animals from strictly aquatic forms to terrestrial types. Today amphibians are represented by frogs and toads (order Anura), newts and salamanders (order Caudata), and caecilians (order Gymnophiona). These three orders of living amphibians are thought to derive from a single radiation of ancient amphibians, and although strikingly different in body form, they are probably the closest relatives to one another. As a group, the three orders make up subclass Lissamphibia. Neither the lissamphibians nor any of the extinct groups of amphibians were the ancestors of the group of tetrapods that gave rise to reptiles. Though some aspects of the biology and anatomy of the various amphibian groups might demonstrate features possessed by reptilian ancestors, amphibians are not the intermediate step in the evolution of reptiles from fishes.

Modern amphibians are united by several unique traits. They typically have a moist skin and rely heavily on cutaneous (skin-surface) respiration. They possess a double-channeled hearing system, green rods in their retinas to discriminate hues, and pedicellate (two-part) teeth. Some of these traits may have also existed in extinct groups.

Members of the three extant orders differ markedly in their structural appearance. Frogs and toads are tailless and somewhat squat with long, powerful hind limbs modified for leaping. In contrast, caecilians are limbless, wormlike, and highly adapted for a burrowing existence. Salamanders and newts have tails and two pairs of limbs of roughly the same size however, they are somewhat less specialized in body form than the other two orders.

Many amphibians are obligate breeders in standing water. Eggs are laid in water, and the developing larvae are essentially free-living embryos they must find their own food, escape predators, and perform other life functions while they continue to develop. As the larvae complete their embryonic development, they adopt an adult body plan that allows them to leave aquatic habitats for terrestrial ones. Even though this metamorphosis from aquatic to terrestrial life occurs in members of all three amphibian groups, there are many variants, and some taxa bear their young alive. Indeed, the roughly 8,100 living species of amphibians display more evolutionary experiments in reproductive mode than any other vertebrate group. Some taxa have aquatic eggs and larvae, whereas others embed their eggs in the skin on the back of the female these eggs hatch as tadpoles or miniature frogs. In other groups, the young develop within the oviduct, with the embryos feeding on the wall of the oviduct. In some species, eggs develop within the female’s stomach.

Watch the video: frogs life cycle video (December 2021).