Evolution of blood sucking insects
Evolution of blood sucking insects is intimately intertwined with the evolution of higher organisms. The class Insecta consists of about three quarters of a million described species. They show wide behavioral and feeding patterns and estimated 14,000 insect species feed on blood (Adams, 1999). These blood sucking insects are important to humans as they act as vectors of various devastating diseases such as malaria, sleeping sickness, filariasis, leishmaniasis, dengue, typhus and plague. In addition these insects cause major economic losses in farming by direct damage to livestock by transmitting veterinary diseases such as various trypanosomiases.
Blood sucking insects
There are four main insect orders of blood-sucking insect (diptera, hemiptera, phthiraptera, and siphonoptera)
The dipterans are the most important and familiar blood sucking insects. They are economically important to humans as vectors of many devastating diseases. They have a single pair of wings and the second pair is modified as halters which are use as balancing organs. Most adult dipterans have large and highly mobile heads bearing well developed eyes.
Dipterans transmit causative agents of human diseases such as malaria, yellow fever, dengue, west nile virus and encephalitis (Japanese, eastern equine, western equine encephalitis). Mosquitoes are the most familiar of all blood sucking insects. Adult mosquitoes are small, rather delicate insects with slender bodies, long legs and elongated forward projecting mouth parts. Only female dipterans take blood meals and feed on wide range of vertebrates. However, males feed on plant juice or nectar. Humans are the major hosts for most mosquitoes. Black flies or buffalo gnats, also dipterans, make life miserable for humans, livestock and wildlife in many parts of the world and their bite transmits pathogens to their host often leading to death. Black flies act as important vectors of the filarial worm Onchocera volvulus , which causes the human disease river blindness. The biting midges like sand flies also transmit several arbo-viruses such as bluetongue, bovine ephemeral fever, African horse sickness and Akabane virus as well as the medically important Oropouche virus. The blood sucking midges also transmit parasitic protozoa including Hepatocystis spp. to mammals, some Leucocytozoon spp and Akiba spp to birds. Female frog biting midges, Corethrella spp. are vectors of certain trypasomiasis and they use the frog mating calls to locate them frogs and obtain a blood meal.
Hemiptera (True bugs)
They range in size from 1 millimeter to around 15 centimeters in size. They are winged insects with five segmented antennae. Most hemipterans are either entomophagous (feed on other insects) or plant feeders, but three families contain several species of blood sucking insects.
In the family Cimicidae (bed bugs), all members feed on blood. Adult bed bugs are wingless dorso-ventrally flattened brownish insects about 4-7 millimeters in length. Commonly known bed bugs are Cimex lectularis and Cimex hemipterus and they normally feed on human blood. They bite at night and heavy infestations have been associated with chronic iron–deficiency anemia. Cimicids are also important economic pests of poultry.
In the family Reduviidae (kissing bugs) all members feed on a variety of vertebrates, including humans and many are intimately associated with the habitual resting sites or nests of birds, mammals and other animals. They can be the vectors of Trypanosoma cruzi the causative agent of Chagas disease. The adult bug is a 10-20mm, brown black nocturnal winged insect.
The third family is the Polyctenidae, known as bat bugs. All members of this group are permanent ectoparasites that are obligate haematophages living on microchiropteran bats of both the New and Old world. They are of no quantified economic importance. They bear superficial resemblance to fleas. Adult bugs lack eyes and ocelli.
Adult lice vary in size from about 0.5mm to about 8mm depending on species. They are dorso-ventrally flattened, wingless, brownish –grey insects with three to five segmented antennae. Their eyes are generally slight or absent. Lice have a tough, leathery cuticle capable of considerable expansion after a blood meal because of the presence of concertina–like folds. These insects are wingless parasites that live on most birds and mammals. Lice deposit their eggs on the hair or feathers of the host. There are about 4000 described species.
Phthirapterans are divided into two groups: chewing lice (Mallophaga) and sucking lice (Anoplura). The chewing lice feed on bits of hair, feathers or skin of the host. The sucking lice feed mainly on blood. They have highly modified mouthparts according to their feeding habit. The sucking lice have modified piercing and sucking mouth parts for blood feeding where as the chewing lice have chewing mouth parts that are used to cut various foods.
Three species of lice are commonly found on the human body: Pediculus humanus (body louse), Pediculus capitis (head louse) and Pthirus pubis (crab or pubic louse). The human body lice are the vectors of Rickettsia prowazekii and Bartonella quintana, the causative agents of epidemic typhus and Quintana fever respectively.
There are about 2500 described species and sub-species. Adult fleas are small, brown, wingless, flattened insects. Their body is generally covered with backward projecting spines and with combs. All adult fleas are ecto-parasitic on warm blooded hosts including about 94% mammals and 6% birds. There are about 20 flea species that will feed on humans. The order also includes Pulex irritants, Ctenocepahlis felis (cat flea) Ctenocephalis canis (dog flea). They may cause moderate to severe pruritic reactions appearing as areas of moist dermatitis on their hosts. The most common flea which carries disease to humans is Xenopsylla cheopis. This species transmit bacterium Francisella tularensis and causes a plague–like disease called tularaemia in mammals. The disease is particularly severe among domesticated animals like cattle, horses, and sheep. Fleas also transmit the causative agent of murine typhus, Rickettsia typhi. The disease is a mild infection in wild rodents throughout the world and may cause significant mortality in affected human populations. Fleas are also act as intermediate hosts of some helminthes (parasitic worms). They also transmit filarial worm of dogs, Dipetalonema reconditum.
Host seeking behavior of blood sucking insects
Blood sucking insects feed from a range of different host animals including humans and other mammals, birds, amphibians, reptiles, fish and even insects, arachnids and annelids. Some have very specific host preferences while the majority of bloods sucking insects are adapted to feed on available blood meals. The location of the host is one of major problems that has for host seeking insects. Different types of blood sucking insects have different kinds of antennal receptors (Chapman, 1982). The number of receptors varies according to the lifestyle. The more independent host seeking insects have larger number of receptors. Thus lice have 10 to 20 antennal receptors and fleas about 50, stable flies nearly 5000 antennal receptors and more adventurous Triatoma infestans has 2900 antennal receptors.
Insects feeding on blood use a variety of cues to find a host. Commonly used olfactory components include carbon dioxide (CO2), lactic acid, acetone, ammonia, butanone, fatty acids, indole, phenolic compounds, urine and heat (Meijerink, et.al., 2000). CO2 is an important olfactory stimulus which almost all blood sucking insects cue on. Vision is also an important activation and orientation stimuli (Thompson, 1976). The main visual organs of insects are the compound eyes. But haematophagous insects also have other visual receptors such as ocelii. However, vision is not used alone but is instead integrated with information from other senses. Sound is the main cue guiding the midges of Corethrella spp to their frog host. Generally insects that feed on blood are stimulate by host odour and then use smell to track the host from a distance, visual information is often used in the final stage of the orientation.
Evolution of the blood sucking insects
The blood sucking nature of insects probably evolved at least six times during the Jurassic and Cretaceous periods (145-65 million years ago) (Balashov, 1984). This supports the diversity of forms and lifestyles seen in modern day insects as well as their complex relationships with vertebrates. Several hypotheses have been proposed to explain the evolution of haematophagous insects (feeding on blood).
One hypothesis proposes that the origin of blood sucking behavior of insect began with close association with vertebrates. This hypothesis suggests that insects were attracted to the nests of birds for their humid warm environments and the presence of organic debris for food. Gradually they may have evolved symbiotic associations with birds and mammals climbing into fur and feathers to be translocated from one nest site to another. Initially they might feed on organic debris at the nest and later accidental ingestion of skin may had lead to selection favoring individuals with adaptations to efficiently use such materials. Morphological and behavioral adaptations would have allowed them to remain on the host body for longer periods of time and increased efficiency feeding on skin and feathers. Chewing mouth parts could have evolved this way. In accord with this hypothesis there is intermediate form of insects. Present day mallophagens, such as the elephant louse, feed on blood at the base of feathers using chewing mouth parts instead of piercing mouth parts. The progression on skin feeding might occur on this route. It is thus believed that haematophagous lice evolved from an original nest dwelling free living ancestors 265-225 million years ago (Kim, 1985).
A second hypothesis suggests that ancestral forms of insects were parasitic on primordial mammals and radiated along the lines of mammalian evolution. The close association of insects and land dwelling vertebrate is thought to have happened during the Mesozoic. This hypothesis proposes that blood sucking habits evolved from attraction to feed on vertebrate secretions as well as host dung as common larval habitats. The dung can be a good environment for insect larvae. However, limited resources and quick dried up nature of host dung may have increased larval competition for this resource. This selective pressure may have increased the association with vertebrates becoming permanent parasites so they would have a chance to deposit eggs on newly deposited dung.
A third hypothesis has been proposed to explain the evolution of blood feeding behavior in male insects. This hypothesis proposes that males that feed on blood may have benefited from increased mating opportunities with the females allied with vertebrates.
Another route of evolution of blood sucking habit suggests that blood feeding evolved in some insect lineages from ancestral insects that were adapted for piercing surfaces. The entomophagous insects are good example to support this hypothesis. Most of these insect groups are predacious on other insects but few species have adapted to blood feeding habit. These entomophagous insects supposed to attract to vertebrates such as amphibians and reptiles as well as higher vertebrates those live in wet areas or regularly visit wet environments. Since these wet areas are breeding grounds for most dipterans, large number of insects associated with these vertebrates. Entomophagous insects resembling snipe flies (Rhagionidae) are attracted to vertebrates with other insects around them. Hematophagy in these individuals was at first probably occasional and opportunistic chance event which led to full hematophagy through continues close association with the vertebrate host. It is thought that evolution of blood feeding behavior of bugs, fleas and dipterans appeared along this line. The lifestyles of several present day insects are consistent with this hypothesis (Whiting, 2002). It has also been argued that hematophagy may have arisen in some insect groups from plant feeding insects that had piercing and sucking mouth parts. Such mouth parts are seen in the mouth Calpe eustrigata of family Noctuidae. Most species of this family have piercing proboscis to penetrate fruit rind, but Calpe eutrigata (Vampire moth) use the proboscis to penetrate vertebrate skin to feed on blood. It is likely that the plant-feeding ancestor of modern day blood feeders have developed hematophagy due to the close association with vertebrates. It can be conclude that the blood feeding behavior evolved in different ways in different evolutionary lineages and therefore no single hypothesis are able to explain the evolution of blood sucking behavior.
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