An Introduction to Medical Entomology
For educational purposes.
Arthropoda - Insecta
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GENERAL CHARACTERISTICS OF THE HEMIPTERA
[Also See: Hemiptera Family Key]
In the Hemiptera <Characteristics> only the Heteroptera are of medical importance. Most species are vegetarian, but predatory species that feed on the blood of other insects, mammals and humans cause problems. Except for some wingless species, most have the fore wings thickened at their bases (e.g., Coreidae). The mouthparts are a beak with joints that enable the insect to pierce the body. All species have a gradual metamorphosis were immatures resemble smaller adults.
In this group of abundant species many cause damage to plants, while others are predatory on harmful insects. Two families, Cimicidae & Reduviidae) have species that cause injury to humans by their blood sucking habit or as intermediate hosts of pathogenic organisms. Although primarily parasites of humans, they can also sustain themselves on birds rabbits, mice and rats. Feeding is predominantly at night.
Hemiptera, meaning "half-wing", are the true bugs that include many species that are destructive to agricultural crops. DNA evidence has shown a close relationship to the Homoptera, so that the old classification under one group, Heteroptera, may eventually be reinstated.
They have one-half of their wings leathery and rough, while the other half is membranous and soft. They are called hemelytra. Sometimes the leathery wings may be reduced or absent. There is a rather large scutellum. Metamorphosis is simple as the nymphs have the same form as adults except for the lacking wings.
The mouthparts arise from the front of the head and they possess true sucking mouthparts whereas other insects that suck have modified mouthparts. Their beak usually has 3-4 segments. Plant feeding species suck plant juices whereas predatory species suck blood from their hosts.
The legs are typically the running type, equipped for rapid movement. Predaceous species have specialized forelegs for grasping. Some species have legs modified for rowing.
The families of Hemiptera can be divided into Landforms and Non-land forms depending on whether they are terrestrial, aquatic or merely inhabit the water surface. Common species of medical and non-medical importance are discussed for distinction purposes, and additional information on <Habits>, <Adults> and <Juveniles> is included when available.
Pentatomidae. -- <Habits>; <Adults> & <Juveniles> -- The true stinkbugs are one of the largest families in the order with a worldwide distribution. They are especially predominant in the tropics. In North America many species are crop pests where they are very destructive to alfalfa seed. The Say's Plant Bug is a green alfalfa feeder. All feed on herbaceous plats, sucking juices. There is one predaceous subfamily.
These insects are flattened, shield-shaped insects, 1/4 - 1/2 inch long usually. Many species have beautiful colorations, especially in tropical areas. Antennae are 5-segmented, from which the family derives its name (Penta tomidae)
Coreidae. -- Leaf footed bugs include the squash bug and box elder bug. This is a moderately sized group whose species have well developed scent glands, which open on the sides of the thorax. These give off a distinctive odor when handled. All are plant feeders and injurious to cucurbits and to some fruit crops. Some are a nuisance as household pests.
Some coreids are brightly colored that often collect in great numbers on trees and shrubs. The box elder bug in Western North America can cause a nuisance from the swarms that frequently occur.
Lygaeidae. -- <Habits>; <Adults> & <Juveniles> -- The "chinch bugs" belong to this family, which includes some brilliantly colored and large species (e.g., milkweed bug). Extremely injurious they prefer to feed on seeds such as cereals, especially wheat, and have been serious pests in Central North America. The main symptom of injury is a wilting and death of grain, especially maize. There are some predaceous species.
Their size ranges from 1/4 to 3/8 inches long. The antennae have four segments. There are only four of five veins in the wing. In cold climates they overwinter in clumps of grasses as adults. Adults migrate to grain fields in springtime and eggs are laid in the boot of the plants. They will emigrate to maize as nymphs, which cannot fly but must walk.
The False Chinch Bug will feed on most wild plants in Western North America but will invade herbaceous crops if present. Overwintering is usually in the nymphal stage.
Miridae. -- <Habits>; <Adults> & <Juveniles> -- The plant bugs, Lygus spp. is a large family with over 5,000 species known. They feed primarily on plant juices and many are economic pests, and a few are predaceous.
They are fragile insects with drab to brilliant coloration. The wing has a unique appearance with two small cells, and the scutellum has a triangle on its back.
Mirids are very active and can run and fly very rapidly. Several species have taken up residence with ants. They injure a wide variety of crops and are especially important on alfalfa seed production, cotton, clover, celery and various fruits. Both nymphs and adults feed mainly on the newly developed parts of plants and distort any fruit that might form. In Western North America they rank as one of the most important economic insects.
Overwintering is in the adult stage. There are approximately four generations per year, with 20-30 days per generation.
Reduviidae. -- <Habits>; <Adults> & <Juveniles> -- These are the assassin bugs and kissing bugs. Their beak lies back between their legs in a groove called the stridulatory groove. When the beak is rubbed against the groove a squeaking sound is produced.
All members of this family are predaceous and bloodsuckers. The group has very harmful aspects, as some species are important medically as vectors of very serious human diseases. In tropical America Chagas Disease, caused by trypanosomes is vectored by assassin bugs. Their bite is very painful due to their toxic saliva. They may even enter dwellings to feed upon bed bugs and other insects.
Nabidae. -- <Habits>; <Adults> & <Juveniles> -- Damsel bugs are small, 3-11.5 mm in length, slender with their front femora somewhat enlarged. They are all predaceous and feed on many different kinds of insects. Their color varies from yellow to brown with well-developed wings.
Cimicidae. -- These are the bedbugs, which possess no wings in any stage of development. They are flattened insects that enable them to squeeze into fine crevices. They are blood feeders that favor tight areas, under belts, etc. Of the more than 45 species known, most feed upon birds, some upon bats and two species upon humans.
Bedbugs are nocturnal insects that occur only where their hosts are present. They require several meals of blood to complete their life cycle, and when blood is unavailable they can stretch their life cycle. Some people develop a terrific rash while others are rather insensitive. There has been no disease associated with Cimicidae, but because of their severe annoyance a quest for control led to the development of Rotenone as an insecticide.
Tingidae. -- These are lace bugs, whose body's dorsal surface shows a very find network of veins, which gives it a delicate lace pattern. The immature stages differ from the adult in possessing many spines.
All species are plant feeders, preferring to reside on the undersides of leaves. Some form galls on the leaves and many cause some defoliation.
For a long time this group was believed to be ancestral to land, but this has now considered false. Certain predatory forms inhabit a shore or littoral environment. Many species are able to spend only a part of their life in water. Some are able to live on the water surface.
Special modifications have enabled them to exist in aquatic environments. They have developed special breathing apparatus, some lost the predatory habit and feed only on plants, and some have had their legs modified so that they do not sink into the water itself.
Belostomatidae. -- <Habits>; <Adults> & <Juveniles> -- Giant water bugs have a flattened form facilitates movement. Most are brownish and leathery in appearance and are found in quiet water or in streams. Their front legs are modified for the predatory habit, and they are very aggressive and active swimmers. Insects, frogs and fish are included in their diet.
Belostomatids have the habit of feigning death when captured, and they can inflict painful bites.
Their eggs are laid on vegetation or glued to objects in water, usually in masses containing up to 100 eggs. Oftentimes the female lays eggs on the back of the male. The glue is water insoluble.
In the water they breathe by means of a breathing tube. Retractile appendages at the end of the abdomen lead to spiracles on the 6th abdominal segment. Air reservoirs are also present.
They have nocturnal flight habits and are attracted to light. They can be a nuisance in tropical areas when they infest buildings.
Notonectidae. -- <Habits>; <Adults> & <Juveniles> -- The backswimmers have a keel-shaped appearance somewhat like a boat. They swim up side down. They rest on water, their legs extended as oars that have fringes of hairs, which allow them to propel themselves rapidly. These insects are difficult to capture as they are very agile and dive rapidly into the water. They inhabit streams and lakes. Some species are brightly colored. They suck the juices of plants and will travel long distances to water.
Gerridae. -- <Habits>; <Adults> & <Juveniles> -- Water striders have long slender legs. Body hairs repel water and their claws have been reposition from apical to pre-apical so that the surface film of the water is not broken. The hind legs are for steering, while the front legs are held under the head and shorter then the rest. They hold their prey with their front legs, and pierce and suck out the liquid contents.
Tropical members of this insect group exist in the marine habitat and can be found a few hundred kilometers out at sea.
Mesoveliidae. -- Water Treaders occur on floating vegetation at the edges of pools or ponds or on logs that project from water. They can run rapidly over the water surface. Their size ranges up to only 5 mm in length. They are slender insects and usually green or yellow in color. Adults may be either winged or wingless. They are all predaceous on small aquatic organisms that occur near the surface of water.
Corixidae. -- Water boatmen are very widespread and probably the most abundant of the Hemiptera. Different species prefer certain kinds of water. They have a very interesting wing pattern, which is black and white with a barred effect. Their size is 1/4 to 3/4 inches in length, and their coloration is protective. Their legs are modified for rapid movement in water. Ent91
This is the only aquatic group that is not predaceous, as it feeds on algae with some exceptions. The larvae also feed on minute vegetable material by scooping it into their mouths. They have spatulate tarsi. Some may feed on mosquito larvae. They are short-lived insects. Specialized masticating structures are present.
Many corixids are very active at night.
The common bedbugs are members of this family. They have flattened bodies that are adapted to hiding in crevices. The wings are degenerated to small pads. The segmented beak is held in a groove on the ventral surface of the head and thorax. The head is broad with two large compound eyes. Ocelli are absent and the antennae have 4 segments. A distinctive odor is associated with bedbugs.
Two widespread species are Cimex lectularius L. of temperate latitudes and Cimex hemipterus Fabr. of tropical areas. Other less common species usually not associated with humans are Leptocimex boueti (Brumpt), Cimex pilosellus Horvath, C. pipistrelli (Jenyns), Oeciacus spp., Cimexopsis nyctalis List and Haematosiphon inodorus (Duges).
DEVELOPMENT & HABITS
Bedbug eggs hatch in 6-7 days at room temperature. The young nymph begins to feed after hatching from the egg. After five molts and about 30 days the adult stage is reached. The number of generations per year depends on the temperature and host availability. Dispersal is primarily by human activity. They can hide in clothing and their eggs are deposited in all their hideouts.
EFFECTS ON HUMANS
The bites of bedbugs affect people in different ways depending on their susceptibility. Ranges are from severe irritation to no noticeable effects even though blood may be lost. Although experiments have shown that bedbugs can transmit bubonic plague (Pasteurella pestis), it is uncertain whether they do so naturally. Other experiments with Leishmania and Spirochaeta failed.
Control of minor bedbug infestations involves cleaning bedding, but in severe cases fumigation of the room or structure is required. For greater detail, please refer to Cimicidae.
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This is a large family of about 410 species that feed on the blood of their hosts. Feeding is largely on other insects or as cannibals. Some species that attack humans are important vectors of human and other animal diseases. Their bites may be severe and sometimes take a long time to heal. Several important species that attack humans are Melanolestes picipes, Rasahus biguttatus and Reduvius personatus.
Their appearance is more elongated than bedbugs, and they are larger and more active. The beak has 3 segments and can cause a painful wound. The elongated head moves freely and eyes are very conspicuous. Prominent ocelli may be present and the antennae have 4 segments.
There are many subfamilies and genera in the group, but only a few attack humans, most of which are of minor importance. Most of the important species are found in the widespread subfamily Triatominae. In America there are many in this group that harbor Chagas Disease.
Panstrongylus megistus (Burm.) of South America is a serious vector of Chagas Disease. The insect is mainly domestic and hides in any available cover during daytime. It feeds at night without inflicting a serious wound. The adults are black with red markings on the prothorax, wings and abdomen. Eggs are deposited in crevices & holes on the walls of dwellings. Duration of the life cycle is 260-300 days.
Triatoma rubrofasciata (DeGeer) occurs from the Orient to Africa and America, and has a similar life cycle as P. megistus.
Triatoma sanguisuga (LeConte) is the large conenose of North and Central America. It is associated with poultry and adults invade human dwellings. The adult is 18-20 mm long, flattened and dark brown with pink areas on the abdomen and the tips of the thickened wings.
EFFECTS ON HUMANS
Although bites of some Triatominae may be severe, others give little reaction. The most important disease associated with this group is Chagas Disease caused by Panstrongylus megistus. The disease is spread from Argentina through Central America and Mexico. Symptoms are fever, swelling of eyelids and face, enlargement of lymphatic glands and destruction of cardiac muscles, spleen cells and the brain. The body endothelial tissue cells are also destroyed gradually. Two forms of the disease are acute and chronic. The acute stage may result in death in 2-4 weeks. The chronic state proceeds variably.
The insects obtain the trypanosomes from infected hosts when sucking blood and retain it for two or more years. The trypanosomes develop in the insect's intestinal tract and infect humans when the insect defecates in the wound where it is feeding. In humans the incubation period is about 12 days and the trypanosomes are found in the blood during this period. Later they disappear from the blood and are found in cardiac muscles and cells of the spleen, liver, brain and most of the tissues, but in a Leishmania form. The trypanosome form may appear periodically in the blood when the person may serve as a reservoir for the disease.
There have been many species of animals capable of harboring the trypanosomes, among which are armadillos, bats, cats and dogs, mice, opossums and wood rats. However, transmission to humans is almost exclusively in tropical and subtropical regions.
For Control, please refer to Chagas Disease.
Many other Reduviidae occasionally attack humans causing annoyance. Rhodnius prolixus Stal of South and Central America readily bites humans and is also a natural vector of Chagas Disease in Venezuela. Reduvius personatus L. , a widespread bug, attacks humans and is known as the "Kissing Bug," Rasahus biguttatus Say and R. thoracicus Say of the Neotropics are called "Corsairs," Arilus cristatus L. is the "Wheel Bug" of North America, where it can cause painful bites. Melanolestes picipes H.S. & M. abdominalis H. S. of North America give severe bites. In the Baja California Peninsula of Mexico a few cases of a suspected Chagas Disease illness have occurred, and people are advised not to sleep outdoors in wilderness areas as the vectors may sequester around rocky boulders.
(Nabidae, Anthocoridae, Pyrochorridae, Lygaeidae
Miridae, Notonectidae and Belostomatidae)
Additional Hemiptera families have species that are known to attack humans, which are susceptible to their bites. These insects may have poisonous glands or certain contaminations on their proboscis may be passed on to the host. Included here are the Nabidae of tropical regions, the cosmopolitan Anthocoridae and Pyrochorridae of tropical regions. However, reports of bites by all members of the Hemiptera are usually vague, so that problem species may be expected in all groups. Following are descriptions of representatives of the whole order:
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