Insects are arthropods of the class Insecta. Insects have an adult stage characterized by a hard exoskeleton, 3 pairs of jointed legs, and a body segmented into head, thorax, and abdomen. Insects comprise the most diverse and numerous class of the animal kingdom and include numerous species of praying mantis, dragonflies, grasshoppers, true bugs, flies, fleas, bees, wasps, ants, lice, butterflies, moths, and beetles. The number of species is estimated at between 6 and 10 million, with more than a million species already described. Insects represent more than half of all known living organisms and potentially represent more than 90% of the differing life forms on Earth. Hence, human contact with insects is unavoidable. Exposure to biting or stinging insects or to their remains can range in severity from benign or barely noticeable to life threatening.
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See When Bugs Feast: What's Causing that Itch?, a Critical Images slideshow, to help identify various skin reactions, recognize potential comorbidities, and select treatment options.
Differentiating between insect bites and stings
Many patients confuse an insect bite with a sting and may use the terms interchangeably. A bite is usually from mouth parts and occurs when an insect is agitated to defend itself or when an insect seeks to feed. Bites from mosquitoes, fleas, bed bugs, and mites are more likely to cause itching than pain.
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A stinging apparatus is usually a sharp organ of offense or defense, especially when connected with a venom gland, and adapted to inflict a wound by piercing, as the caudal sting of a scorpion. The stinger is typically located at the rear of the animal. Animals with a stinger include bees, wasps, hornets, and scorpions.
Most stinging insects are of the order Hymenoptera, which is made up of multiple families, including 3 that are clinically important: Apidae (bees), Vespidae (wasps), and Formicidae (ants). Bees have barbed stingers that disengage, causing them to die after a single sting. Wasps, hornets, and yellow jackets (Vespidae family members) do not have barbed stingers and, as such, can sting multiple times. 
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Since their introduction into the southern United States in the 1920s, imported fire ants anchor themselves with their mandibles and subsequently inflict a sting. Fire ants often pivot or re-anchor themselves only to sting again and again resulting in a sensation of fire at the site. When fire ants swarm, they often position themselves on their victim and sting simultaneously in response to an alarm pheromone released by one or several individuals. The response is very aggressive and results in cutaneous pustule formation from formic acid deposition. Immobilized or elderly patients can become rapidly covered by swarms of these ants, resulting in severe stings and even death (See Fire Ant Bites).
Hymenoptera stings result in more fatalities than stings or bites from any other arthropod.
Bites by classes Insecta
This article is limited to bites by insects and not arachnids. Stings by members of the order Hymenoptera and order Scorpionida are discussed in other articles, as are bites of venomous arachnids in the class Arachnida (spiders) and bites of the order Acarina (mites and ticks).
Injuries from exposure to millipedes (class Diplopoda), centipedes (class Chilopoda), and caterpillars (order Lepidoptera) also are discussed in other articles (see Differentials); however, many of the principles that guide diagnosis and treatment of insect bites also apply to bites and stings of these other organisms.
Bed bugs 
Bed bugs (Cimex lectularius) were essentially a nonissue in the 1970s and 1980s, owing to effective insecticides like DDT. Increasing resistance and limitation of those insecticides have caused bed bug infestations to become an almost ubiquitous issue in the United States and other developed countries. A multitude of studies have shown an alarming increase of bed bug infestations over the last several years.  Bed bugs tuck away in clothing and shoes or easily migrate through walls of shared housing. Bed bugs can usually be found initially in the inner workings or the base of box springs but also like to hide in mattresses, under baseboards, along crevices in the walls, in vents, and even behind picture frames.  Infestations can be identified by fecal spotting (seen below).
Bed bugs are flightless and are ovoid and flattened in shape. They are photophobic and all are obligatory blood feeders on vertebrates, with their preferred host being humans. Bed bugs have 5 juvenile stages, which are pictured below, along with the adult male and female forms. 
When bitten, cutaneous reactions (as seen below) usually appear within several hours of the bite, and patients usually notice these reactions the morning after having been bitten.
The reactions themselves are myriad in character and vary widely between individuals, with some patients showing no reaction at all.  Distinguishing a bed bug lesion from those caused by other insects is difficult; however, the classic "bed bug wheal" is an extremely pruritic papular urticaria and may be as large as 2-6 cm. For the clinician, the history of bug or insect bites may be unknown to the patient and this should always be in the differential, especially in the setting of a new travel location for sleep or worsening lesions just after sleep. These lesions can be in linear or bunched configurations and are usually found on exposed skin.  Other cutaneous reactions can be more pronounced, with anything from bullous, hemorrhagic reaction  to a more targetoid appearance.  Anaphylaxis from bed bug bites is also possible, necessitating prompt recognition and treatment.
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Treatment of cutaneous reactions does not differ from the general treatment for other insect envenomations discussed later. Although difficult, identifying cutaneous lesions from bed bugs versus other causes is important to prevent misdiagnosis and subsequent unneeded treatments and procedures (eg, scabicides, skin biopsies). Eradication of bed bug harborages is expensive, specialized, and challenging to say the least. It should only be undertaken by professionals.
Although illness related to insect exposure in a particular locale may be easily recognizable, the emergency physician must also be aware of more exotic insect-related diseases as humans travel to more remote areas of the country and the world. Additionally, exotic insects are often kept as pets (sometimes illegally) or can be encountered in shipments of foreign origin (eg, mantises, stick insects).
Anaphylactic shock is the most notable immediate risk associated with insect exposures. Hypersensitivity to otherwise harmless insect saliva, venom, body parts, excretions, or secretions can cause systemic responses in some individuals. Diagnosing the early phases of a systemic allergic reaction preceding anaphylactic shock is of paramount importance in treating any patient in whom insect exposure is suspected. Severe anaphylaxis can be fatal in as little as 10 minutes.
The reoccurrence rate is 40-60% for insect stings. Hence, the patient should be instructed on how to avoid future exposure to the causative agent, if possible. A prescription and clear instructions on the use of an epinephrine autoinjector should be provided to patients when the risk of another reaction is judged to be substantial. 
Diseases transmitted by insect bites
The need to be aware of diseases transmitted by insect bites is crucial; Lyme disease, transmitted by ticks, and malaria, transmitted by mosquitoes, are discussed in other articles (see Tick-borne Diseases, Lyme; Malaria).
Chagas disease, increasingly found in the desert southwest and in persons residing in or traveling to Central America or South America, should be considered, particularly when the bite site is on the soft skin of the periorbita or lips.  The kissing bug (see below) can be a vector for this infection. Because this infection may produce an acute and chronic illness with notable morbidity and mortality, especially in pediatric patients, clinicians should maintain a high index of suspicion (see Trypanosomiasis).
Of note, some illnesses transmitted by insects do not produce symptoms until long after the infecting bite. In South America and parts of Africa, blackflies (Simuliidae) are responsible for transmission of onchocerciasis. This illness is also known as river blindness and eventually can produce blindness years after the initial infection. This disease is extremely rare in the United States. Chagas disease, a leading cause of cardiomyopathy in the world, may present latently as well. Obtaining a history of international travel is important because this information can lead to a diagnosis that would otherwise be overlooked. Determining the destination, time of year, length of stay, and time since travel are all important pieces of history to obtain.
Exposure to arthropods may produce dermatitis, cellulitis, urticaria, or blistering unrelated to biting or stinging. Some species of moths, caterpillars, centipedes, beetles, and spiders have urticating hairs or secretions that can cause cutaneous irritation. For further information, please refer to the respective articles on these exposures (see Differentials).
An uncommon occurrence in North America is myiasis by fly larvae. Fly larvae enter the host through varying mechanisms ranging from oviposition of live, burrowing larvae on the host, on or near open wounds, to attachment to other bloodsucking insects. While not generally the result of an insect bite, myiasis can produce pustules and lesions similar to insect bites. These lesions generally contain one or more developing fly larvae. Severe cases of myiasis can cause seizures.
Plant-eating phytophagous insects can bite in self-defense, and their bites are not predatory. This article is limited to discussion of organisms that bite to feed on blood or to catch prey.
Relatively harmless insects
Cockroaches have been reported to bite humans, but their bite generally is harmless. Continued repeated exposure to their remains and feces poses a greater health threat, such as increased incidence of asthma, especially in inner cities, and their remains and feces are possible vectors for transmission of viral and bacterial diseases.
Earwigs generally are harmless insects that have earned an unpleasant reputation. This may be because of their depiction in popular culture, such as in the television series, "The Night Gallery." Although they appear to have a large pincer on the posterior abdomen, it is not capable of rendering anything more serious than a mild pinch. Additionally, and contrary to popular belief, they do not routinely enter the human ear canal and parasitize humans. Cockroaches are much more likely to be found lodged in a patient's auditory passage.
Mouthparts of biting insects can be classified into 3 broad groups: piercing, sponging, and biting. Tremendous diversity exists in the morphology of these groups. Insects discussed in this article generally are nonvenomous, yet many species inject saliva while biting. Their saliva may aid in digestion, inhibit coagulation, increase blood flow to the bite, or anesthetize the bite locus. Most lesions are the result of the victim's immune response to these insect secretions. In the case of Chagas disease, the infective organism is transmitted via the feces of a reduviid bug, which enters through the bite site when the wound becomes pruritic and is scratched. Most insect bites are minor and can result in superficial puncture wounds to the skin. Horseflies feed with a large scissorlike proboscis that can cause a relatively deep and painful wound.
In the United States, the American Association of Poison Control Centers (AAPCC) reported 30,738 cases of single exposures to insects in 2014.  Just more than 1,500 of these were listed as resulting in moderate and less than 40 described as major reactions. A moderate reaction is defined as signs or symptoms that were more pronounced or systemic, whereas a major reaction is life-threatening or leads to significant residual disability. It is important to understand that the numbers mentioned in this report are only those reported to American Poison Centers, and it should be of no surprise that the actual numbers of uncaptured data are much higher. Fatalities among these exposures are rarely reported to poison centers and usually result from allergic reactions to Hymenoptera stings. Because of underreporting, these numbers are only a glimpse as to what is actually occurring. 
The AAPCC 2014 Annual Report published the following single-incident exposures for insects  :
Ant or fire ant bites: 757 total; 42 moderate, 1 major, 0 deaths
Bee, wasp, or hornet stings: 3,968 total; 245 moderate, 15 major, 1 deaths
Mosquitos: 164 total; 9 moderate, 0 major, 0 deaths
Other insect bites and/or stings: 6,049 total; 348 moderate, 5 major, 0 deaths
Reliable statistics are not available for insect bite exposures because most cases are not reported and do not require hospital care.  A study in tropical Zimbabwe, where biting insects are common, found that 1.5% of hospital admissions were related to insect exposure, including both bites and stings. A vast majority of these were arachnid or Hymenoptera related.
No race predilection is known.
No sex predilection is known.
No age predilection is known.
Prognosis generally is good except in patients with severe untreated anaphylaxis or in those with chronic or invasive infections.
Mortality associated with insect bites is from hypersensitivity reactions, either anaphylactic (immunoglobulin E [IgE] mediated) or anaphylactoid (non-IgE mediated), or from complications resulting from infection. The US Centers for Disease Control and Prevention estimates an annual rate of 90-100 deaths from insect venom anaphylaxis.  In patients with anaphylaxis secondary to insect venom, risk factors for increased severity of reaction include older age, preexisting cardiovascular disease or mast cell disorder, concomitant treatment with beta-adrenergic blockage or ACE inhibitors, previous severe reactions, and the type of insect (honeybees presenting the highest risk). 
Biting insects are ubiquitous in nearly all parts of the world, yet certain measures can be taken to minimize risk of exposure. Periodic pest control may eliminate nests and minimize reproduction of biting insects.
Wear protective clothing (ie, long pants, long sleeves), especially when outdoors. Many insects are incapable of biting through clothing. Additionally, light-colored clothing appears to be less attractive to many biting insects, including mosquitos.  Avoid dark colors or brightly colored floral patterns. Wear protective footwear. Wear gloves when working with soil or in areas of heavy infestation.
Avoid use of heavy perfumes, scented soaps, sprays, or lotions that may attract insects. Be aware of surroundings; for example, avoid dense vegetation or animals suspected of carrying fleas, chiggers, or ticks. Prudent use of insect repellent can help minimize exposure to insect bites and stings.  Be aware of the potential for bees or other foraging insects to enter opened soft drink containers that are left idle.
For a guide to recognizing common stinging hymenoptera, please see this CDC pictorial guide.
For a guide to recognizing common scorpion species in the United States, please see the CDC pictorial key to common US scorpion species.
For patient education resources, see the First Aid and Injuries Center. Also see the patient education articles Insect Bites, Allergy: Insect Sting, Severe Allergic Reaction (Anaphylactic Shock), Black Widow Spider Bite, Brown Recluse Spider Bite, and Ticks.
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