Medically-significant
Guides
Acharia
Saddleback Caterpillar Moths and Allies
Acharia is a genus of moths in the family Limacodidae, commonly known as saddleback caterpillar moths. The genus is notable for larvae with urticating (stinging) hairs and distinctive saddleback patterning. Some species, particularly Acharia stimulea, are well-known in North America for their painful stings. Other species such as Acharia fusca have become invasive agricultural pests in oil palm plantations.
Centruroides sculpturatus
Arizona Bark Scorpion
Centruroides sculpturatus, commonly known as the Arizona bark scorpion, is a small, light brown scorpion native to the Sonoran Desert. It is the only scorpion species in North America considered dangerously venomous to humans, with stings capable of causing severe pain, numbness, and in rare cases life-threatening symptoms in young children. The species exhibits fluorescent properties under ultraviolet light due to compounds in its exoskeleton. It is frequently used in educational outreach programs due to its docile handling behavior in captivity.
Cheiracanthium
Longlegged Sac Spiders, Yellow Sac Spiders
Cheiracanthium is a genus of araneomorph spiders in the family Cheiracanthiidae, first described by Carl Ludwig Koch in 1839. Commonly known as longlegged sac spiders or yellow sac spiders, the genus contains over 230 accepted species. These spiders are notable for their pale coloration, nocturnal hunting behavior, and habit of constructing silken retreats rather than permanent webs. They are frequently encountered in human dwellings, particularly in temperate regions, and have been the subject of controversy regarding their medical significance.
Latrodectus
widow spiders, black widow spiders, brown widow spiders
Latrodectus is a genus of cobweb spiders (Theridiidae) containing approximately 34-35 species distributed worldwide except Antarctica. Females are typically dark-colored with distinctive reddish ventral abdominal markings, often hourglass-shaped, while males and juveniles frequently display dorsal red or red-and-white patterns. The genus is medically significant due to potent venom containing the neurotoxin latrotoxin, which causes the condition latrodectism. Despite their reputation, bites rarely cause death in humans. The common name "widow spiders" derives from observations of sexual cannibalism, though this behavior is more common in laboratory settings than in nature.
Latrodectus geometricus
Brown Widow, Brown Button Spider, Grey Widow, Brown Black Widow, House Button Spider, Geometric Button Spider
Latrodectus geometricus, commonly known as the brown widow, is a widow spider in the family Theridiidae native to Africa with a cosmopolitan distribution due to human-mediated introduction. It is smaller and lighter in color than black widow species, with distinctive black-and-white geometric patterns on the dorsal abdomen and an orange-yellow hourglass marking ventrally. The species is notable for its highly aggressive behavior toward native black widow spiders in invaded ranges, actively seeking out and killing them rather than competing for resources. While possessing neurotoxic venom, its bites are significantly less dangerous to humans than those of black widows due to lower venom delivery.
Latrodectus variolus
Northern Black Widow, Northern Widow
Latrodectus variolus is a venomous spider in the family Theridiidae, native to northeastern North America. It is closely related to the southern black widow (L. mactans) and western black widow (L. hesperus). The species is medically significant due to its neurotoxic venom, though bites are rarely fatal to healthy adults. It has been observed expanding its range northward beyond previously documented boundaries.
Loxosceles
Recluse Spiders, Brown Spiders, Fiddle-backs, Violin Spiders, Reapers
Loxosceles is a genus of venomous spiders in the family Sicariidae, comprising approximately 149 species of cryptic, reclusive, nocturnal arachnids. These spiders are characterized by their six eyes arranged in three pairs, a distinctive violin-shaped marking on the cephalothorax in many species, and cytotoxic venom that can cause necrotic lesions in some bite victims. The genus has a primarily neotropical distribution, with significant diversity in Brazil and the southwestern United States, though some species have been introduced to other regions through human activity.
Loxosceles apachea
Apache Recluse
Loxosceles apachea, the Apache recluse, is a species of recluse spider in the family Sicariidae. It is found in the southwestern United States and Mexico. Like other recluse spiders, it possesses cytotoxic venom that can cause necrotic lesions in rare cases, though bites are uncommon and typically occur only when the spider is trapped against skin. The species is poorly studied compared to the more widespread brown recluse (L. reclusa).
Loxosceles arizonica
Arizona brown spider, Arizona Recluse
Loxosceles arizonica, commonly known as the Arizona brown spider or Arizona Recluse, is a recluse spider species in the family Sicariidae. Field studies indicate that ants constitute its primary prey, particularly species in the genera Novomessor and Camponotus. Like other Loxosceles species, it possesses cytotoxic venom capable of causing necrotic lesions, though documented cases of envenomation in humans appear limited compared to more widespread relatives such as L. reclusa.
Loxosceles blanda
Big Bend Recluse
Loxosceles blanda, commonly known as the Big Bend recluse, is a species of recluse spider in the family Sicariidae. It was described in 1983 by Gertsch and Ennik. The species is native to the United States, with its common name suggesting association with the Big Bend region of Texas. Like other members of the genus Loxosceles, it possesses cytotoxic venom that can cause tissue necrosis in humans, though documented cases specific to this species are limited.
Loxosceles deserta
Desert Recluse
Loxosceles deserta, commonly known as the desert recluse, is a venomous brown spider in the family Sicariidae. It is native to the southwestern United States and northwestern Mexico. The species is frequently misidentified as the brown recluse (L. reclusa) or L. unicolor, despite occupying a geographically distinct range. Its venom contains cytotoxic compounds that can cause necrotic skin lesions in humans, though bites are rare and typically defensive.
Loxosceles devia
Texas Recluse
Loxosceles devia, commonly known as the Texas recluse, is a species of recluse spider in the family Sicariidae. It is native to the southern United States and Mexico. Like other members of the genus Loxosceles, it possesses venom that can cause necrotic skin lesions in humans, though bites are rare and typically occur only when the spider is trapped against skin. The species is closely related to other recluse spiders and shares their characteristic shy, reclusive behavior.
Loxosceles rufescens
Mediterranean recluse spider, violin spider, brown spider
Loxosceles rufescens is a globally invasive recluse spider native to the Mediterranean region. It has established populations across six continents through human-mediated dispersal, often in synanthropic environments. The species is medically significant due to cytotoxic venom containing phospholipases D that can cause loxoscelism—characterized by dermonecrosis, hemolysis, and potential systemic complications. Despite its fearsome reputation, bites are defensive and relatively uncommon even where populations are established.
Megalopyge crispata
Black-waved Flannel Moth, Crinkled Flannel Moth, White Flannel Moth
Megalopyge crispata is a flannel moth in the family Megalopygidae, found in the eastern United States from the Atlantic coast westward to Oklahoma. Adults are active from May to October, with one generation annually in northern populations and two or more in southern populations. The species is notable for its venomous caterpillars, which possess hollow urticating hairs capable of delivering painful stings to humans.
Megalopyge opercularis
southern flannel moth, puss caterpillar, asp, Italian asp, fire caterpillar, woolly slug, opossum bug, puss moth, tree asp, asp caterpillar
Megalopyge opercularis is a moth in the family Megalopygidae, notable for its highly venomous larval stage and strikingly different adult form. The caterpillar, known as the puss caterpillar, is densely covered in hair-like setae that conceal venomous spines capable of delivering extremely painful stings. The adult, called the southern flannel moth, is covered in soft fur ranging from dull orange to lemon yellow. The species has two broods annually in much of its range, with late-season larvae sometimes overwintering in cocoons.
Megalopyge opercularis bissesa
Southern Flannel Moth, Puss Caterpillar
Megalopyge opercularis bissesa is a subspecies of southern flannel moth, commonly known as the puss caterpillar in its larval stage. The caterpillar is covered in dense, soft-looking hair that conceals venomous urticating spines capable of delivering painful stings. Found in the southeastern United States from the Mid-Atlantic to Florida and Texas, this insect feeds on various woody plants including oak, hickory, and apple. The caterpillar's distinctive appearance—resembling a tiny, hairy cat or 'Cousin Itt' from The Addams Family—belies its defensive capabilities.
Megalopygidae
New World Flannel Moths, Flannel Moths, Crinkled Flannel Moths
Megalopygidae is a family of moths occurring in North America and the New World tropics, with larvae commonly known as puss caterpillars or asps. The caterpillars are covered in long, soft hairs that conceal hollow venomous spines capable of delivering severe stings. The family contains two subfamilies in the United States: Megalopyginae (genus Megalopyge, 7 species) and Trosiinae (genera Norape and Trosia, 5 species). The venom system is unique among Lepidoptera, consisting of aerolysin-like pore-forming toxins called megalysins that were recruited via horizontal gene transfer from bacteria.
Paederus
Whiplash Beetles
Paederus is a large genus of rove beetles comprising over 600 species. Members are distinguished from most other rove beetles by bright aposematic coloration, diurnal activity, and attraction to lights. Several species contain pederin, a potent vesicant toxin produced by endosymbiotic bacteria, which causes paederus dermatitis when beetle hemolymph contacts human skin. Both larvae and adults are predatory on other insects.
Scorpiones
scorpions
Scorpiones is an ancient order of predatory arachnids, representing the oldest known terrestrial metazoans with fossil records dating back approximately 435 million years. Members are characterized by a pair of large pedipalps modified into grasping pincers, four pairs of walking legs, and a segmented metasoma (tail) terminating in a telson bearing a venomous stinger. Scorpions are the only arachnids that give birth to live young, with females carrying offspring on their backs until their first molt. They exhibit a unique ability to fluoresce under ultraviolet light due to fluorescent compounds in their exoskeletons.
Sicariidae
Sixeyed Sicariid Spiders, recluse spiders, violin spiders, sand spiders, assassin spiders
Sicariidae is a family of haplogyne spiders comprising three genera—Loxosceles, Sicarius, and Hexophthalma—with approximately 177-180 species. Members are characterized by six eyes arranged in three dyads (pairs), a distinctive trait among spiders. The family includes medically significant species such as the brown recluse (Loxosceles reclusa) and six-eyed sand spiders. All genera produce sphingomyelinase D or related tissue-destroying substances in their venom, unique among spiders. The family exhibits diverse ecological strategies: Loxosceles species are synanthropic and found nearly worldwide in warmer regions, while Sicarius and Hexophthalma are specialized desert dwellers with self-burying behavior.
Solenopsis
fire ants, thief ants, ginger ants, tropical fire ants, red ants
Solenopsis is a genus of ants in the family Formicidae comprising over 200 species worldwide, including the notorious fire ants and thief ants. Members of this genus are characterized by their ability to deliver painful stings using a venom-injecting stinger. The genus includes some of the most economically and ecologically significant invasive ant species, particularly Solenopsis invicta (red imported fire ant), which has spread from South America to become a major pest across the southern United States and beyond. While fire ants represent a minority of Solenopsis species, they have disproportionate ecological and economic impacts due to their aggressive behavior, large colony sizes, and potent venom.
Steatoda nobilis
Noble False Widow
Steatoda nobilis is a large theridiid spider native to Madeira and the Canary Islands, now established as one of the world's most invasive spider species. It has spread across Europe, North and South America, and other temperate regions, often becoming the dominant spider in urban environments. The species is of moderate medical significance: bites typically cause symptoms similar to bee or wasp stings, though some cases involve more severe reactions including bacterial infections from pathogenic bacteria carried on the spider's body. It constructs strong, irregular cobwebs with concealed tubular retreats and employs an 'attack wrap' hunting strategy to subdue prey.
Theridiidae
tangle-web spiders, cobweb spiders, comb-footed spiders
Theridiidae is a large, globally distributed family of araneomorph spiders comprising over 2,600 species in more than 130 genera. Members are characterized by a comb of serrated bristles (setae) on the tarsus of the fourth leg, used to manipulate silk. The family exhibits remarkable diversity in web architecture, ranging from irregular tangle webs to specialized gumfoot webs and sheet-like structures. Theridiids are ecribellate, producing sticky capture silk rather than woolly silk, and are entelegyne with females possessing a genital plate. The family includes medically significant species such as widow spiders (Latrodectus), model organisms for social behavior research (Anelosimus), and specialized kleptoparasites (Argyrodinae).