Megalopygidae

Megalopygidae caterpillars are distinguished from similar-appearing tussock moths (Erebidae: Lymantriinae) and tiger moths (Erebidae: Arctiinae) by their concealed hollow venom spines beneath soft hairs rather than urticating hairs or setae. They differ from Limacodidae (slug caterpillars) in body shape—flannel moth larvae are more cylindrical and hair-covered, while slug caterpillars are flattened and slug-like. Within the family, M. opercularis and M. crispata are differentiated by leg coloration: M. o. opercularis has black setae along the entire tarsus, while M. crispata has black setae only on distal tarsomeres (excluding tarsomere 1).

Temperate deciduous and mixed forests, disturbed land, upland habitats, pine-dominated forests (less common), subtropical forest, bayou systems, arid shrubland at moderate elevations, wet tropical forest, Madrean Sky Islands at high elevations, Florida Everglades, Chisos Mountains, Guadalupe Mountains.

North America, South America, Central America, and the Caribbean. In the United States: eastern, southeastern, south-central, Mid-Atlantic, southern Great Plains, Florida coastal plain, Texas, Arizona, and New Mexico. Ranges extend into Mexico (Sierra Madre Occidental, Veracruz, northern Sonora) and Central America. Puerto Rico hosts M. krugii in tropical forests and M. opercularis (first documented in 2023).

Larval stages most commonly observed. Megalopyge opercularis shows peak larval abundance in September (southeastern US) and October (south-central US), with two peaks in south-central US. Adults observed year-round in tropical regions; temperate populations show seasonal activity corresponding to host plant availability.

Larvae feed on leaves of woody plants including oak (Quercus), hickory (Carya), apple (Malus), birch (Betula), pecan (Carya illinoinensis), persimmon (Diospyros), black locust (Robinia pseudoacacia), and rose (Rosa). Specific host associations vary by species and region.

• Quercus - larval food plantoak species widely used across range
• Carya - larval food plantmockernut hickory and other hickory species
• Malus - larval food plantapple
• Betula - larval food plantbirch
• Diospyros - larval food plantpersimmon
• Robinia pseudoacacia - larval food plantblack locust
• Rosa - larval food plantrose

Complete metamorphosis with egg, larva, pupa, and adult stages. Eggs laid on host foliage. Larvae pass through multiple instars; early instars typically white and densely hairy. Pupation occurs in silk cocoon. Adults are short-lived and do not feed; their primary function is reproduction. The larval stage is the longest and most conspicuous phase.

Caterpillars are slow-moving and rely on crypsis via their hairy appearance. When disturbed, venom spines inject defensive venom causing immediate and building pain. Envenomation produces characteristic grid-like skin lesion patterns. Pain radiates from contact site and may persist for hours or days, accompanied by regional lymph node ache. Adults are nocturnal and attracted to light.

Larvae function as herbivores on woody plants, contributing to nutrient cycling. They serve as prey for various predators, though their venomous defense reduces predation pressure. The family has been documented as host for parasitoid wasps including Calliephialtes grapholithae (Ichneumonidae), which attacks concealed larvae.

Medically significant due to severe envenomations from caterpillar contact. Stings cause intense burning pain, localized swelling, redness, and blistering; systemic reactions including headache, nausea, and shock-like symptoms may occur. Contact with eyes or nasal membranes is particularly dangerous. Medical attention is advised for systemic symptoms. The puss caterpillar (Megalopyge opercularis) is considered among the most venomous caterpillars in North America.

• LimacodidaeSlug caterpillars also possess venomous spines, but differ in flattened, slug-like body form rather than cylindrical hairy shape. Limacodidae venom system evolved independently and contains different toxins.
• Erebidae: LymantriinaeTussock moths have hairy larvae but lack hollow venom spines; their urticating hairs cause irritation through mechanical means and different chemical compounds.
• Erebidae: ArctiinaeTiger moth larvae often hairy but lack the concealed hollow spine venom apparatus; setae may cause irritation but not the severe envenomation of Megalopygidae.

The soft, hairy appearance of puss caterpillars leads many to assume they are harmless, resulting in unnecessary stings. The name 'flannel moth' suggests a soft, benign nature that belies the medical significance of larval envenomation. Some believe all hairy caterpillars sting similarly, but Megalopygidae venom is distinct in mechanism and severity.

• venomous
• medically significant
• stinging caterpillar
• puss caterpillar
• asp
• horizontal gene transfer
• New World
• defensive venom

• BugGuide
• GBIF taxonomy match
• iNaturalist taxon
• NCBI Taxonomy
• Catalogue of Life
• Megalopygidae | Beetles In The Bush
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• Look but do not touch: puss caterpillar, Megalopyge opercularis; White flannel moth, Norape ovina; Io moth, Automeris io; buck moth Hemileuca spp.; and hag moth, Phobetron pithecium — Bug of the Week
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• Geographical and temporal distribution of Megalopygidae in the United States and Puerto Rico
• New insights and renewed focus on the behavioural and mechanical traits of the larvae of the limacodid-group and related families (Lepidoptera: Aididae, Anomoeotidae, Dalceridae, Himantopteridae, Limacodidae, Megalopygidae, and Somabrachyidae)