|Male small emperor moth, Saturnia pavonia (Saturniinae)|
The family Saturniidae, commonly known as saturniids, by most measures include the largest species of moths. They are a family of Lepidoptera, with an estimated 2,300 described species worldwide. The Saturniidae include such Lepidoptera as the giant silkmoths, royal moths and emperor moths.
Adults are characterized by large size, heavy bodies covered in hair-like scales, lobed wings, reduced mouthparts, and small heads. They lack a frenulum but the hind wings overlap the forewings, producing the same effect of an unbroken wing surface. These moths are sometimes brightly colored and often have translucent eyespots or "windows" on their wings. Sexual dimorphism varies by species, but males can generally be distinguished by their larger, broader antennae. Most adults possess wing spans between 1 and 6 in (2.5 and 15 cm), but some tropical species, such as the Atlas moth (Attacus atlas), may have wing spans up to 12 in (30 cm). Together with certain Noctuidae (chiefly Calpinae and Catocalinae, such as the genera Ascalapha, Erebus, or Thysania), the Saturniidae contain the largest Lepidoptera, and some of the largest insects alive today.
The majority of saturniid species occur in wooded tropical or subtropical regions, with the greatest diversity in the New World tropics and Mexico, though they are found all over the world. About 12 described species live in Europe, one of which, the emperor moth, occurs in the British Isles, and 68 described species live in North America, 42 of which reside north of Mexico and Southern California.
Some saturniids are strictly univoltine, producing only one generation a year, whereas others are multivoltine, producing more than one brood a year. Spring and summer broods hatch in a matter of weeks; autumn broods enter a state known as diapause and emerge the following spring. How the pupae know when to hatch early or hibernate is not yet fully understood, though research suggests day length during the larval fifth instar plays a major role, as well as cooling temperatures. Longer days may prompt pupae to develop early, while shorter days result in pupal diapause. The number of broods is flexible, and a single female may produce both fast-developing and slow-developing individuals, or they may produce different numbers of broods in different years or parts of the range. In some species – e.g. the luna moth (Actias luna) or Callosamia securifera (both Saturniinae) –, spring and summer broods look different, with different genes activated by environmental conditions.
Depending on the moth, a single female may lay up to 200 eggs on a chosen host plant. Others are laid singly or in small groups. They are round, slightly flattened, smooth and translucent or whitish.
Saturniid caterpillars are large (50 to 100 mm in the final instar), stout, and cylindrical. Most have tubercules that are often also spiny or hairy. Many are cryptic in coloration, with countershading or disruptive coloration to reduce detection, but some are more colourful. Some have urticating bristles. A few species have been noted to produce clicking sounds with the larval mandibles when disturbed (e.g. Saturniini like Actias luna and the polyphemus moth, Antheraea polyphemus). The clicks may serve as aposematic warning signals to a regurgitation defense.
Most are solitary feeders, but some are gregarious. The Hemileucinae are gregarious when young and have stinging hairs, those of Lonomia containing a poison which may kill a human. A. armida is another well-known example, and are infamous for their large conspicuous masses during the day. Their coloration is not cryptic, instead exhibiting aposematism.
The other caterpillars in this size range are almost universally Sphingidae, which are seldom hairy and tend to have diagonal stripes on their sides. Many Sphingidae caterpillars bear a single curved horn on their hind end. These are actually not dangerous, but large-haired caterpillars should generally not be touched except by experts.
Most saturniid larvae feed on the foliage of trees and shrubs. A few, particularly Hemileucinae such as Automeris louisiana, A. patagonensis, and Hemileuca oliviae, feed on grasses. They moult at regular intervals, usually four to six times before entering the pupal stage. Prior to pupation, a wandering stage occurs, and the caterpillar may change colour, becoming more cryptic just before this stage.
Most larvae spin a silken cocoon in the leaves of a preferred host plant or in leaf litter on the ground, or crevices in rocks and logs. While only moderately close relatives to the silkworm (Bombyx mori) among the Lepidoptera, the cocoons of most larger saturniids can be gathered and used to make silk fabric. However, larvae of some species – typically Ceratocampinae, like the regal moth (Citheronia regalis) and the imperial moth (Eacles imperialis), burrow and pupate in a small chamber beneath the soil. This is common in the Ceratocampinae and Hemileucinae. Unlike most silk moths, those that pupate underground do not use much silk in the construction. Once enclosed in the cocoon, the caterpillar sheds the larval skin and becomes a pupa, and the pupa undergoes metamorphosis for about 14, at which point it either emerges or goes into diapause. During metamorphosis, respiratory systems will stay intact, the digestive system will dissolve, and reproductive organs will take form.
Adult females emerge with a complete set of mature ova and "call" males by emitting pheromones (specific "calling" times vary by species). Males can detect these chemical signals up to a mile away with help from sensitive receptors located on the tips of their feather-like antennae. The males fly several miles in one night to locate a female and mate with her; females generally will not fly until after they have mated.
Since the mouthparts of adult saturniids are vestigial and digestive tracts are absent, adults subsist on stored lipids acquired during the larval stage. As such, adult behavior is devoted almost entirely to reproduction, but the end result (due to lack of feeding) is a lifespan of a week or less once emerged from the pupa.
One specific species in the Saturniidae family with a special mating pattern is Callosamia promethea, the promethea slikmoth. The females still use pheromones to attract males during their "calling time" in the evening. What sets C. promethea apart is that it is the only moth in the family where the females are nocturnal and the males and diurnal.
A few species are important defoliator pests, including the orange-striped oakworm moth (Anisota senatoria) on oaks, the Pandora pinemoth (Coloradia pandora) on pines and Hemileuca oliviae on range grasses.
Other species are of major commercial importance in tussah and wild silk production. These notably include the Chinese tussah moth (Antheraea pernyi), its hybridogenic descendant Antheraea × proylei, and the Ailanthus silkmoth (Samia cynthia). In 2015, Adarsh Gupta K of Nagaraju's research team at Centre for DNA Fingerprinting and Diagnostics, Hyderabad, India discovered the complete sequence and the protein structure of Muga Silk Fibroin and published it in Nature (journal) Scientific Reports 
Many species of Saturniidae are a valuable food source for man in sub-Saharan Africa, examples being the Mopane worm (Gonimbrasia belina), Gonimbrasia zambesina, the cabbage tree emperor moth (Bunaea alcinoe), Gynanisa maia, Imbrasia epimethea, Imbrasia oyemensis, Melanocera menippe, Microgone cana, Urota sinope and the pallid emperor moth (Cirina forda).
Most Saturniidae are harmless animals at least as adults, and in many cases at all stages of their lives. Thus, some of the more spectacular species – in particular Antheraea – can be raised by children or school classes as educational pets. The soft, silken cocoons make an interesting keepsake for pupils.
In terms of absolute numbers of species, the Saturniidae are most diverse in the Neotropics. Also, their most ancient subfamilies occur only in the Americas. Only the very "modern" Saturniidae are widely distributed across most parts of the world. Thus, it is quite safe to assume – even in the absence of a comprehensive fossil record – that the first Saturniidae originated in the neotropical region. Note that at least two of the subfamilies included below are commonly treated as separate families (Oxyteninae and Cercophaninae).
The following list arranges the subfamilies in the presumed phylogenetic sequence, from the most ancient to the most advanced one. Some notable genera and species are also included.
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