The sun-moths belong in the Castniidae Family, which in the Australian Region presently contains the single genus Synemon Doubleday, 1846. They are day-flying moths, small to medium in size with a wing expanse of about 20-55 mm, and have clubbed antennae similar to butterflies.

The moths are found in most areas of mainland Australia, but are absent from the very arid inland areas and Tasmania. They inhabit grasslands and low sedge-matrush lands or very open woodlands that contain their hostplants. There are about 24 recognised species in Australia, and there are possibly more than 20 unnamed species. Many of the latter comprise populations that are very similar and difficult to separate, and will likely require DNA analysis to establish their true taxonomic relationships.

Some isolated small populations of putative Synemon selene in Victoria are known to form parthenogenetic female groups. There are many closely related specie-groups within the sun-moths based on similar larval host-plants, similar body and wing morphology etc, but the ultimate grouping will be based on DNA phylogeny parameters when the latter are eventually elucidated.

The forewings of the sun-moths typically have a cryptic camouflage pattern of brown or grey-black, with streaks of white, while the hindwings have bright display-patterns of either red, orange or yellow that are used as aposematic flash displays to startle potential predators.

The genus consists of many closely related cryptic morphologic groups akin to mimicry groups although the sun-moths are not known (as yet) to be poisonous or distasteful, and there is no similar poisonous moth or butterfly group that could form a Batesian mimicry group, except perhaps the Tiger Moths (Arctiidae). Birds are known to take adult Synemon plana (Golden Sun-moth) without ill effects.

Sun-moths are often mistaken for butterflies and skippers because of their diurnal flight, bright hindwing colour and clubbed antennae. However, they are easily differentiated from butterflies by their habit of settling with the forewings covering the hindwings. This can either take on a position typical of many moths with wings tightly folded along their bodies in the shape of a tent, or if on the ground, with wings flattened and partially open. In some species like Synemon plana the sexes are strongly dimorphic with the female having bright yellow hindwings, and the male having dull brown hindwings. The clubbed antennae are longer in males than in the females. The club is also unusual in possessing a tiny spine-like process called the mucro, at the end of the club.

The eye-catching adults usually fly in full sun in warm to hot weather, during the heat of the day from about 10:00am to 4:00pm, (cloud cover may cause the moths to be inactive, similar to skippers). They tend to be more locally common around midday as they become fully active from their overnight nap or because they have newly emerged from their underground pupae, taking in nectar or undergoing mating. Then later on in the day they disperse over the area of their habitat, where they can be seen flying slowly; the males looking for unmated females, or the females looking for host-plants to lay eggs on. The moths rest either on the ground or on low plants and grasses. In hot conditions, settled moths are very timid and can be very difficult to approach. Their flight is swift when disturbed, usually near ground level in the case of small species, which are then quickly lost from sight, but large species can fly 1-2 m above ground level particularly when there are low shrubs in the habitat.

Some of the sun-moth species during their evolution have lost the use of their feeding proboscis and consequently only live a short period until their reserves of body fat have been used up, which may only be a few days. The flight times and the length of the flight period of the moths are not well known in SA but it seems the proboscis (nectar) feeding species as a local group can fly for long periods of a month or more, while the non-feeding species tend to have very short flight periods of 1-2 weeks at any one location and they usually have coincident emergences so that all the moths are available at once for mating. Flight periods of individuals have been documented from 1-10 days. The females are normally larger than males, except in the case for Synemon plana. Flight times are mainly during spring and summer, but a few fly during autumn.

The females are gravid at pupal eclosion, having their full compliment of developed eggs. They also have a long extendible ovipositor (a tubular abdominal egg laying device at the end of the abdomen), which is used to lay a (usually) single large egg deep within or on the base of the host-plant, or within cracks in the ground next to the host plant. Eggs are elongate-ellipsoidal spindle shaped with prominent longitudinal and finer lateral ridges. The number of longitudinal ridges is a useful taxonomic parameter. Eggs are usually pale yellow when freshly laid, but turn white or reddish if fertile. In temperate areas the eggs can take up to 13 weeks to hatch. Depending on the species, the larvae feed on grasses, low tussock sedges and mat-rushes (monocotyledons). Usually each species is monophagous, but in some species the females have been seen to lay eggs on both grasses and sedges. Females are reputed to have the capacity to lay up to 200 eggs. One female after copulation laid 45 eggs over one day while in temporary captivity.

The larvae are of witchetty or bardi grub form, usually white when young but are sometimes red, but the former often turn pink, red or orange near maturity. They usually live and feed underground at the base of the hostplant when immature, either within the basal growth of the plant in a hollowed out chamber in the case of sedges and mat-rushes, or within the grass stem in the case of grasses. They tunnel deeper underground to the root zone as they get more mature, to feed on roots, where they live in a hollowed out chamber. This latter adaptation would likely protect the larvae from the above-ground effects of drought, dry summer conditions and infrequent fires, but has not protected them from the modern farming practices of tillage, repeated chemical and fertilizer applications, overgrazing and trampling by stock, use of broad-acre insecticides and herbicides, and frequent fires. Being underground also possibly helps them from being parasitised by flies and wasps, although it places them in a position where they can suffer from predators like echidnas, bandicoots and carnivorous beetle larvae. Pupation either occurs within the root zone or deeper part of the hostplant, or in the soil within a silk-lined tunnel peripheral to the hostplant, and the empty pupa case (exuvium) is left protruding from the tunnel at ground level after the moth emerges. The pupa is brown, elongate with one or two transverse rows of low dorsal spines on each segment of the abdomen, the latter a common morphology in certain moth groups that are used to anchor the pupae in their silk cocoons, shelters or tunnels. The pupa is reputed to be able to move within the silk lined tunnel, presumably by using the abdominal ridges.

Studies indicate the early-stage brood period generally requires two years to complete, but may be less for smaller species, or longer for larger species depending on the environmental or climatic conditions at the time. Mature larvae are known to become torpid and delay pupation during adverse conditions.

The sun-moths are similar to many butterflies as regards conservation and many species are threatened. Their habitat has historically been subjected to large-scale clearance for urban and farming activity, leaving small fragmented patches of habitat remaining for the use of the sun-moths. Those sun-moths that use grasses for hostplants are particularly threatened and are in direct competition with farmers' livestock and have suffered the most from historical habitat clearing and stock degradation. Subsequent degradation processes such as overgrazing, weed-infestations and herbicide spraying, fire-management and mosquito-locust eradication spraying programmes have contributed to the demise of many of the sun-moth populations in fragmented habitats.

Colonising, herbivorous burrowing animals like rabbits and wombats can also have an impact on sun-moth habitat if the animals become too prolific. Some of the moths have ancestrally developed defences from fires and exposure, by having their mature larvae and pupae live underground, and these stages can survive fires provided they are not repetitive. However, young larvae occur near ground level and are cooked by fires. Many sun-moth populations (particularly the non-feeding adult types) have very restricted dispersal ability and are therefore confined to their present habitat fragments. If the habitat fragment is destroyed or even degraded, then so too is the sun-moth population.

*Notes: A good reference for South Australian and Victorian sun-moths is provided by DOUGLAS, F. 2008: The Sun-moths (Lepidoptera: Castniidae) of Victoria, with a detailed study of the Pale Sun-moth (Synemon selene Klug, 1850). Master of Applied Science Thesis, University of Ballarat, Accepted June 2008. 323pp.