Earwigs make up the insect order Dermaptera. With about 2,000 species in 12 families, they are one of the smaller insect orders. Earwigs have characteristic cerci, a pair of forceps-like pincers on their abdomen, and membranous wings folded underneath short, rarely used forewings, hence the scientific order name, "skin wings". Some groups are tiny parasites on mammals and lack the typical pincers. Earwigs are found on all continents except Antarctica.
Earwigs are mostly nocturnal and often hide in small, moist crevices during the day, and are active at night, feeding on a wide variety of insects and plants. Damage to foliage, flowers, and various crops is commonly blamed on earwigs, especially the common earwig Forficula auricularia.
Earwigs have five molts in the year before they become adults. Many earwig species display maternal care, which is uncommon among insects. Female earwigs may care for their eggs; the ones that do will continue to watch over nymphs until their second molt. As the nymphs molt, sexual dimorphism such as differences in pincer shapes begins to show.
Extant Dermaptera belong to the suborder Neodermaptera, which first appeared during the Cretaceous. Some earwig specimen fossils are placed with extinct suborders Archidermaptera or Eodermaptera, the former dating to the Late Triassic and the latter to the Middle Jurassic. Dermaptera belongs to the major grouping Polyneoptera, and are amongst the earliest diverging members of the group, alongside angel insects (Zoraptera), and stoneflies (Plecoptera), but the exact relationship among the three groups is uncertain.
Etymology
The scientific name for the order, Dermaptera, is Greek in origin, stemming from the words , meaning , and (plural ), meaning . It was coined by Charles De Geer in 1773. The common term, earwig, is derived from the Old English , which means , and , which means , or literally, . Entomologists suggest that the origin of the name is a reference to the appearance of the hindwings, which are unique and distinctive among insects, and resemble a human ear when unfolded. The name is more popularly thought to be related to the old wives' tale that earwigs burrowed into the brains of humans through the ear and laid their eggs there. Earwigs are not known to purposefully climb into ear canals, but there have been at least two anecdotal reports of earwigs being found in the ear.
Distribution
Earwigs are abundant and can be found throughout the Americas and Eurasia. The common earwig was introduced into North America in 1907 from Europe, but tends to be more common in the southern and southwestern parts of the United States. The only native species of earwig found in the north of the United States is the spine-tailed earwig (Doru aculeatum), found as far north as Canada, where it hides in the leaf axils of emerging plants in southern Ontario wetlands. However, other families can be found in North America, including Forficulidae (Doru and Forficula being found there), Spongiphoridae, Anisolabididae, and Labiduridae.
Few earwigs survive winter outdoors in cold climates. They can be found in tight crevices in woodland, fields and gardens. Out of about 1,800 species, about 25 occur in North America, 45 in Europe (including 7 in Great Britain), and 60 in Australia.
Morphology
Most earwigs are flattened (which allows them to fit inside tight crevices, such as under bark) with an elongated body generally long. The largest extant species is the Australian giant earwig (Titanolabis colossea) which is approximately long, while the possibly extinct (declared extinct in 2014) Saint Helena earwig (Labidura herculeana) reached . Earwigs are characterized by the cerci, or the pair of forceps-like pincers on their abdomen; male earwigs generally have more curved pincers than females. These pincers are used to capture prey, defend themselves and fold their wings under the short tegmina. The antennae are thread-like with at least 10 segments.
Males in the six families Karschiellidae, Pygidicranidae, Diplatyidae, Apachyidae, Anisolabisidae and Labiduridae have paired penises, while the males in the remaining groups have a single penis. Both penises are symmetrical in Pygidicranidae and Diplatyidae, but in Karschiellidae the left one is strongly reduced. Apachyidae, Anisolabisidae, and Labiduridae have an asymmetrical pair, with left and right one pointing on opposite directions when not in use. The females have just a single genital opening, so only one of the paired penises is ever used during copulation.
The forewings are short oblong leathery plates used to cover the hindwings like the elytra of a beetle, rather than to fly. Most species have short and leather-like forewings with very thin hindwings, though species in the former suborders Arixeniina and Hemimerina (epizoic species, sometimes considered as ectoparasites) are wingless and blind with filiform segmented cerci (today these are both included merely as families in the suborder Neodermaptera). The hindwing is a very thin membrane that expands like a fan, radiating from one point folded under the forewing. Even though most earwigs have wings and are capable of flight, they are rarely seen in flight. These wings are unique in venation and in the pattern of folding that requires the use of the cerci.
Internal
The neuroendocrine system is typical of insects. There is a brain, a subesophageal ganglion, three thoracic ganglia, and six abdominal ganglia. Strong neuron connections connect the neurohemal corpora cardiaca to the brain and frontal ganglion, where the closely related median corpus allatum produces juvenile hormone III in close proximity to the neurohemal dorsal arota. The digestive system of earwigs is like all other insects, consisting of a fore-, mid-, and hindgut, but earwigs lack gastric caecae which are specialized for digestion in many species of insect. Long, slender (excretory) malpighian tubules can be found between the junction of the mid- and hind gut.
The reproductive system of females consist of paired ovaries, lateral oviducts, spermatheca, and a genital chamber. The lateral ducts are where the eggs leave the body, while the spermatheca is where sperm is stored. Unlike other insects, the gonopore, or genital opening is behind the seventh abdominal segment. The ovaries are primitive in that they are polytrophic (the nurse cells and oocytes alternate along the length of the ovariole). In some species these long ovarioles branch off the lateral duct, while in others, short ovarioles appear around the duct.
Life cycle and reproduction
Earwigs are hemimetabolous, meaning they undergo incomplete metamorphosis, developing through a series of four to six molts. The developmental stages between molts are called instars. Earwigs live for about a year from hatching. They start mating in the autumn, and can be found together in the autumn and winter. The male and female will live in a chamber in debris, crevices, or soil deep. After mating, the sperm may remain in the female for months before the eggs are fertilized. From midwinter to early spring, the male will leave, or be driven out by the female. Afterward the female will begin to lay 20 to 80 pearly white eggs in two days. Some earwigs, those parasitic in the suborders Arixeniina and Hemimerina, are viviparous (give birth to live young); they would be fed by a sort of placenta. When first laid, the eggs are white or cream-colored and oval-shaped, but right before hatching they become kidney-shaped and brown. Each egg is approximately tall and wide.
Earwigs are among the few non-social insect species that show maternal care. The mother pays close attention to the needs of her eggs, such as warmth and protection. She faithfully defends the eggs from predators, not leaving them even to eat unless the clutch goes bad. She also continuously cleans the eggs to protect them from fungi. Studies have found that the urge to clean the eggs persists for only a few days after they are removed, and does not return even if the eggs are replaced; however, when the eggs were continuously replaced after hatching, the mother continued to clean the new eggs for up to three months.
Studies have also shown that the mother does not immediately recognize her own eggs. After laying them, she gathers them together, and studies have found mothers to pick up small egg-shaped wax balls or stones by accident. Eventually, the impostor eggs were rejected for not having the proper scent.
The eggs hatch in about seven days. The mother may assist the nymphs in hatching. When the nymphs hatch, they eat the egg casing and continue to live with the mother. The nymphs look similar to their parents, only smaller, and will nest under their mother and she will continue to protect them until their second molt. The nymphs feed on food regurgitated by the mother, and on their own molts. If the mother dies before the nymphs are ready to leave, the nymphs may eat her.
After five to six instars, the nymphs will molt into adults. The male's forceps will become curved, while the females' forceps remain straight. They will also develop their natural color, which can be anything from a light brown (as in the tawny earwig) to a dark black (as in the ringlegged earwig). In species of winged earwigs, the wings will start to develop at this time. The forewings of an earwig are sclerotized to serve as protection for the membranous hindwings.
Behavior Most earwigs are nocturnal and inhabit small crevices, living in small amounts of debris, in various forms such as bark and fallen logs. Species have been found to be blind and living in caves, or cavernicolous, reported to be found on the island of Hawaii and in South Africa. Food typically consists of a wide array of living and dead plant and animal matter. For protection from predators, the species Doru taeniatum of earwigs can squirt foul-smelling yellow liquid in the form of jets from scent glands on the dorsal side of the third and fourth abdominal segment. It aims the discharges by revolving the abdomen, a maneuver that enables it simultaneously to use its pincers in defense. Under exceptional circumstances, earwigs form swarms and can take over significant areas of a district. In August 1755 they appeared in vast numbers near Stroud, Gloucestershire, UK, especially in the cracks and crevices of "old wooden buildings...so that they dropped out oftentimes in such multitudes as to literally cover the floor".Cowan, Frank, The Project Gutenberg EBook of Curious Facts in the History of Insects; Including Spiders and Scorpions, 1865, EBook issue 15 December 2012, (citing Gentleman's Magazine, Aug. 1855, vol xxv, p. 376.) Accessed 1 September 2021 A similar "plague" occurred in 2006, in and around a woodland cabin near the Blue Ridge Mountains of the eastern United States; it persisted through winter and lasted at least two years.Kevin Hathorne, Earwig invasion mystery, Pest Management Professional, (specialist trade journal) accessed 1 September 2021
Ecology Earwigs are mostly scavengers, but some are omnivorous or predatory. The abdomen of the earwig is flexible and muscular. It is capable of maneuvering as well as opening and closing the forceps. The forceps are used for a variety of purposes. In some species, the forceps have been observed in use for holding prey, and in copulation. The forceps tend to be more curved in males than in females.