Most studies about the European earwig Forficula auricularia L. (Dermaptera: Forficulidae) assume that four larval moults occur from hatching to adult. In this note, we use observational and quantitative approaches to reveal the presence of an additional intermediate moult occurring immediately after egg hatching. We then briefly discuss how this finding may open up new possibilities for follow-up studies to improve our general understanding of the taxonomy, biology, and evolution of this species.

Egg-laying patterns of the European earwig, Forficula auricularia Linnaeus (Dermaptera: Forficulidae), from three climatologically different areas; Montréal, Québec, Canada, Vancouver, British Columbia, Canada, and San Francisco, California, United States of America; were compared in laboratory. Three different egg-laying patterns were observed. Among the biotic parameters studied; previtellogenic follicular atresia, number of oocytes per ovariole at imaginal moult, female life expectancy, and ovarian activity explained female successful responses to local weather conditions. Follicular atresia, combined with a low number of oocytes, impeded oviposition in a low percentage of females from Montréal. It also impedes the production of a third brood in the Vancouver population, and of a fourth brood in the San Francisco population. Both female life expectancy and length of ovarian activity also had a significant impact on the oviposition pattern in the three climatic regions. Under the cold temperate climate of the Montréal area, only one brood was produced due to the short period of female fertility. Longer period of fertility and life expectancy enabled females to produce two broods in the milder temperate climate of Vancouver, and three broods in the San Francisco climate. Our results strongly suggest an exaptation enabling the native European populations of F. auricularia to adapt successfully in North America.

The population structure of the European earwig, Forficula auricularia Linnaeus (Dermaptera: Forficulidae), was investigated in three sites in eastern Canada (Montréal, Québec; Fredericton, New Brunswick; Truro, Nova Scotia), with two goals; describe the seasonal trends of the epigeal phase, and ascertain if the three studied populations belong to sibling species “A”, as opposed to sibling species “B” found in Vancouver, British Columbia, Canada. Insects were collected using wooden grooved traps placed on lawns and tree trunks; traps were checked weekly from the first spring melt of the frost barrier until after autumn first frost (epigeal phase). The epigeal phase was short, about five months. The different instars and adults showed a single peak of abundance. No first instars, few second instars, and mostly fourth instars and adults were collected in the arborescent stratum. The data demonstrated that these three populations have the same epigeal phenology. The interbreeding experiment established that the three studied populations belong to the same sibling species (A) of F. auricularia, and differ from sibling species (B) from Vancouver. I suggest that the climatic conditions in eastern Canada are like those in other world regions where sibling species “A” is present, and that these regions represent the edge of the bioclimatic environment of this species because of its limits on reproduction.

In North America, the life cycle of the European earwig (Forficula auricularia L.) can be divided into a nesting phase (hypogean phase) and a free-foraging phase (epigean phase) (Crumb et al. 1941; Behura 1956; Lamb and Wellington 1975). Adults spend the nesting phase in the soil; females burrow into the ground at the onset of the cold weather, lay eggs, and then care for the eggs. Hatching occurs in spring; first- or second-instar nymphs move to the soil surface for the free-foraging period. The earwig, a nocturnal insect, spends the entire daylight period of hiding under trash or in dark crevices. Where two broods occur, females reenter the ground a second time (Lamb and Wellington 1975). Stomach content analyses (Crumb et al. 1941; Sunderland and Vickerman 1980) and food preference tests (McLeod and Chant 1952; Buxton and Madge 1976) revealed that the European earwig is omnivorous. Under laboratory conditions, nymphs fed freshly frozen aphids, Rhopalosiphum padi (L.) (Hemiptera: Aphididae), survive better than those fed green algae or carrots, develop faster, and produce heavier females (Phillips 1981; Carrillo 1985).

Principal components analysis was used to assess niche partitioning between four aphid predators on the basis of oviposition strategies. The study was conducted by sampling abundance and position of the eggs of these predators in corn monocultures in two locations of southern Quebec. The results indicated that the chrysopid Chrysopa occulata Say laid its eggs on corn leaves usually without aphid colonies, and late in the season. All other predators reacted to aphid concentrations. The syrphid Sphaerophoria philanthus (Mg.) oviposited close to the ground, early in the season, and among colonies of the aphid Rhopalosiphum padi (L.). The two coccinellids laid their eggs during July and August with a maximum during tasseling for Coleomegilla maculata lengi (Timberlake) and after pollination for Hippodamia tredecimpunctata tibialis Say. Of these two species, the latter one selected sites at higher levels on the plant, and was more abundant at the edges of the field. It is suggested that the observed niche partitioning cannot be the result of interspecific competition in variable environments such as maize monocultures.

Oogenesis and associated ovarian development were studied from the onset of adult eclosion to oviposition in the European earwig, Forficula auricularia L., by examining histological sections of ovarioles and whole mounts of ovaries. Each pair of ovaries contains a mean of 85.4 ovarioles. Chronologically, oogenesis has been divided into four phases, adult emergence, previtellogenesis, vitellogenesis, and choriogenesis, each described here in detail. The mean size of the basal follicle increases from 15 μm at time of adult emergence to 0.9 mm at time of choriogenesis, and the oocyte to trophocyte ratio increases from 1.0 to 4, with a minimum of 0.5 at the onset of previtellogenesis. The oocyte to trophocyte ratio of 0.7 marks the onset of vitellogenesis, which lasts about three times (60 days) as long as previtellogenesis. The extended vitellogenesis is still under way when females bury themselves in the ground, in October, in the Montréal area.

The effects of various diets composed of millet leaves and stems, millet anthers, and animal prey on the development of Forficula senegalensis Serville were studied in the laboratory using mortality, duration of development, and weight at eclosion and at imaginal moult as comparative parameters. A diet of millet leaves and stems did not allow larvae to reach the adult stage. A diet of millet anthers allowed the production of adults, but with high mortality. When fed animal prey, the larvae developed adequately into adults, but with the mixed diet (prey and millet anthers) development was more rapid and the weight of the adults produced was greater. Millet leaves are probably not an important part of larval earwigs diet in their natural environment. They do, however, eat millet anthers and they may also feed on the insect fauna that lives on millet.

The influence of diet on oviposition and survival of Forficula senegalensis Serville was studied in laboratory conditions using four different regimens: millet leaves and stems, lepidopteran eggs and larvae, millet pollen, and no food at all. Four parameters were monitored: female weight, survival time, time to oviposition, and number of eggs produced. Starving females and females fed millet leaves and stems lost weight and survived for an average of 30–40 days without any of them laying eggs. Females fed millet pollen or lepidopteran eggs and larvae gained weight and survived for more than 60 days. Forty percent of the females fed pollen laid an average of 49 eggs the first time, and 50% of those fed lepidopteran eggs and larvae produced an average of 67.5 eggs. In both cases, less than 2% of the females oviposited a second time (average of 30 eggs). Earwigs fed lepidopteran eggs and larvae oviposited sooner. A diet of millet leaves is insufficient for oviposition in F. senegalensis; females need a richer diet based on pollen or animal preys in order to lay eggs.

The reproduction of Forficula senegalensis Serv. was studied in fields of bulrush millet (Pennisetum typhoides Burm.), 40 km south of Maradi, Niger. Flight activity, as determined by light trapping, follows a seasonal cycle in two phases: first the arrival of adults of the previous year in the fields at the onset of tillering; and second, the dispersal of adults of the new generation at the time of blooming and maturation of the plant, and their departure at harvest time. Females lay their eggs at the collar of the plants during tillering; hatching and nymphal dispersal occur soon afterward. From the second instar on, nymphs migrate to the intermediate and upper levels of the plant. The reproductive cycle of F. senegalensis thus appears closely synchronized with the development of bulrush millet.

The egg laying regime of Forficula auricularia L., defined by different parameters (number of broods, proportion of females that produce two broods, number of eggs produced at each batch, and the proportion of the total number of eggs represented by the first batch), was studied in the laboratory and under seminatural conditions for a population from Montréal. Approximately half of the females produced two batches of eggs in the laboratory; however the number of eggs produced in the first batch was substantially higher than that of the second batch. In the field a single batch of eggs was produced. Comparison of our data with the literature showed that the Montréal population appears to have a greater resemblance to one of a cold temperate climate (Font-Romeu, Eastern Pyrénées) than one of a warmer climate (Rennes, Bretagne) for all parameters studied.

We studied the effects of frost on the survival rate of European earwigs, Forficula auricularia L., during the underground phase of their cycle and determined the timing of oviposition, hatching, and death of the adults; we selected couples and inserted them into the ground to a depth of 150 cm in an open area and along the foundations of a heated suburban house in the Montréal area. Oviposition occurred in November and December, and eggs hatched between January and early June, after an average incubation period of 79–189 d depending on experimental conditions. In the open area all adults and eggs at ground surface died, killed by frost, whereas those in the soil along the house foundation did not die. Males died during the fall and early winter; their longevity was shorter than that of females, which survived until shortly after egg hatch. Females and eggs survived temperatures as low as − 3 and − 2 °C, respectively. For successful reproduction, females must keep their developing eggs at a temperature low enough to delay hatching until mid-April.

Growth, number of fusions, and polyploidy of ovarian trophocytes were studied in the European earwig, Forficula auricularia L., from the imaginal moult to oviposition, by examining sections of ovarioles and whole mounts of ovaries. During a reproductive cycle, the nucleus of the basal trophocyte enlarges its mean diameter from 5 to 170 ± 33 μm, successively absorbing the trophocyte nuclei of both adjacent follicles and undergoing seven endoreplications. The nucleus thus increased its DNA content 192 times, which is 66.7% more than in the artioploid type of polyploidy. However, vitellogenesis lasted about three times longer than previtellogenesis. This type of polyploidy accelerates oogenesis, allows an earlier oviposition, and may constitute an adaptation specific for the Montréal population of earwigs.