Ovoviviparity and genital evolution: a lesson from an earwig species with coercive traumatic mating and accidental breakage of elongated intromittent organs

The Biology and Social Life of Earwigs (Dermaptera)

Earwigs are often known for the forceps-like appendage at the end of their abdomen, urban legends about them crawling into human ears, and their roles as pest and biological control agents. However, they are much less known for their social life. This is surprising, as many of the 1,900 species of earwigs show social behaviors toward eggs, juveniles, and adults. These behaviors typically occur during family and group living, which may be obligatory or facultative, last up to several months, and involve only a few to several hundred related or unrelated individuals. Moreover, many individuals can alternate between solitary and group living during their life cycle, an ability that probably prevailed during the emergence of social life. In this review, I detail the diversity of group living and social behavior in earwigs and show how further developing this knowledge in Dermaptera can improve our general understanding of the early evolution of social life in insects.

Potential of Marava arachidis, a Newly Recorded Earwig Species in Egypt as a Biological Control Agent of Rhipicephalus annulatus Tick in Laboratory

In Egypt, only five species of Dermaptera (earwigs) have been reported. Based on both the morphological and molecular data of the earwig samples collected from a bakery in Beni-Suef, Egypt, we identified the species as Marava arachidis (Spongiphoridae), a cosmopolitan species with no prior records in Egypt. The current study was designed to analyze its predation capability on newly emerged eggs and larvae of the Rhipicephalus annulatus tick. A laboratory functional response study was set up by applying a predation test with various predator-prey ratios as treatments. This experiment was applied using the undefined mix of collected earwigs and the laboratory-collected eggs and the larvae of R. annulatus. The laboratory results showed that the mean number of predated tick eggs was 18.64 ± 11.18 in 24 h under the highest predator-prey ratio (1:10) examined, accompanied by 12.04 ± 4.38 broken but unconsumed eggs. Moreover, M. arachidis predated an average of 12.32 ± 9.07 tick larvae per day. In contrast, the mean dead larvae increased to 38.4 ± 2.30 per day with the highest predator density (1:10). The number of eggs and larvae consumed increased with the predator density. A linear relationship was detected between earwig density and the consumption rates of tick eggs (R² = 0.99; p = 0.0001) and larvae (R² = 0.96; p = 0.003). In conclusion, M. arachidis was first recorded in Egypt. This earwig has predation capability on R. annulatus tick eggs and larvae.

Genetic basis for the evolution of pelvic-fin brooding, a new mode of reproduction, in a Sulawesian fish

Modes of reproduction in animals are diverse, with different modes having evolved independently in multiple lineages across a variety of taxa. However, an understanding of the genomic change driving the transition between different modes of reproduction is limited. Several ricefishes (Adrianichthyidae) on the island of Sulawesi have a unique mode of reproduction called "pelvic-fin brooding," wherein females carry externally fertilized eggs until hatching using their pelvic fins. Phylogenomic analysis demonstrated pelvic-fin brooders to have evolved at least twice in two distant clades of the Adrianichthyidae. We investigated the genetic architecture of the evolution of this unique mode of reproduction. Morphological analyses and laboratory observations revealed that females of pelvic-fin brooders have longer pelvic fins and a deeper abdominal concavity, and that they can carry an egg clutch for longer than non-brooding adrianichthyids, suggesting that these traits play important roles in this reproductive mode. Quantitative trait locus mapping using a cross between a pelvic-fin brooder Oryzias eversi and a non-brooding O. dopingdopingensis reveals different traits involved in pelvic-fin brooding to be controlled by different loci on different chromosomes. Genomic analyses of admixture detected no signatures of introgression between two lineages with pelvic-fin brooders, indicating that introgression is unlikely to be responsible for repeated evolution of pelvic-fin brooding. These findings suggest that multiple independent mutations may have contributed to the convergent evolution of this novel mode of reproduction.

A new species of Diplatys (Insecta, Dermaptera, Diplatyidae) earwig from the Western Ghats of India

To explore diversity of earwigs (Dermaptera) in different agricultural ecosystems of South India, an extensive taxonomic survey was conducted in 2020 during which an undescribed species of Diplatys was collected. Twenty-one species of the genus Diplatys (Diplatyidae, Diplatyinae) have been reported to date from India, of which six species are known from Karnataka, South India. Based on a male specimen collected from a sugarcane field in Karnataka, a new species, Diplatyssahyadriensissp. nov., is described as the twenty-second species of this genus from India. A revised key to the males of Diplatys species from India and Sri Lanka is provided. This new record adds to the known species diversity in the Sahyadri Ranges of the Western Ghats in Shivamogga District, Karnataka, part of the Southern Plateau and Hills agro-climatic region of India.

Exploration of the homology among the muscles associated with the female genitalia of the three suborders of Psocodea (Insecta)

By using μCT technology, we reconstructed 3D models of the female genital structures and associated muscles of seven species from three suborders of Psocodea (free-living species only, formerly known as independent insect order "Psocoptera"). The homology of the female genital structures and associated muscles of different species is discussed. A total of 21 muscle groups were observed, and except for one muscle, all were homologized throughout the order. Moreover, some of the homologous muscles could be identified confidently in holometabolan insects. Using the muscles as landmarks, we discuss the homology of the ovipositor valves between Psocodea and other neopteran insects. Most importantly, the ovipositor of the suborder Trogiomorpha was identified to consist of the well-developed external valve (V3) plus a remnant of the dorsal valve (V2). We also examined the phylogenetic information included in the female genital muscles and found that certain muscles provide useful information and support deeper nodes (e.g., monophyly of the suborder Psocomorpha). The present study of female genital muscles not only helps us to better understand the phylogeny of Psocodea but also provides a solid foundation for research on muscle evolution.

Penetration mechanics of elongated female and male genitalia of earwigs

We unveiled the penile penetration mechanics of two earwig species, Echinosoma horridum, whose intromittent organ, termed virga, is extraordinarily long, and E. denticulatum, whose virga is conversely short. We characterised configuration, geometry, material and bending stiffness for both virga and spermatheca. The short virga of E. denticulatum has a material gradient with the stiffer base, whereas the long virga of E. horridum and the spermathecae of both species are homogeneously sclerotised. The long virga of E. horridum has a lower bending stiffness than the spermatheca. The virga of E. denticulatum is overall less flexible than the spermatheca. We compared our results to a previous study on the penetration mechanics of elongated beetle genitalia. Based on the comparison, we hypothesised that the lower stiffness of the male intromittent organ comparing to the corresponding female structure is a universal prerequisite for the penetration mechanics of the elongated intromittent organ in insects.

Fitness advantages of the biased use of paired laterally symmetrical penises in an insect

The evolution of laterality, that is the biased use of laterally paired, morphologically symmetrical organs, has attracted the interest of researchers from a variety of disciplines. It is, however, difficult to quantify the fitness benefits of laterality because many organs, such as human hands, possess multimodal functions. Males of the earwig Labidura riparia (Insecta: Dermaptera: Labiduridae) have morphologically similar laterally paired penises, only one of which is used for inseminating the female during a single copulation bout, and thus provide a rare opportunity to address how selection pressure may shape the evolution of population-level laterality. Our population studies revealed that in 10 populations, located at 2.23-43.3° north, the right penis is predominantly used for copulating (88.6%). A damaged penis was found in 23% of rare left-handers, suggesting that the left penis can function as a spare when the right one is damaged. By pairing L. riparia females with surgically manipulated males, we found that males forced to use the right penis outperformed left-handed males in copulation (the probability of establishing genital coupling during the 1-hr observation period: odds ratio [OR] of 3.50) and insemination (probability of transferring a detectable amount of sperm: OR of 2.94). This right-handed advantage may be due to the coiled morphology of the sperm storage organ with a right-facing opening. Thus, female genital morphology may play a significant role in the evolution of handedness and may have acted as a driving force to reduce penis number in related taxa.

Traumatic mating by hand saw-like spines on the internal sac in Pyrrhalta maculicollis (Coleoptera, Chrysomelidae, Galerucinae)

Morphology of the aedeagus and vagina of Pyrrhaltamaculicollis and its closely related species were investigated. The internal sac of P.maculicollis bears hand saw-like spines, which are arranged in a row. Healing wounds were found on the vagina of this species, whose females were collected in the field during a reproductive season. However, the number of the wounds is low in comparison to the number of the spines. In addition, males of P.tibialis bear one spinous sclerite on the internal sac, but the female of this species show no wounds on the vagina. The vaginal wall is thicker in P.maculicollis and P.tibialis in comparison to other studied species, whose males bear no spinous sclerite. This thickening in P.maculicollis is hypothesized that they prevent damaging their own internal sac during everting and withdrawing the internal sac with the spines.

Earwigs from Brazilian caves, with notes on the taxonomic and nomenclatural problems of the Dermaptera (Insecta)

Based on samples collected during surveys of Brazilian cave fauna, seven earwig species are reported: Cylindrogaster cavernicola Kamimura, sp. n., Cylindrogaster sp. 1, Cylindrogaster sp. 2, Euborellia janeirensis, Euborellia brasiliensis, Paralabellula dorsalis, and Doru luteipes, as well as four species identified to the (sub)family level. To date, C. cavernicola Kamimura, sp. n. has been recorded only from cave habitats (but near entrances), whereas the other four organisms identified at the species level have also been recorded from non-cave habitats. Wings and female genital structures of Cylindrogaster spp. (Cylindrogastrinae) are examined for the first time. The genital traits, including the gonapophyses of the 8th abdominal segment shorter than those of the 9th segement, and venation of the hind wings of Cylindrogastrinae correspond to those of the members of Diplatyidae and not to Pygidicranidae. This is the first synopsis of cave-dwelling earwigs of Brazil, one of the most species-rich areas of Dermaptera in the world.