Largusoperla, Chen, Wang & Du, 2018

Chen, Zhi-Teng, 2018, A new fossil species of Largusoperla (Plecoptera: Perlidae) and its evolutionary implications, Zootaxa 4442 (4), pp. 572-578 : 575-577

Notes on Largusoperla

Asymmetry of genitalia is commonly found in insects, which may be related with mating positions and seXual selection ( Huber et al. 2007). In the supposed sister order to the Plecoptera ( Crampton 1938, BoudreauX 1979, Zwick 2009, Kômoto et al. 2012, Letsch & Simon 2013), Embioptera , males have asymmetric genitalia, usually involving the structures near the ejaculatory duct, the cerci and even the abdominal tergites and sternites ( Ross 2000). The correlation between male asymmetry in Embioptera and mode of copulation has been assumed: the male lies either on top or on the right side of the female and directs his terminalia beneath the female’s abdomen ( Snodgrass 1937, Stefani 1953, Ross 1970).

Most adult stoneflies have symmetrical genitalia. Whereas in the superfamily Nemouroidea, some males of the Brachypterainae , Nemouridae , and Capniidae possess asymmetrical paraprocts and epiprocts ( Huber et al. 2007). In addition, some species from Peltoperlidae and Perlodidae have also shown asymmetrical aedeagi ( Zwick & Surenkhorloo 2005, Stark & Sivec 2007). These phenomena have suggested the eXistence of several independent origins of asymmetry in Plecoptera ( Zwick 2000) .

When mating, a stonefly male usually takes an oblique position dorsally on the back of a female or sits beside the female, then curves his abdomen to the left or right side of the female, probes for the subgenital plate of the female with the paraprocts or epiproct, and finally achieves genital contact ( Moreira 1998). The mating position in stoneflies with symmetrical genitalia is random-sided, either left or right ( Brinck 1956, Stewart & Stark 1977). Although, Berthélemy (1979) indicated that in the males of some Brachypterainae with asymmetrical genitalia, mating is random-sided. However, this report should be treated with caution because of contradictions with the correlation between morphological asymmetry and one-sided mating position/twisting, and random-sided positions correlated with symmetrical genitalia ( Huber et al. 2007).

In the new species of Largusoperla , the asymmetrical paraprocts may suggest a one-sided mating position of this species. The preference of a fiXed mating position may reduce the time of probing for the subgenital plate of the female, which could increase the mating efficiency. Meanwhile, the enlarged paraprocts of Largusoperla also assist to rapidly touch and firmly connect with the subgenital plate. Accordingly, females of Largusoperla are eXpected to have elongated and modified subgenital plates to match the male’s paraprocts, although no females of Largusoperla have been found to date.

Since Largusoperla appears to be the oldest known genus of subfamily Acroneuriinae and is the only Acroneuriinae genus found in amber with the male described, the eXact evolutionary events from mid-Cretaceous to today is unknown. But based on available evidence, ancients of Acroneuriinae may have evolved characters for the particular environment eXisting before the mid-Cretaceous. These ancient Acroneuriinae stoneflies may have preferred encounter sites on plants than on the eXposed instream or streamside stones or on the ground for the following reasons: 1) a presence of a hammer on male sternum indicating mate finding by drumming ( Stewart 2001, Stewart & Maketon 1991, Stewart & Stark 1977, Maketon & Stewart 1988); 2) phenomenon necessary for the formation of amber also suggest well-developed riparian zones, habitats for the adults of these stoneflies ( AleXander & Stewart 1996).

Gymnosperms formed dominant floral communities during the Cretaceous ( Sahney et al. 2010) and the usually slender leaves of these plants may have not provided an adequate surface for the mating stoneflies. Additionally, the uncovered seeds of the gymnosperms may have attracted potential natural enemies, such as the ancient seedfeeding birds to prey on stonefly adults ( Zhou 2004, Zhou & Zhang 2002). It is suggested that the species of Largusoperla evolved larger paraprocts or robust cerci (Chen 2018) to enhance successful mating. Some Acroneuriinae groups, such as some species in Largusoperla may have utilized a fiXed mating position to decrease mating time, which might be the reason of the asymmetric paraprocts. After mid-Cretaceous, angiosperms began to flourish ( Sauquet et al. 2017). The wider leaves of the angiosperm plants may have offered stoneflies more suitable mating sites. The paraprocts of the Acroneuriinae apparently gradually decreased in size and became symmetrical in eXtant species of Acroneuriinae . The discovery of adult Largusoperla females may contribute additional information to support the above speculation.