Observation of heterospecific mating attempt by b lue chaser Libellula fulva Müller, 1764 and b road-bodied chaser L. depressa Linnaeus, 1758 (Odonata: Libellulidae)

. A successful copula formation between a Libellula fulva male and a L. depressa female was documented photographically on 23. 6. 2022 along a small stream at the Natura 2000 site Ličenca pri Poljčanah in NE Slovenia. This represents the first record of an anomalous mating attempt with copula formation between the species involved. Their distribution in Slovenia as well as syntopic and syntemporal observations in the country are presented and briefly discussed, as are the site - specific factors and aged female colouration that may have contributed to the described rare attempt of heterospecific mating


Introduction
Dragonflies (Insecta: Odonata) have a unique mode of reproduction among insects with indirect insemination and delayed fertilization.In order to reproduce, sexually mature dragonflies must encounter a conspecific of the opposite sex, recognize it and mate with it.Prior to copulation, sperm is transferred from the male's primary genitalia at the tip of the abdomen to his secondary genitalia at the base of the abdomen, and during the formation of the heartshaped copula or mating wheel, it is transferred to female's genitalia.The eggs are not fertilized until they are laid.Recognition of sex and species in most odonates is primarily based on visual NATURA SLOVENIAE 25(2): 53-60 characteristics such as size, body shape, flight style, colour and colour pattern (Corbet 1999;Wildermuth & Martens 2019).
Reports of anomalous mating attempts between different dragonfly species are not extremely rare, and comprehensive reviews on this topic have been presented by e.g.Bick & Bick (1981), Utzeri & Belfiore (1990), and Corbet (1999).They include both observations of homosexual heterospecific male tandems, as well as heterosexual heterospecific pairing attempts.The latter usually include observations of interspecific tandems within the same genus (e.g.Heidemann 1982;Miller & Fincke 2004;Chovanec 2022), less frequently combinations between representatives of different genera (e.g.Bedjanič 2006;Wildermuth 2015), or even between different families within the same suborder (e.g.Corbet 1999;Kosterin et al. 2001;Tamm et al. 2015).In most instances, the anomalous mating attempt ends in the tandem stage, before the contact of male's secondary copulation organ and female's genitalia and thus prior to actual sperm transfer.Less commonly, the heterospecific mating wheel is formed, which is an obligatory stage in the functional framework for potential insemination and subsequent fertilization.The reports of subsequent oviposition after heterospecific copulation are much rarer (e.g.Kunz 2010), while reports on confirmed interspecific hybrids in dragonflies are only occasional (Corbet 1999;Futahashi & Hayashi 2004, Okude & Futahashi 2022;Solano et al. 2018).
Nevertheless, with rare exceptions (e.g.Kunz 2010), the great majority of reports only documents tandem formation, without actual copulation, i.e. without the formation of a mating wheel.In the genus Libellula, individual cases of heterospecific copulation have been reported in the Nearctic (Bick & Bick 1981) and Oriental species (Utzeri & Belfiore 1990), while only a few scattered reports are known for European representatives of the genus -e.g.Seggewiße (2008) documented an apparently unsuccessful attempt of copula formation between a L. fulva male and a L. quadrimaculata female, while Wildermuth & Martens (2019) briefly mention exceptional cases of anomalous mating attempts between L. depressa and L. quadrimaculata.A further observation of heterospecific copulation between a L. fulva male and a L. depressa female, not previously reported in the literature, is added in this article.

Materials and methods
The observations described in the sequel were made on 23. 6. 2022 at the Natura 2000 site Ličenca pri Poljčanah in NE Slovenia.The locality is a small right tributary of the Ličenca stream at the bridge of the Ponevnik-Zgornje Laže side road, 200 meters N of the settlement of Zgornje Laže (WGS 84 Lat./Long.: 46.3223 °N, 15.5396 °E).The aforementioned small tributary of the Ličenca stream is less than 1 meter wide, the water current is slow and the richly overgrown riparian vegetation almost completely covers the water surface.On the upstream side of the 25(2): 53-60 bridge, a small widening is present, forming a shallow 1.5 × 0.5 m wide pool, overgrown with Sparganium sp. and some clumps of Carex sp. and Iris pseudacorus, but still with some open water surface.The surrounding landscape is an open mosaic agricultural land, upstream of the bridge on one side there is a regularly mown semi-intensive meadow, while the other side is bordered by a field.Along the stream there is a narrow, unmown belt of vegetation with a few small Salix shrubs.
The observations on site were made from the road bridge.In the afternoon, between 15:15 and 15:25 p.m. (Central European Summer Time), the weather was predominantly sunny with 25 o C, very light cloud cover and no wind.Photographs were taken with the Sony Cyber-Shot RX10 IV digital camera.The precise time of each individual photograph was subsequently determined at home using the automatically saved image properties.

Results and discussion
On arrival at the site, already known from previous visits, regular rapid dragonfly inventory was carried out from the road bridge.Soon, a somewhat strange, restlessly flying tandem was spotted, and only seconds later I realized that it consisted of a L. fulva male and a L. depressa female.Apparently, the female was trying to free herself from the grasp of male's appendages, but was unsuccessful.The pair in tandem was restless, the potential mates repeatedly settled down for a few seconds and then flew off again and changed position.After observing this behaviour for about a minute and being unable to take a photo, the first observed copula was formed at around 15:19:40 (Fig. 1a) and lasted less than 15 seconds.Then the copula disbanded and the pair still remained in tandem at the same location.From above, at a distance of a few centimetres and without any contact, it was harassed for a few seconds by a flying L. depressa male (Fig. 1b).After changing position and flying around at short intervals, the pair successfully formed the second copula at around 15:20:25, whereupon several successful photos were taken (Fig. 1c, d).The pair in the copula changed position on Sparganium leaves once or twice, but remained in the copula at least until 15:22:12, when the last photo was taken.After that, visual contact with the pair was lost, and they did not return to the same location for the next two or three minutes before I left.Other dragonfly specimens observed during my brief visit to this The basic prerequisite for potential heterospecific sexual interaction is clearly the simultaneous occurrence of adult dragonflies in space and time.Regarding the known occurrence of both species in Slovenia, the database of the Slovene Dragonfly Society and the Centre for Cartography of Fauna and Flora (as of October 2023) contains 571 localities with 937 faunistic data for L. fulva, while L. depressa is much more common with 2,200 known localities and 3,768 faunistic data (Fig. 2).More importantly, both species were recorded at the same locality in 337 cases with 814 faunistic data.Their co-occurrence on the same date was recorded at 279 localities for which 375 such faunistic data are available, the latter meaning the simultaneous observation of both species at the same location and on the same date.
NATURA SLOVENIAE 25(2): 53-60 As for the observations of mating activity of both species, considering conspecific pairs with copula or tandem formation, the above-mentioned database contains 164 data for L. depressa and 147 for L. fulva, but with an inverse ratio of only 220 individual conspecific pairs observed in the former and 471 pairs observed in the latter.Regardless of the commonness of L. depressa, the above numbers are not surprising due to the different mating behaviour.In L. fulva, copulation usually lasts between 10-15 minutes, with the pair usually settling in a sunny place after a short zigzag flight, while in L. depressa, copula formation is very short and copulation completed in flight usually lasts only 4-30 seconds (Sternberg 2000;Sternberg et al. 2000;Wildermuth & Martens 2019).However, the presented data from Slovenia show that the observations of conspecific mating attempts extracted from all faunistic data of the species are not rare, but nevertheless much more frequent in L. fulva (15.7 %) than in L. depressa (4.4 %).
Other factors that may have contributed to the rare anomalous mating attempt are the specifics of the microlocality, where the road bridge and overgrown upstream section direct the adult dragonflies to concentrate around a small shallow pool and stream section with some open water surface only a few square meters in size.In such small-size environment, conspecific and heterospecific interactions are more likely to occur.It should also be noted that the end of June falls towards the end of flight period for both species, which means that the individuals are NATURA SLOVENIAE 25(2): 53-60 already old, and have much less vivid colour patterns, as evident from the dull brownish coloration of the L. depressa female, which has also developed a slight light blue pruinescence on the dorsum of the middle abdominal segments (Fig. 1b).Neglecting considerably broader abdomen and the larger dark basal spots on the fore and hind wings, this colouration somewhat resembles the colouration of old L. fulva females and may have contributed to the observed mating confusion.
As the Slovenian odonatological records clearly show, it is obvious that L. fulva and L. depressa occur together spatially and temporally and that they often share the same habitats at the same time.The situation is probably similar in many other parts of their range in Central Europe at least.It is therefore surprising that the present observation is only the first evidence of an anomalous mating attempt with copula formation between the species involved.In the present case, the locality specifics and the older age colouration of the involved L. depressa female could have been the decisive factors.In any case, the complete lack of similar reports from elsewhere suggests that the mate recognition and other mechanisms that prevent heterospecific mating attempts between the two species are apparently very effective.