Length Variation and Distribution of the Lesser Stag Beetle Dorcus Parallelipipedus (Coleoptera: Lucanidae)

58 entomologische berichten 73 (2) 2013 Length variation and distribution of the lesser stag beetle Dorcus parallelipipedus (Coleoptera: Lucanidae) Paul Hendriks KEYWORDS Biometry, body length, sexual dimorphism Entomologische Berichten 73 (2): 58-67 The lesser stag beetle (Dorcus parallelipipedus) varies considerably in length. To learn more about this variation, an analysis was made of the length of 1282 individuals throughout its distribution range in Europe. Specimens were from museum collections and from populations. The length distribution of this beetle is clearly affected by sexual dimorphism, the males having larger mandibles than the females and thus greater lengths. Therefore, length distributions of this beetle were analyzed separately for males and females. The maximum and minimum lengths found in this study match with the lengths given in the literature. The information given in this study allows comparison of lengths of beetles under different (environmental) conditions or populations to its general distribution of lengths. Introduction D. parallelipipedus, but these data appeared not to be available. The lesser stag beetle, Dorcus parallelipipedus Linnaeus (fig- In various publications, minimum and maximum lengths were ures 1-2), occurs throughout nearly the whole of Europe, south- given, but these differed considerably (see table 5). The aim ern Scandinavia, Turkey and into southern Russia. The beetle of the current study was to provide a length distribution of lives in rotting wood (Klausnitzer 1995). Although related to D. parallelipipedus across Europe that will make it possible to the stag beetle, Lucanus cervus (Linnaeus), its appearance is far compare the lengths of reared and wild beetles, and to compare less impressive, being two to three centimeters long, uniformly the lengths of beetles from different populations or different black in colour and with much smaller mandibles. Males and locations. To this end, specimens from museum collections females of D. parallelipipedus appear similar in length, but the and from the field were measured, besides using data from males have larger mandibles and broader heads (figures 1-2). the internet. Based on these three data sets, I provide a length Large numbers of adults and larvae can be found in rotting distribution and maximum and minimum lengths for D. paral- wood of standing trunks, stumps, thick branches or logs of de- lelipipedus, which I checked against the literature. ciduous trees (Klausnitzer 1995 and personal observation). There, the beetles (figure 3) live closely together in tunnels Material and methods that they have created by splintering and moving the wood with their mandibles. The places in between the tunnels are Data sets inhabited by the larvae. Larvae also create tunnels, but these are densely packed with wood splinters and their fecal pellets To assess the length distribution of D. parallelipipedus, I collected on which they feed occasionally. The larvae pupate in or near data from three sources. First, 950 specimens from various the harder parts of the wood. Unlike L. cervus, the adults of museum collections were measured, with samples throughout D. parallelipipedus may live for several years (Fremlin & Hendriks the species’ distribution range. This group is referred to as 2011). We have an incomplete picture of the species’ biology and ‘collections’. The second data set consisted of beetles measured behaviour due to its secluded life in decaying wood. This makes in the field, such as beetles found together in dead wood, or in it difficult to spot and observe the species. a restricted area, such as a forest or group of trees. It was made I am performing rearing experiments to study D. paralleli- sure that this group consisted of beetles that were collected pipedus behaviour and biology, including whether adult length randomly within populations and not with the aim to collect depends on the conditions under which the larva develops. the largest or smallest beetle (table 1). In this way a bias in the Earlier studies (Hendriks 2007, Rink 2006, Sprecher-Uebersax measured length due to potential preference of the collector 2001) have revealed that fungal activity in white rotted wood was minimized. The groups of beetles measured in restricted is important for the development of larvae of stag beetles areas were named ‘populations’ and consisted of a total of 332 (Lucanus cervus) and rhino beetles (Oryctes nasicornis). To com- individuals. In addition, I placed an appeal for length data of pare the lengths of reared beetles with those of beetles in the D. parallelipipedus on the internet site of the ‘European stag- field, I searched the literature for the length distribution of beetle discussion group’. entomologische berichten 59 73 (2) 2013 1. Female of D. parallelipipedus. Photo: Paul Hendriks 1. Vrouwtje van D. parallelipipedus. 2. Male of D. parallelipipedus. Photo: Paul Hendriks 2. Mannetje van D. parallelipipedus. The total length of all beetles was taken in millimeters Mann-Whitney U-test. I provide descriptive statistics (the mean, from the end of the elytra to the end of the mandibles because medium, 25 and 75 percentiles and standard deviations as well lengths in literature are given this way. Finally, I gathered re- as maximum and minimum length) for collections and popu- cords of maximum and minimum lengths of D. parallelipipedus lations separately, and for the combined data set to provide from the internet, only using measurements that could be veri- general insight in the beetles’ length distribution throughout fied, for example, from photos with a ruler alongside the beetle. Europe. To compare the data obtained in my study with literature, I carried out a second analysis of length data to establish I carried out a literature survey, starting from the reference lists whether the sexual dimorphism of D. parallelipipedus can be of several studies on Lucanidae, such as those of Klausnitzer explained by a difference in mandible length between the (1995) and Franciscolo (1997). male and the female. For this, I took the total length of 91 available beetles (58 males and 33 females), as well as the length from the end of the elytra until the front part of the Statistical analysis head (mouth parts) of these beetles, thus excluding the man- Length data per group (collections or populations) were first dibles. I refer to this latter measurement as body length. Via analyzed with a normality test (Kolmogorov-Smirnov) for males correlation and regression, I established the relation between and females separately. Male and female length of the collec- total length (including mandibles) and body length (without tion and population groups were then compared using the mandibles) for these 91 beetles. This relationship was then 60 entomologische berichten 73 (2) 2013 3. Length variation in D. parallelipipedus. Males, left row, 29-18 mm. Females, right row, 26-18 mm. Photo: Paul Hendriks 3. Lengte variatie van D. parallelpipedus. Mannetjes, linker rij, 29-18 mm. Vrouwtjes, rechter rij, 26-18 mm. used to calculate body length from total length for the entire somewhere in between. This gave a problem in the analysis set of beetles. Total length and body length of males and fe- of the results; fully opened or closed mandibles sometimes males for the entire set of beetles were statistically analyzed, leading to a difference in total length of 3 to 4% in the largest using the Mann Whitney U test. One set of length data from beetles (see figure 4); in females, this difference is 1 to 2%. As English beetles (C. Hawes, Ipswich, table 1) consisted of body it was not always clear whether the beetles were measured lengths. Table 1 shows the estimated minimum and maxi- with open or closed mandibles, the extra length of beetles mum total lengths of these beetles to allow comparisons with with open mandibles was added to the total length of the the other groups of beetles. largest beetle in the descriptive statistics of the total group of While measuring the dried individuals in museum col- beetles (table 3). This was done to make sure that the maxi- lections, I noticed that the mandibles varied in position, mum length was established. The largest male beetle in the being either folded to the head, open wide or in a position entire data set was 30 mm. Adding 4% extra length results in entomologische berichten 61 73 (2) 2013 a b 4. Difference in total length of a large male beetle with (a) open or (b) closed mandibles. Photos: Paul Hendriks 4. Verschil in lengte van een groot mannetje met (a) open en (b) gesloten kaken. a b 5. Difference in total length of a small male beetle with (a) open and (b) closed mandibles. Photos: Paul Hendriks 5. Verschil in lengte van een klein mannetje met (a) open en (b) gesloten. Results a maximum length for male beetles of 31 mm. In females the added 2% led to a maximum length of 27 mm (largest meas- Beetles from collections and field populations ured female was 26 mm). Adding extra length was not done for the smallest beetles, as there is no discernible difference Table 1 shows the collected length records of D. parallelipipedus in length when measured with opened or closed mandibles from various European countries. In Sweden and England, in- (figures 5). Lastly, I compared the mean values and standard ternet groups were asked to collect length data. In total, 1282 deviations of the population groups from the various coun- beetles were measured, 332 from field populations and 950 from tries with these parameters for the total group (717 males collections (table 1). and 565 females). It indicatively shows the variation of length It was difficult to measure large numbers of beetles from between countries. field populations. Most beetles are found within decayed wood 62 entomologische berichten 73 (2) 2013 Table 1. Collected length data of D.

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