Isopods were collected by hand in Sežana, Slovenia. Live isopods were imaged using a DigiMicro Profi digital microscope and MicroCapture Pro software (both from DNT). All collected specimens were preserved in 96% ethanol. For light microscopy, pleopods and mouthparts were dissected with a pair of entomological pins and mounted in Euparal (Waldeck). Pereopods, uropods, antennae and mandibles were dissected with tweezers and mounted in glycerol. Whole specimens were imaged in 96% ethanol using an MZ FLIII stereomicroscope, equipped with a DFC425 C digital camera (both from Leica). Dissected appendages were imaged using an AxioImager Z.1 microscope equipped with HRc and MRm digital cameras (all from Zeiss), which was also used for fluorescence imaging with blue excitation (450-490 nm; Zeiss filter set 09) to visualise the structuring of the epicuticle. Drawings were prepared from micrographs using Illustrator software (Adobe). For scanning electron microscopy (SEM), the appendages and cephalothorax were transferred from 96% ethanol to pure acetone, air dried, sputter-coated with platinum and observed using a JSM-7500F field emission SEM (JEOL) at 5 kV acceleration voltage.

All specimens illustrated in the description, as well as three aditional females, were deposited in the Slovenian Museum of Natural History (PMSL-Isopoda-000001–000009). Other specimens are kept in the collection of the author at the University of Ljubljana, Biotechnical Faculty, Department of Biology.

Additional individuals were observed by chance in Paklenica National Park in Croatia, but were not collected. The observed isopods were photographed with a Samsung Galaxy S9 smartphone. To estimate their body-length, they were measured on photographs in comparison to features on the author’s hand.

The examined specimens of T. vespertilio were collected in Sežana in the coastal area of Slovenia. The isopods were found under wooden boards and large stones in green areas between multi-storey residential buildings. The presence of juveniles and ovigerous females demonstrates that the isopods in Sežana reproduce, suggesting that they are permanent residents of the region, although found only in a synanthropic habitat so far. Our observations from Croatia indicate that this species also lives in forests and its choice of habitat may depend on the local climate. The isopods collected in Sežana were slightly smaller than the specimen previously described (Budde-Lund 1896, Schmidt 1997), as their body length did not exceed 12 mm. By contrast, individuals observed in Croatia were larger than the specimens from Slovenia, approximately 14 mm in length and comparable with the reported size of Budde-Lund’s specimen.

The lack of records of T. vespertilio after its original discovery may be due to the species being overlooked for a time. Prior to the revision of the genus Trachelipus (Schmidt 1997), there were no illustrations of T. vespertilio and only the original description could be used for its identification. In addition, T. vespertilio was not included in most identification keys, including the influential key by Schmölzer (Schmölzer 1965). As a result, it was difficult to identify it if it had been collected. Nevertheless, the region of Slovenia where T. vespertilio is present was well studied less than twenty years ago without finding the species (Vilisics and Lapanje 2005). It is, therefore, likely that T. vespertilio is expanding its range northwards in synanthropic habitats and has only recently established populations in Slovenia. Although new records expand the distribution range of T. vespertilio, it remains small, making the species vulnerable to extinction.

As discussed by Schmidt, male characters important to species identification in the genus Trachelipus include the shape of male pereopod 7, especially the carpus and the shape of male pleopod 1 (Schmidt 1997). Other taxonomically significant characters in this genus include the size and position of the glandular pore fields on the epimera, the structure of pleopodal lungs, the size and shape of the cephalic lobes and granulation of the tergites. In his revision of the genus, Schmidt considered T. vespertilio a valid species, based on the shape and relative size of the cephalic lobes, with the lateral lobes “far surpassing the median lobe in size and about twice as long as eyes”. This certainly describes the species well; although its cephalic lobes are not remarkably large, the eyes of T. vespertilio are relatively smaller than in other Trachelipus species with well-developed cephalic lobes. This, in combination with other characters, such as the convex edges of lung fields with numerous setae, the positions of glandular pore fields near the edges of pereon epimera, the straight edge of the concavities on the posterior edge of pereonite 1, makes T. vespertilio easily distinguishable from other Trachelipus species inhabiting the region.

Some other European species of Trachelipus, such as Trachelipus camerani (Tua, 1900) and Trachelipus rhinoceros (Budde-Lund, 1885) may have similarly-shaped cephalic lobes as T. vespertilio and overlapping distribution ranges. The median cephalic lobe usually has a straight front edge in T. camerani, differentiating it from T. vespertilio, but it is reportedly highly variable in T. rhinoceros (Schmidt 1997). Nevertheless, these species can be easily distinguished from T. vespertilio. In both T. camerani and T. rhinoceros, the posterior edges of the tergites are serrated. Furthermore, the distal article of the antennal flagellum is less than twice as long as the proximal article in these two species. Other differences are apparent in male characters, such as the shape of pereopod 7 and the position of the tuft of setules on the endopod of pleopod 1, which is subapical in T. camerani and T. rhinoceros.

Another morphologically similar species is Trachelipus ratzeburgii (Brandt, 1833), which resembles T. vespertilio in characters such as the position of glandular pore fields, the relative position of noduli laterales, the appearance of the cephalic lobes and the shape of the antenna. An important difference between T. vespertilio and T. ratzeburgii is the relative size of the eyes, which are longer than half the length of both the cephalothorax the lateral cephalic lobes in T. ratzeburgii. Furthermore, the margins of lung fields on most pleopods are not convex in T. ratzeburgii and the tuberculation of the tergites is less pronounced. The two species also differ in colouration, although this is likely to vary to some extent. Further differences are in the crest on the carpus of male pereopod 7, which is highest proximally in T. ratzeburgii and highest in the middle in T. vespertilio, but this feature is also expected to vary, at least in T. ratzeburgii (Tomescu et al. 2015). Other male characters are similar in the two species.

Although a total of 17 specimens were collected in this study, only one male was unfortunately available for microscopic examination. This makes it impossible to assess the possible variability of male characters between individuals of T. vespertilio, which may be considerable in some species of the genus (Tomescu et al. 2015).