Apatetica, Westwood, 1848

Zaytsev, A. A. & Tokareva, A. S., 2021, Larval description of Nodynus leucofasciatus Lewis, 1879 (Coleoptera: Staphylinidae: Apateticinae) with discussion of Staphylinidae chaetotaxy, Russian Entomological Journal 30 (4), pp. 482-499 : 492-496

Difference between Apatetica and Nodynus larvae

Major differences between Apatetica and Nodynus larvae were pointed out before [ Grebennikov, Newton, 2012], such as antenna exceeding head capsule length (much shorter in Nodynus ) and 1-jointed urogomphi, exceeding abdomen length (much shorter and 2-jointed in Nodynus ). However, larvae of Apatetica , studied by Grebennikov and Newton, had both urogomphi broken, so the number of joints should be revised. Additional difference is ligula apically covered in numerous microtrichia in Apatetica (smooth in Nodynus ).

Taxonomic position of Apateticinae based on larval characters

As was mentioned above,there are several principal views on taxonomic position of Apateticinae , which will be briefly discussed here on the basis of larval characters. It should be noted that these results are preliminary and separate study with additional data on larval morphology (including chaetotaxy) of various groups of Staphylinoidea is needed.

A group within Silphidae , part of Silphinae [ Madge, 1979].

Apateticinae larvae share some common features with those of Silphinae , such as six pairs of stemmata, similar shape of mandibles and hypopharyngeal sclerotization as well as presence of multiporose structures on epipharynx. But most of these attributes can also be found in other staphylinid subfamilies, so cannot be considered as the proof of close relationship. For instance, similar mandible structure is characteristic for some Aleocharinae , Omaliinae , Trigonurinae , etc [ Steel, 1970; Pototskaya, 1976]; six pairs of stemmata are present in some Scaphidiinae , all Trigonurinae , Glypholomatinae and other groups [ Thayer, 2005]; multi-

Figs 54–65. Nodynus leucofasciatus , microsculpture: 54, 57, 60, 63 — larva I instar; 55, 58, 61, 64 — II instar, 56, 59, 62, 65 — III instar; 54–56 — frontale; 57–59 — parietale; 60–62 — labium, submentum and mentum; 63–65 — pronotum. Not to scale.

Рис. 54–65. Nodynus leucofasciatus , микроскульптура: 54, 57, 60, 63 — личинка I воЗраста; 55, 58, 61, 64 — II воЗраста; 56, 59, 62, 65 — III воЗраста: 54–56 — фронтальный склерит; 57–59 — париетальный склерит, сверху; 60–62 — нижнЯЯ губа, субментум и ментум; 63–65 — пронотум. Не в масШтабе.

porose structures on epipharynx also developed in some Tachyporinae , Aleocharinae and Phloeocharinae [ Ashe, Newton, 1993; Ashe, 2005]. Hypopharyngeal sclerotisation is poorly studied in staphylinid larvae and its similar structure can be possibly found in other representatives of the family. At the same time, Apateticinae larvae differ from those of Silphinae by presence of modified (apically frayed) setae as well as epicranial and tergal glands.

Moreover, typical Silphinae larval features such as

Figs 66–80. Nodynus leucofasciatus , microsculpture: 66, 69, 72, 75, 78 — larva I instar; 67, 70, 73, 76, 79 — II instar; 68, 71, 74, 77, 80 — III instar; 66–68 — mesonotum; 69–71 — abdominal tergite V; 72–74 — abdominal sternite VI; 75–77 — fore leg, trochanter and femur, anterior view; 78–80 — basal joint of urogomphi, dorsal view.

Not to scale.

Рис. 66–80. Nodynus leucofasciatus , микроскульптура: 66, 69, 72, 75, 78 — личинка I воЗраста; 67, 70, 73, 76,

79 — II воЗраста; 68, 71, 74, 77, 80 — III воЗраста; 66–68 — меЗонотум; 69–71 — V тергит брюШка; 72–74 — VI стернит брюШка; 75–77 — нога первой пары: вертлуг и бедро, спереди; 78–80 — баЗальный членик урогомфы, сверху. Не в масШтабе. 80

the development of extending laterotergites, mala with large galeal lobe covered in numerous setae and apically bilobed ligula were not found in both Nodynus and Apatetica larvae. So, based on larval morphology, we cannot confirm close relations of Apateticinae and Silphinae .

Sister-group of Trigonurinae or Scaphidiinae [ Grebennikov, Newton, 2012; McKenna et al, 2015].

Most of the shared features of Apateticinae and Trigonurinae , such as six pairs of stemmata, shape of mandibles and epipharynx with multiporose structures couldn’t indicate close relations, as was mentioned above. Although Grebennikov and Newton [2012] mentioned the presence of frayed setae on head and body segments of Trigonurus as shared feature with Apateticine larvae, judging by figures they are actually not frayed, but clavate pubescent ones [ Pototskaya, 1976; Lawrence, 1991], which are very rare in Staphylinidae . Moreover, Trigonurus larva differs from Apateticinae by the bilobed ligula and epipleurites fused with abdominal tergites I–IX [ Pototskaya, 1976]. Another inconsistent character is the number of urogomphi joints, which is two in Nodinus and Trigonurus , but single one in Apatetica (see remark above). Besides, urogomphal joints in Trigonurus have distinct ring-shaped sculpture [ Grebennikov, Newton, 2012] which is absent in Apateticinae .

According to recent molecular phylogenetic studies, Apateticinae and Scaphidiinae considered to have a sister-group relations [ McKenna et al, 2015; Gusarov, 2018]. Shared features of larvae of these two subfamilies are rather hard to point out because of variable morphology of Scaphidiinae , which larvae can have five or six pairs of stemmata, one- or two-jointed urogomphi, various shape of mandibles, etc. [ Kasule, 1966; Kompantsev; 1987]. Nonetheless, larvae of these two subfamilies have both epicranial and tergal glands and the structure of thoracic endoskeleton is nearly identical in Scaphidium and Nodynus larvae. However, glands mentioned above are also found in Glypholomatinae , Aleocharinae , Oxytelinae and Staphylininae larvae [ Kilian, 2007]. Identity of the thoracic endoskeleton potentially could be of taxonomic significance, but knowledge of its structure within Staphylinoidea is fragmentary, so this similarity should be treated with caution. Scaphidiinae larvae differ from Apateticinae by crenulate emargination on anterior margin of labrum, round- ed ligula, epipleurites fused with abdominal tergites II– IX and hypopleurites fused with abdominal sternites VIII–IX [Ashe, 1984; Kompantsev, 1987]. Moreover, Scaphidiinae larvae lack frayed (and modified in general) setae and multiporose structures in epipharynx. So, in our opinion, there is no direct confirmation of close relations between Apateticinae and Scaphidiinae or Trigonurinae based on larval morphology.

Separate family in Staphylinoidea [ Hansen, 1997]

Based even on preliminary comparative analysis given above, it becomes quite evident, that most morphological features of Apateticinae larvae (number of stemmata, shape of mandibles, presence of apically frayed setae and mutiporose structures, set of thoracic endoskeleton elements etc.), can be found in different combinations in various groups within the family. The only unique feature that can be established for Apateticinae is “tetra-lobed” ligula, so in our opinion there are not enough morphological evidences to count this group as separate family in Staphylinoidea. It is interesting to mention though, that very similar shape of ligula is known in some Hydraenidae [ Deler-Hernández, Delgado, 2017], but it requires further elaboration.

At the present time it is rather difficult to make any conclusions about the taxonomic position of Apateticinae within the Staphylinidae (except that it’s almost certain they are not part of Silphinae , which are often treated as staphylinid subfamily nowadays) due to combination of characters mentioned above in various representatives within the family. Thus, we preliminary agree with Grebennikov and Newton [2012], who refer this "basal" group as Staphylinidae incertae sedis.

Notes on the chaetotaxy of Staphylinidae

Nowadays two main nomenclatural systems of chaetotaxy are used in studies of various groups of Coleoptera . The first one is by Ashe and Watrous [1984], proposed initially for Aleocharinae and used later for other subfamilies of Staphylinidae , as well as, with certain degree of modification, different Staphyliniformia families, such as Leiodidae , Hydraeinidae, Catopidae , Ptiliidae [ Wheeler, 1990; Delgado, Soler 1997; Sörensson, Delgado, 2019]. More recently this system was proposed for such distant groups as Helotidae and Monotomidae (Cucujoidea) [ Lee et al, 2007; Wagner et al, 2020].

However, application of this system meets some difficulties in establishing of homologies within the family or Staphylinoidea in general. Firstly, in Ashe and Watrous paper chaetotaxy nomenclature for antennae, mandibles, and labio-maxillar complex have not been provided, although their setal pattern is rather constant within the family. Secondly, this system undergo various modifications in studies of other staphyliniform families, e.g. change in the setal rows orientation from transverse to longitudinal [ Wheeler, 1990; Kilian, 2007] and different principle of coding [ Delgado, Soler, 1997]. It resulted in the fact that obviously homological setae were coded differently,which could lead to confusion, especially when one tries to establish the common chaetome pattern for Staphyliniformia in general. For example, seta Al2 [ Ashe, Watrous, 1984] located on trochanter closely to pore c4, and typical for all Staphyliniformia (and most likely beetles in a whole), is coded as Ad1 for Leiodidae [ Kilian, 2005] and Hydraenidae [ Delgado, Soler, 1997]; frontal seta Fd 3 in Aleocharinae is coded as Da2 for Leiodidae and Fd2 for Ptiliidae [Sörensson, Delgado, 2019], etc. So, despite significant number of homological setae in larvae of these families, this similarity is unobvious due to the different coding. Moreover, the designation of chaetome elements based on the location on certain side of the body or appendage (marginal, lateral, anterolateral, posteroventral, etc) could also become the reason of different names appearing for the same setae. Such situation is rather natural, because the position of certain setae may change in comparison with Dalotia (Atheta) coriaria Kraatz , the model of Ashe’ system, or even if orientation of the object on slide mount is different. In fact, it occurs even in studies within Aleocharinae , e.g., seta Pv1 on tibiotarsus may be coded as V1 [Staniec et al, 2018a] or Av2 [Staniec et al, 2018b]. All these difficulties resulted in the situation, when some authors rejected Ashe and Watrous system, suggesting consistent numeration of chaetome elements instead [ Solodovnikov, Newton, 2005; Pietrykowska-Tudruj et al, 2011].

Also, Ashe and Watrous system is based on study of third-instar larvae, whose chaetotaxy is characterized by appearing of “secondary”setae, which number and position is significantly variable. Wheeler, in his study of Leiodidae [1990] pointed out the necessity of studying the chaetotaxy of first-instar larvae, and in subsequent works on Leiodidae , Hydraeinidae and other Staphyliniformia families [ Delgado, Soler, 1997; Kilian, 1998] this was performed. Unfortunately, in most papers on larval chaetotaxy of Staphylinidae , data are provided only for last-instar larvae, which makes certain difficulties in understanding common chaetome elements within the family.

The second nomenclatural system have been proposed by Bousquet and Goulet [1984] for Carabidae , and later improved by Makarov [1996]. It was subsequently used in studies of other adephagan families, such as Dytiscidae , Haliplidae , Rhysodidae [Alarie, Harper, 1990; Makarov, 2008; Michat et al, 2020], as well as Cupedidae (Archostemata) [ Grebennikov, 2004], and Hydrophilidae (Hydrophiloidea) [ Fikacek et al, 2008].

In our opinion this system have the following advantages over that of Ashe and Watrous: chaetotaxy of all the body regions and appendages was coded and the designation of homological setae carried out with the help of additional methods, such as sigillotaxy and diagnostic of rather stable complexes (e.g. “seta-pore”) [ Makarov,1996].

Based on our study, it appears that significant part (about 70%) of Nodynus chaetome elements can be reliably homologized with those of Carabidae (Table). The highest degree of congruence can be traced on body segments (except abdominal IX–X) and leg joints (except coxa). Less obvious homologies can be established for structures which undergo significant transformation comparing with Carabidae , such as labrum, labium, maxilla with galea and lacinia fused, as well as abdominal segment IX with two-jointed urogomphi. However, we believe that even on such structures more homological elements of chaetome could be found, if comprehensive study of first-instar staphylinid larvae would be performed. So, we conclude that Bousquet and Goulet system can be applied for Staphyliniformia larvae and, potentionally, other groups of beetles. It can be the first step to unified nomenclatural system of chaetotaxy for beetles larvae in general, based on homological elements, which can have a certain value for phylogenetic study within the order. Moreover, if we agree with the basal position of Apateticinae within Staphylinidae (see Discussion above), it makes us suggest that their chaetome pattern can be close to original for the family, thus making it the “starting point” when analyzing other staphylinid larvae chaetotaxy.

Acknowledgements. We are sincerely grateful to K.V. Makarov (MPGU) for permission to use photo of adult Nodynus and for valuable comments during our study.

Competing interests. The authors declare no competing interests.