A common terminology for the external anatomy of centipedes (Chilopoda)

Abstract A common terminology for the external morphological characters of centipedes (Chilopoda) is proposed. Terms are selected from the alternatives used in the English literature, preferring those most frequently used or those that have been introduced explicitly. A total of 330 terms are defined and illustrated, and another ca. 500 alternatives are listed.


A common terminology for the external anatomy of centipedes (Chilopoda) Introduction
Th is contribution is intended to propose a common terminology for the external morphological characters of centipedes (Chilopoda). Students still use diff erent terms to describe the same or similar structures in centipedes, even limiting our survey to papers in English. Consequently, the terminology is heterogeneous, redundant, and sometimes ambiguous. Th e lack of standardization hinders comparative analysis and integration of information scattered through the literature, and discourages new students from undertaking taxonomic and morphological investigations on this arthropod group.
Eff orts to revise the terminology have been rare and limited to either particular character sets or selected chilopod sub-groups, as exemplifi ed by the terms for integumental projections discussed by Crabill (1960a) and those proposed for major taxonomic characters in Scolopendromorpha (Lewis et al. 2005). A common English terminology encompassing all external features and applicable to the Chilopoda as a whole has never been proposed.

Methods
Th e terminology recommended here encompasses major structural features of the body and all external characters recognized under light microscopy. Internal characters are not addressed herein, because the terminology in use is more consistent and uniform. We also exclude fi ne structural details, including those of peristomatic structures (epipharynx and hypopharynx), because they have been documented only recently, by histology and scanning electron microscopy, so that a consistent terminology is available (Edgecombe and Giribet 2006;Edgecombe 2006, 2008). Our recommended terminology mainly focuses on adult morphology of extant chilopods, but is intended to be applicable to other post-embryonic stadia and to extinct taxa as well.
We considered all publications in English dealing with centipedes since Lewis' (1981) treatise on chilopod biology (the most recent, comprehensive synthesis on the morphology of this group in English) and a selection of older works also in English (listed in the additional fi le: Pre-1981 publications) that seemed most relevant for the morphological terminology. We omitted XIX century publications, because their terminologies were often based on erroneous or unwarranted homologies with other arthropods and have long been superseded. We retrieved all applicable terms and assessed counterparts.
To maximize future applicability, alternative criteria of selection have been discussed with authors who are either currently the most active centipede systematists publishing descriptions in English and/or have already addressed issues of terminology standardization. In order to identify and recommend a single term for each character, we applied the following criteria: (i) we selected a term already used in the literature, except when all alternatives are either ambiguous or inconsistent with other selected terms; (ii) among alternatives, we selected either the term used most frequently (by most authors and/or in most publications) or the one explicitly introduced and defi ned by an infl uential author; (iii) we applied minor emendations to selected terms (in endings, prefi xes, hyphenations between elements of compound words) when necessary for consistency and uniformity. We refrained from revising the terminology based on homology hypotheses with other arthropods (Edgecombe 2008), because many relationships remain under debate.
Major anatomical diff erences exist between the six centipede orders, fi ve extant -Scutigeromorpha, Lithobiomorpha, Craterostigmomorpha, Scolopendromorpha, and Geophilomorpha -and one extinct, Devonobiomorpha. Morphological and taxonomical investigations by diff erent authors have sometimes been and still are limited to single orders, leading to diff erent terminological traditions. While we propose a consistent terminology for the entire class, we specify the order(s) to which each term is applicable to facilitate usage by students interested in single orders; when no orders are specifi ed, it is meant that the term is applicable to all orders; when an order is specifi ed, it is meant that the term is applicable to at least some taxa in the order.

Results
After reviewing the relevant literature as explained above, we retrieved roughly 830 terms that apply to 330 anatomical features. By applying the criteria described above, we obtained the recommended terminology presented herein.

Abbreviations and formulae
Conventional abbreviations recommended for describing particular elements and patterns of elements are described below.
Antennal articles in Geophilomorpha. Each article is indicated by a Roman number, from the most basal article (I) to the most distal one (XIV).

Arrangement of ocelli in Lithobiomorpha.
Th e number of ocelli in diff erent rows are indicated from the most dorsal row to the most ventral row, separated by commas [1+ n 1 , n 2 , … where 1 is the posterior ocellus and n 1 , n 2 , …are the numbers of seriate ocelli in the rows].
Pattern of teeth on the anterior margin of the forcipular coxosternite in Lithobiomorpha and on the tooth-plates in Scolopendromorpha. Th e number of teeth is indicated for the right and the left side, separated by a plus [n right + n left ].
Leg-bearing segments and pairs of legs. Each leg-bearing segment, or the corresponding pair of legs, is indicated by an Arabic number, from the most anterior one (1) to the most posterior one.
Tergites and sternites of the leg-bearing segments. Each tergite and sternite is indicated by T and S respectively (TT and SS for multiple tergites and sternites, respectively), followed by an Arabic number, from the most anterior ones (T1 and S1) to the most posterior ones.
Arrangement of spurs on the legs in Li thobiomorpha (plectrotaxy; Table 3). Th e position of the spurs on each article of the legs is indicated in a tabular form (Table 4). Spurs on the ventral side are indicated on the left part of the table, those on the dorsal side on the right part. Th e pairs of legs are indicated by Arabic numbers, from the most anterior one (1) to the most posterior one (15), as described above. Th e articles are indicated by the following abbreviations: C = coxa, t = trochanter, P = prefemur, F = femur, T = tibia (upper case letter, except for trochanter). Th e position of each spur relative to the antero-posterior axis is indicated by the following abbreviations: a = anterior, m = median, p = posterior (lower case letter). Leg articles without  F  T  C  t  P  F  T  1  --mp  amp  am  --mp  ap  a  2  --mp  amp  am  --amp  ap  ap  3  --mp  amp  am  --amp  ap  ap  4  --mp  amp  am  --amp  ap  ap  5  --mp  amp  am  --amp  ap  ap  6  --mp  amp  am  --amp  ap  ap  7  --mp  amp  am  --amp  ap  ap  8  --mp  amp  am  --amp  ap  ap  9  --mp  amp  am  --amp  ap  ap  10  --mp  amp  am  --amp  ap  ap  11  --mp  amp  am  --amp  ap  ap  12  --amp  amp  am  a  -amp  p  ap  13  -m  amp  amp  am  a  -amp  p  p  14  -m  amp  amp  am  a  -amp  p  p  15  -m  amp  amp  am  a  -amp  p  -spurs are indicated by a dash. Spurs that could be absent (variation) are indicated in parentheses. Spurs that could be present on one side only are marked by an asterisk. Additional spurs are indicated by the abbreviation of their position (a, m, p) typed as a superscript to the corresponding spur (e.g., a a ). Legs with the same plectrotaxy in both ventral and dorsal side can be indicated in distinct rows or in a single, common line. Th is convention was fi rst proposed by Ribaut (1921) and introduced in the English literature by E.H. Eason and R.E. Crabill (Eason 1951(Eason , 1964Crabill 1953Crabill , 1962Crabill and Lorenzo 1957). Each individual spur is indicated in a text with a formula comprising the following abbreviations: the pair of legs (1-15), the side of the leg (V = ventral, D = dorsal), the position relative to the antero-posterior axis (a, m, p), the leg article (C, t, P, F, T) (e.g., 15VaC). Pattern of coxal pores on the legs in Lithobiomorpha. Th e number of coxal pores is indicated from anterior to posterior legs, without separation between the numbers […n 13 n 14 n 15 ].