Agathodesmus Silvestri, 1910,
treatment provided by
|Agathodesmus Silvestri, 1910|
Agathodesmus : Silvestri 1910:362. Attems 1914:282, 1940:487. Brölemann 1916:547, 587. Jeekel 1971:310; 1982:11; 1983:146; 1985:50, 51; 1986:46. Hoffman 1980:184. Simonsen 1990:57. Golovatch et al. 2009:2. Mesibov 2009:92.
Atopogonus : Carl 1926:386. Attems 1940:477. Verhoeff 1941:406. Jeekel 1971:314, 1984:88, 1986:46. Hoffman 1980:186, 1999:480. Simonsen 1990:57. Golovatch et al. 2001:185, 2009:2, 44. Mesibov 2009:92 (synonymised).
Other assigned species: Agathodesmus adelphus sp. n., Agathodesmus aenigmaticus sp. n., Agathodesmus agnus sp. n., Agathodesmus anici sp. n., Agathodesmus bonang sp. n., Agathodesmus bucculentus (Jeekel, 1986), Agathodesmus carorum sp. n., Agathodesmus chandleri sp. n., Agathodesmus gayundah sp. n., Agathodesmus hahnensis sp. n., Agathodesmus johnsi Mesibov, 2009, Agathodesmus kerensis sp. n., Agathodesmus kirrama sp. n., Agathodesmus millaa sp. n., Agathodesmus morwellensis sp. n., Agathodesmus parapholeus sp. n., Agathodesmus quintanus sp. n., Agathodesmus sagma sp. n., Agathodesmus summus sp. n., Agathodesmus yuccabinensis sp. n.
Small Polydesmida with head and 19 or 20 rings; body not curling in spiral; head and telson facing downwards; metatergites with numerous tubercles of varying sizes, sometimes bearing a single seta; ring 2 tergite extended laterally, basally and anteriorly, and edged with large tubercles; no paranota on posterior rings, but 'pseudo-paranota' of metatergal tubercles sometimes present above leg bases; gonopod with neither cannula nor prostatic groove, telopodites separate, each consisting of a more or less cylindrical proximal portion tipped with a thin tab, and a lamellar distal portion arising near the apex of the proximal portion and directed posterobasally or laterobasally.
The diagnosis above slightly amends the one given in Mesibov (2009). Agathodesmus as a genus is easily recognised by the distinctive structure of the gonopod telopodite (Fig. 2). The proximal portion (pp) is typically straight and usually more or less cylindrical, but with the medial and posterior surfaces flattened. The pp arises from the distomedial corner of the small, oblong gonocoxa, where its base is partly contained in a small concavity. The telopodite base may extend basally as a short, rounded projection (be) to overlap the apex of the gonocoxa in ventral or posterior view. The apex of the pp extends distally as a thin tab (at), and on or just below the tab on the posterior surface there are three long, closely adjacent, apical setae (as) in a row; scattered smaller setae may be present on the posterior surface of the pp. Arising just basal to the as on the posterodistal surface of the pp is the distal portion (dp) of the telopodite. Jeekel (1986) used the word 'complicated' three times in his description of the dp ('acropodite') in Agathodesmus bucculentus , and while the details of its structure can be very hard to put into words, some generalities are clear and are applicable to all known Australian Agathodesmus species. The dp is always directed posterobasally or laterobasally, giving the telopodite as a whole the appearance of a leg tightly bent at the knee. The main branch (mab) of the dp is flattened into a lamella and is usually divided into lobes. The lamella is typically curved so that the surface seen in posterior view is slightly convex, and the distal margin of the mab and portions of its inner, concave surface may be thickened or folded. A smaller, medial branch (meb) of the dp arises near the base of the dp on its medial side and usually curves laterally so that its tip is hidden behind the mab in ventral view. Portions of the meb are sometimes densely covered with fine, hair-like structures.
In this paper I provide posterior or posterolateral gonopod views of all species described here. These views are convenient when examining males with tightly flexed telopodites, and with careful attention to shape and position, a posterior or posterolateral view is diagnostic. However, readers should be aware that important diagnostic details of the telopodites may be hidden in these views, and that in the SEM views published here, thin portions of the mab may have been distorted by drying. Agathodesmus spp. are unusual among Australian Polydesmida in their 'hidden' gonopod complexity, and manipulation of the telopodite to partially unbend it and separate the meb is often necessary for a positive identification.
When an Agathodesmus sp. gonopod is cleared with 80% lactic acid, the telopodite sometimes extends so that the angle between the pp and dp is greater than 90°. It is easy to imagine that the telopodite extends in this way during mating, and that the apical tab (at) serves as a check on the rotation of the telopodite 'knee'. A tab in a similar location is present anterior to the 'joint' in the pseudo-articulated gonopod telopodite of species in the Australian genus Ginglymodesmus ( Dalodesmidae ) ( Mesibov 2005). However, I have as yet no evidence to support the idea that the telopodite extends in living males. No Agathodesmus have been collected and preserved in copula, and in all of the 430+ Agathodesmus males I have examined the uncleared gonopod telopodites are flexed.
Variation in most non-gonopod characters across the genus is minor and the redescription of the type species Agathodesmus steeli in Mesibov (2009) applies to most details in the new species. The 'diagnostic descriptions' given below include only those characters known to vary significantly among Australian Agathodesmus spp.
In 12 of the 21 known Australian species, males and females have 19 body rings, in five species 20 rings, and in one species males have 19 rings and females 20; three species known only from males have 19, 19 and 20 rings. The smaller species generally have 19 rings and the larger 20 rings, but the correlation of ring number and body size is loose, and two of the largest Australian species have 19 rings ( Agathodesmus kirrama sp. n. and Agathodesmus yuccabinensis sp. n.).
There is also no apparent relationship between body size and the development (or absence) of the 'pseudo-paranota' formed by lateral metatergal tubercles on posterior rings (Fig. 3).
Although the metatergal setae in most Australian Agathodesmus spp. are short with slightly flared tips (Fig. 4A), three of the newly described species have very long setae (Fig. 4B) and appear 'hairy' at low magnification.
While examining the new Agathodesmus species I was able to identify what appear to be spiracular openings, something I failed to do earlier ( Mesibov 2009). The openings are minute (Fig. 5) and on diplosegments are both located close to the anterior leg base.
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