Sphaerochthonius gemma

van der Hammen, L., 1959, Berlese's Primitive Oribatid Mites, Zoologische Verhandelingen 40, pp. 1-93 : 26-27

Sphaerochthonius gemma (Oudemans, 1909)

Hypochthonius gemma Oudemans , 190g, pp. 319, 320.

Sphaerochthonius elegans Berlese , 1910a, p. 266; 1916b, p. 337.

Cosmochthonius gemma Oudemans 1917, P. 343: 1917a, p. 2.S, figs. 51, 52.

For the present I place the name Sphaerochthonius elegans in the synonymy of S. gemma , although it is impossible to give certainty about this supposition without remounting of the type-specimen of the first-mentioned species.

The types of S. elegans and S. gemma originate from Java; in both cases they have been collected by Jacobson. Oudemans as well as Berlese have been under the impression that their specimens belonged to the adult stage; they regarded the presence of three dorsal "coupures" as an important specific character. For this reason Oudemans (1917) even classified the species in the genus Cosmochthonius .

To our knowledge, only juvenile specimens of Sphaerochthonius have 3 dorsal "coupures". The fact that in this genus the notogaster of the juveniles is already partly scleritized, easily leads to the erroneous conclusion that nymphs are adults; this happened indeed in the present case.

In 1910 Berlese characterized S. elegans as a species from Java, strongly different from S. splendidus by the presence of T-shaped dorsal hairs (!) and 3 dorsal "coupures". Evidently Berlese later no more remembered this description because in 1916 he described S. elegans again as a new species, by this time not only from Java but also from Italian Somaliland. In this second description the species is characterized as closely resembling (!) S. splendidus but different by the division of the notogaster into 4 segments.

In the Berlese Collection 3 slides are present: no. 129/50 from Java (designated as type) and nos. 160/43, 44 from former Italian Somaliland.

All specimens distinctly have 3 dorsal "coupures" and monodactyle legs. I compared the preparations with the type of S. gemma (Oudemans) and observed a distinct similarity in habitus.

Oudemans (1909) described S. gemma as a Hypochthonius . He stated a close relationship with Hypochthonius splendidus (later Sphaerochthonius ), but observed the presence of 3 dorsal "coupures", although in his opinion 2 of these might be caused by pressure of the cover-glass. In 1917 he attached more value to the number of "coupures" and thought that an alteration of the generic diagnosis of Cosmochthonius should allow to include H. gemma into this genus; a detailed description followed shortly afterwards (Oudemans, 1917a). Oudemans regarded the type-specimen as adult, and this Impression is also suggested by the description, especially because Oudemans figures the reticulate microsculpture as rather heavy. Grandjean (1932, p. 34) already supposed that gemma probably is a Sphaerochthonius ; he expressed as his opinion that the posterior two "coupures" would appear to be nothing but dorsal ridges.

After remounting the single slide of S. gemma , the type speeimen appeared to be in a condition sufficient to establish the following characters in addition to Oudemans's description. The animal is nearly colourless, and the reticulate microsculpture is faint. The notogaster distinctly shows 3 dorsal "coupures". The legs are monodactyle but for the rest the claw-segment has nearly the same remarkable strueture as in the adults of S. splendidus . The strip of soft skin, laterally of the "unguinal" pieces, is much larger than in Oudemans's figure, the surface of this part is striated. The genital opening is small, and each of the Covers bears only 3 hairs; there are only one pair of genital suckers. Consequently, it is unmistakable that the type is a juvenile speeimen. Because in some Enarthronota the development of the number of genital suckers differs from the normal scheme, it is not certain to which stage the speeimen belongs, but judging from facts known in related species, it most likely is a deutonymph.

In summarizing the above-mentioned facts, I arrive at the conclusion that S. gemma and S. elegans possibly are synonymous, that both have been described after juvenile specimens, but that the adults of the species are unknown.