Trhypochthonius americanus ( Ewing, 1908 )

Weigmann, Gerd & Raspotnig, Günther, 2009, Comparative morphological and biometrical studies on Trhypochthonius species of the tectorum species group (Acari: Oribatida: Trhypochthoniidae), Zootaxa 2269 (1), pp. 1-31 : 8-9

Redescription of Trhypochthonius americanus ( Ewing, 1908)

( Figs. 3 , 4b , 5b )

Tumidalvus americanus Ewing, 1908: 244

Trhypochthonius americanus ( Ewing, 1908) : Banks (1915: 99), Hammen (1956: 403); Woolley (1956: 288); Seniczak & Norton (1994: adult p. 40).

Diagnosis. Body length about 570–680 µ m. Most notogastral setae with spinose setulae and distally broadened (except d 2, p 3); seta c 2 as long as c 3 (about 60 µm), c 1 and d 1 smaller (about 30 µm), d 2 the smallest (about 20 µm), p 1 the longest (about 110 µm) poorly broadened; p 3 -seta acuminate and sparsely spinose. 7 pairs of genital setae (rarely 6 or 8). Notogaster with more or less posterior boss.

General characters. Mean body length 634 µm, measured range 590-680 µm (about 670 µm length after Woolley 1956; even smaller after Ewing 1908); mean length of notogaster 449 µm, mean width 380 µm (n=5; cf. tab. 1).

Prodorsum. Rostral seta about 85 µm long, acuminate, with setulae (seta type 3: cf. Fig. 1a ); lamellar seta about 95 µm, distally broadened; interlamellar seta about 125 µm, bacilliform with setulae (type 4).

Notogaster. Detailed length values of all setae in table 1; shapes of setae in Fig. 4b . Normally, the ventral postanal area forming a distinct boss (it may be absent, as figured in Seniczak & Norton 1994: 40).

Ventral region. As typical for the genus. The genital setation ( Fig. 5b ) is slightly variable within the range from 6-8 pairs, predominant number is 7 (one specimen at one side 6), Woolley (1956) indicated 8 pairs. Mostly the postanal area exceeds the posterior notogastral edge.

Material examined. We are grateful for some specimens donated by Dr. Roy Norton from his collection, which allowed us to study the typical American form of this species: USA, Wisconsin, Dane Co., Univ. Wisconsin-Madison campus. Leg. M. Evans 19.10.198 4, from litter under Russian Olive.

Remarks. This species has the same size range and is morphologically similar to T. tectorum , as it shares the small notogastral setae c 1, d 1, d 2. But there are some obvious differences in the setation: On the notogaster, setae f 2, p 1 and p 2 are significantly larger; p 2 is of type 4 (in tectorum : type 3), and h 3 is distally broadened, type 5 (in tectorum : type 3). An important differentiating character is the number of genital setae, with 7 (usually) to 8 pairs (in tectorum : mostly 9–11). Aoki (2000) described a Japanese record of “ T. tectorum ” with six pairs of genital setae which more resembles T. americanus than the European T. tectorum ; we regard it as an unconfirmed finding.

The two similar Japanese species, T. fujinitaensis and T. stercus , described by Fujikawa (2000), differ in detail in the characteristics of the notogastral and genital setation: all p -setae acuminate, e 1, d 1 and p 1 distinctly shorter; more than 10 pairs of genital setae. Their possible taxonomical relations are discussed below.

Distribution and ecology. This species was collected in several states of the USA ( Marshall et al. 1987), originally in Illinois from moss, often in the litter of pine.

Figures

FIGURE 1. a—Shape types of notogastral setae in the Trhypochthonius tectorum species-group; b—Length measures of notogaster (L NG) and of prodorsum (L PD) on a shrunken specimen; c—the same on a swollen specimen.

FIGURE 3. Trhypochthonius americanus, dorsal view, legs omitted. Scale bar 100 µm.

FIGURE 4. Profiles of all notogastral setae. a—Trhypochthonius tectorum; b—T. americanus; c—T. silvestris; d—T. silvestris europaeus subsp. nov.; e—T. japonicus forma occidentalis.

FIGURE 5. Genital plates. a—Trhypochthonius tectorum; b—T. americanus; c—T. silvestris; d—T. silvestris europaeus subsp. nov.; e—T. japonicus forma occidentalis. Scale bar 100 µm.