Torpacarus omittens omittens Grandjean, 1950

Schatz, H. & Schuster, R., 2012, First Records Of Lohmanniidae (Acari: Oribatida) From The Bermuda Islands, Acarologia 52 (3), pp. 247-257 : 253-255

Torpacarus omittens omittens Grandjean, 1950

Torpacarus omittens Grandjean 1950: 126 , f. 4-5; Balogh 1961: 30, f. 37-38; Wallwork 1962: 486; Bischoff de Alzuet 1971: 101. T. omittens omittens: Balogh and Mahunka 1981: 56 ; Balogh and Balogh 1987: 343, pl. 26 C-D; 1988: 58, pl. 39 A-B; 2002a: 71; 2002b: pl. 124: 10-11; Schatz 1994a: 171.

Dimensions (values for each specimen given in table 1 ): adult (n=7) 601 (560 – 660) x 278 (255 – 310), deutonymph (n=2) 420 – 425 x 200 – 205, tritonymph (n=3) 515 (490 – 550) x 253 (230 – 280) µm.

Sensillus with 10-14 branches in adult, 10 branches in deuto- and tritonymph. Transverse lines mt, nt, pt on notogaster in most specimens present although very weakly developed, but in some specimens absent. Measurements of notogastral setae in adult: c1, d1, e1 10 – 17 µm, c2 44 – 90 µm, d2 70 – 80 µm, f1 thickened, 65 – 100 µm, lateral and posteromarginal setae 90 – 120 µm. Setae c1 and d1 smooth, p1 bilaterally ciliate, other dorsal setae unilaterally ciliate. Length ratio of notogastral setae in deuto- and tritonymph as in adult. Juvenile instars with some transverse rows of foveolae on notogaster. Inner epimeral setae smooth, shorter than the outer rows. Solenidion ω 1 on tarsus I with a distinct dilation distally and frequently with one weaker dilation basally.

Remarks: The specimens found on Bermuda correspond to the original description ( Grandjean 1950) in most respects and are considered conspecific. Balogh and Mahunka (1981) have described a subspecies from Paraguay, T. omittens paraguayensis (length 570-595 µm), Schatz (1994a) described another subspecies from the Galapagos Islands, T. omittens galapagensis (length 620-660 µm). The separation of both subspecies is based on different combinations of morphological characters (see Schatz 1994a). Some characters show a wide variability among the population on Bermuda ( table 1 ):

1. Body size: Most specimens in Bermuda differ from the original description in having smaller body size (560 – 580 µm) which correspond with a specimen from Ghana, West Africa (570 µm, Wallwork 1962). Grandjean (1950) gives a body length of 600 – 650 µm for the type population from Venezuela. Schatz (1994a) recorded a single individual from an island off the Belizean coast with a length of 680 µm. The specimens of sample BE 308 are distinctly larger (length of adult 650 – 660 µm, tritonymph 550 µm) than the rest of the Bermudan population, and the adults correspond in that respect with the type population.

2. Sensillus: The number of branches on sensillus is variable: 9 in specimens from Venezuela, 10-14 in specimens from Bermuda; 15-16 in other described subspecies.

3. Transverse lines on notogaster: Among the described subspecies this character is present only in T. omittens omittens . In most specimens from Bermuda the transverse lines are present but very weakly developed; in some specimens the lines are invisible.

4. Length of notogastral setae: The size ratio of c2: c3 is about 1: 2 in most Bermudan specimens, as is usual in T. omittens omittens , but in specimens of sample BE 308 the setae c2 are very long, causing a ratio of almost 1: 1 (in adult and tritonymph). In the other described subspecies the setae c2 are maximally half as long as setae c3. The setae f1 are long in all Bermudan specimens as is usual in T. omittens omittens and T. omittens paraguayensis .

5. Shape of inner epimeral setae: The epimeral setae of the median row (1a, 2a, 3a, 4a) are long and smooth in most Bermudan specimens as is usual in T. omittens omittens , but in one adult of sample BE 288 setae 1a are long and ciliate. In the other described subspecies these epimeral setae are long and ciliate in T. omittens paraguayensis resp. short and smooth in T. omittens galapagensis .

6. The solenidion ω1 on tarsus I has a distinct dilation distally in all populations of T. omittens omittens investigated in this respect, but one additional weaker basal dilation in most specimens in the population on Bermuda. The other described subspecies have 2 weaker dilations each.

These differences do not justify the separation of the population or part of the population of T. omittens omittens on Bermuda as a separate taxon. However, the variability of characters in different populations of this widespread species might be explained by isolation and the parthenogenetic reproduction mode of the Lohmanniidae ( Norton and Palmer 1991) .

Records from Bermuda: BE 184: 1 adult, 1 tritonymph, 2 deutonymphs. BE 225: 2 adults, 1 tritonymph. BE 288: 2 adults. BE 308: 2 adults, 1 tritonymph .

General distribution: In several places around the Caribbean Sea ( Mexico, Belize, Panama, Cuba, Guadeloupe, St. Lucia, Venezuela) as well as in West Africa (detailed records listed in Grandjean 1950, Wallwork 1962, Mahunka 1985a, 1985b, Schatz 1994a, 2006, Starý 1998, Prieto and Schatz 2004, Subías et al. 2004); first record for Bermuda. Balogh (1962) and Bischoff de Alzuet (1971) recorded T. omittens also from Peru and Argentine. It is uncertain whether these individuals belong to T. omittens paraguayensis Balogh et Mahunka, 1981 or to T. omittens omittens Grandjean, 1950 ( Schatz 1994a) .