Megisthanus papuanus Womersley, 1937

Seeman, Owen D., 2019, The megisthanid mites (Mesostigmata: Megisthanidae) of Australia, Zootaxa 4563 (1), pp. 1-40 : 12

Megisthanus papuanus Womersley, 1937

Figure 5

Megisthanus papuanus Womersley, 1937: 178 .

Material examined. 1 female, Finschafen, Papua, 1933, Rev. Wagner ( J17326 ). In SAM .

Diagnosis. Female (male unknown). Sternal setae st4 probably present (all setae broken). Seta pd2 of femur IV on large projection. Dorsal shield finely punctate, ovate, length 3.35 mm, maximum width 2.00 mm (not including secondary sclerotisation), most dorsal shield setae 80–100 in length, smooth; posterolateral region with 35–40 larger, thicker setae, some of which are very long, forming a pair of tufts; podonotum and opisthonotum densely setose. Sternogynal shields fused posteriorly, each bearing five setae; internal genitalia with rudimentary latigynal element. Ventrianal shield large, strongly trapezoidal, widest posterior to anus.

Differential diagnosis. This species is unique in having both an extremely large, trapezoidal ventrianal shield (ca. 700 x 1200) and two posterolateral patches of very long setae on the dorsal shield ( Fig. 5 Womersley 1937; broken off in examined specimen). As noted by Womersley (1937), the most similar species appears to be Megisthanus gigantoides Stoll, 1893 , which shares a similarly shaped sternoventral and ventrianal shield, but is easily distinguished by lacking the posterolateral patches of very long setae. Instead, M. gigantoides has short dorsal setae interspersed with about 10 pairs of long setae inside the lateral and anterior margins of the dorsal shield.

Remarks. The specimen examined is probably one of the syntypes examined by Womersley (1937), but it is not marked as such, and was not even identified as M. papuanus . It may not be the specimen illustrated by Womersley (1937) as legs II–III are missing, most ventral setae are broken and all the long dorsal setae are also lost. However, the specimen could be a remount of the drawn specimen because it is ringed. Most specimens mentioned in Womersley (1937) are not ringed, but specimens acquired by Womersley after 1937 are ringed, suggesting that the specimen was, at some time, remounted. Much of Womersley’s megisthanid collection is in poor condition, with the medium so degraded that, on some slides, psocids have crawled beneath the coverslip and become trapped. The only other specimen of M. papuanus in the SAM collection is a specimen in ethanol, which is also considerably damaged.

Megisthanus papuanus is a highly distinctive species. Diagnostic features obvious in Womersley’s (1937) description are confirmed here: an extremely large, trapezoidal ventrianal shield ( Fig. 5D ) and the two posterolateral patches of very long setae on the dorsal shield (now only apparent from their large, sclerotised sockets; Fig. 5A ). The species is also distinctive by having an exceptionally broad sternoventral shield (posterior width 740) that is highly porose or hypertrichous, having ca. 180 small setae or pores ( Fig. 5C ); due to all setae being broken, it is difficult to differentiate these, but they appear to be setal bases each with their own associated pore.

Figures

FIGURE 5. Megisthanus papuanus Womersley, 1937, female, photomicrographs. (A) Posterior dorsal shield, most setae broken; arrows indicated patches of sclerotised sockets that presumably represent broken long setae; (B) sternogenital region; (C) posterior sternoventral shield, setae broken; (D) ventral and anal region. Scale bars = 200 µm.