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Polydesmida: miscellaneous H+20 species 


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Dasytigma

margaretae

The four species of Dasystigma are brown H+20 endemics which reach 20-22 mm in length. The spiracles in Dasystigma species are very odd. In most Polydesmida a spiracle is a small, inconspicuous, crater-like structure just above the base of a leg. Spiracles are the entrances to the system of tubes (tracheae) which carry air to the millipede's tissues. In Dasystigma, dense brushes of 'hair' emerge from each spiracle. To the naked eye these 'hairy' spiracles look white and slightly convex. In the top specimen in the image above of D. margaretae, you can clearly see a pair of these peculiar spiracles above the base of each leg-pair. The images below show spiracle 'hairiness' and spacing on a midbody ring of each species.

spiracles

b = D. bonhami, h = D. huonense, m = D. margaretae, t = D. tyleri

Dasystigma species form a tightly fitted distribution mosaic on the map of Tasmania, and it is possible to make an informed guess about the identity of a female or juvenile Dasystigma based on exactly where it was collected. However, only mature male Dasystigma can be positively identified to species, based on gonopod and spiracle structure.

gonopod

The image above labels the femoral process on the side of the gonopod (f) and the more central prefemoral process (p); scale bar = 0.25 mm. The femoral process is tall and narrow with a thin, inwardly pointing spike in D. bonhami. In D. margaretae the femoral process is broad and divided into a marginally toothed anterior portion and a posterior spike, the spike crossing below the anterior portion like a thumb bent slightly towards the palm. In D. tyleri the posterior spike on the femoral process is longer than the anterior portion and not bent over. Finally, in D. huonense the outer edge of the prefemoral process is distinctly S-shaped.

A peculiarity of D. margaretae populations in the Derwent valley is that the rear corners of the paranota are extended into points. This species could be confused at first glance with Lissodesmus modestus, which also occurs in the Derwent valley and also has pointed paranotal corners. Check the spiracles: large, white and close together in D. margaretae, small, crater-like and well-separated in L. modestus.

All four Dasystigma species are forest-dwellers that shelter in leaf and woody litter. The eastern species D. bonhami and D. margaretae and the southern D. huonense are much more abundant than the western D. tyleri, and are present (sometimes in large numbers) in eucalypt regrowth forest arising from clearfall-and-burn silviculture.

Jeekel (1984), Mesibov (2003c), Mesibov et al. (1995)


Tasmania mapbonhami     gonopodsbonhami

D. bonhami Mesibov, 2003

Tasmania maphuonense     gonopodshuonense

D. huonense Mesibov, 2003

 

 

 

 

Tasmania mapmargaretae     gonopodsmargaretae

D. margaretae (Jeekel, 1984)

Tasmania maptyleri     gonopodstyleri

D. tyleri Mesibov, 2003

 


Tasmanodesmus hardyi Chamberlin, 1920

tasmanodesmus

Tasmanodesmus hardyi resembles the Northeast's Tasmanopeltis grandis (see below), but the ranges of these two endemic H+20 dalodesmids overlap only slightly. Both species are purplish-brown with prominent paranota and long, thin legs. T. hardyi lacks the long paranotal seta found in T. grandis and is distinctly smaller, rarely exceeding 20 mm in length. In the gonopod image below, the head is to the right.

T. hardyi is related to certain Victorian and New Zealand dalodesmid millipedes. It was first collected by G.H. Hardy ca 1915, but the type locality within Tasmania is unknown. Although widespread and common in eastern Tasmania, T. hardyi is largely absent from the Northeast. Its western limit coincides with the biogeographical divide known as Tyler's Line. It occurs in a wide range of forest types, and generally shelters under logs and in the upper layers of forest litter.

Bonham et al. (2002), Chamberlin (1920), Eberhard et al. (1991), Mesibov (1997c, 1999, 2004c), Mesibov et al. (1995) 


Tasmania mapTasmanodesmus     gonopodsTasmanodesmus

Tasmanopeltis grandis Mesibov, 2006

tasmanopeltis

Tasmanopeltis grandis is the largest polydesmidan in Tasmania, with some females reaching 35 mm in length. This species could be confused with Tasmanodesmus hardyi (see above), but like Dasystigma species (see above), T. grandis has large, 'hairy' spiracles and a prominent seta on the rear paranotal corner, and T. hardyi has neither of these features. T. grandis is found in all forest types in northeast Tasmania. Adults are generally found in or under rotting logs.

Mesibov (1997c, 2006a)


Tasmania mapTasmanopeltis     gonopodsTasmanopeltis