Arachnid Science: Article Reviews

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Perez-Miles, F., F.G. Costa, C. Toscano-Gadea & A. Mignone. 2005. Ecology and biology of the 'road tarantulas' Eupalaestrus weijenberghi and Acanthoscurria suina (Araneae, Theraphosidae) from Uruguay. Journal of Natural History 39: 483-498.

Eupalaestrus weijenberghi and Acanthoscurria suina are the most prominent Uruguayan tarantula species and likely the dominant invertebrate predators in their native grasslands. Not well studied, these large nocturnal tarantulas are often found while avidly traveling in search of mates during the breeding season, a cruising habit that dubs them 'road tarantulas'. This paper discusses the distribution of the two species throughout Uruguay, ecological importance, predation, reproductive strategies, and burrow construction and maintenance.

Researchers trekked 6,000 km [3728 mi] along routes in Uruguay, stopping at regular intervals to survey the area for tarantulas and burrows. E. weijenberghi was broadly distributed throughout the country, while A. suina was found only in the southern region. The densest populations were found in environments where the soil was fertile and deep. Populations of A. suina were always sympatric with E. weijenberghi.

During the breeding season, dispersal of cruising road tarantulas can predict populations of the beetle pest Diloboderus abderus. This beetle population peaks just as road tarantulas begin to hunt for mates and increase their sexual activity. Though these beetles are not a mainstay of their diets year round, both tarantula species were observed feeding heavily on them during the reproductive season. The tarantulas seem to take advantage of the increased numbers of beetles as a nutritional boost to maintain energy stores during breeding. With this burst of increased consumption, these tarantulas effectively keep that beetle population in check.

Extensive measurements, drawings, and cast moldings were made of underground burrows to study their construction and what benefits that structure likely confers. Many similarities were found between burrows of both species and the environment maintained within. Burrows were usually found in open meadows. These deep (10 or 11 cm; ca. 4 in) tube-shaped dwellings had a relatively tall chamber as main living quarters, which angled away from the vertical tunnel entrance. The burrows of E. weijenberghi were deeper and larger on average than those of A. suina. An even more noteworthy difference was the existence of a slender terminal tube extending from the chamber in E. weijenberghi burrows; this is the only Uruguayan tarantula species that constructs such terminal tubes.

Burrows were often closed with silk and/or clumps of soil. Burrow covering likely protects the occupant from predators (most notably wasps) and parasites, especially critical when molting and when an eggsac is incubating within. Burrow covers also serve an important role in humidity and temperature control. Humidity stays at 100% in the depths of the burrows of both species. In captivity, road tarantulas will build silken burrow covers when temperatures begin to fall with the arrival of winter, suggesting that another benefit of the silk cover is as insulation material.

Burrow entrance diameter was quite variable but, not surprisingly, was correlated with the size of the occupant. In both species, males are typically larger than females so, capitalizing on this dimorphism, the researchers attempted to estimate the distribution of the sexes in the field. When multiple burrows were found together at a field sites the distance between each burrow was measured. The minimum distance showed much variability at these sites yet there was always a trend for A. suina to cluster their burrows more closely together.

Both species had very stereotyped defensive postures, which they could maintain for several minutes, as if frozen in space. Interestingly, laboratory specimens of E. weijenberghi lost the tendency to adopt threat postures after living in captivity for a while, but A. suina was not affected in this manner. The use of urticating hairs and escape attempts as defense mechanisms were much less common than these defensive postural displays.

Many more interesting morphological traits and reproductive behaviors are discussed. This article is highly recommended reading.

For more information: http://www.tandf.co.uk/journals/titles/00222933.asp

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