Morphology, evolution and usage of urticating setae by tarantulas (Araneae: Theraphosidae)

Urticating setae are exclusive to New World tarantulas and are found in approximately 90% of the New World species. Six morphological types have been proposed and, in several species, two morphological types can be found in the same individual. In the past few years, there has been growing concern to learn more about urticating setae, but many questions still remain unanswered. After studying individuals from several theraphosid species, we endeavored to find more about the segregation of the different types of setae into different abdominal regions, and the possible existence of patterns; the morphological variability of urticating setae types and their limits; whether there is variability in the length of urticating setae across the abdominal area; and whether spiders use different types of urticating setae differently. We found that the two types of urticating setae, which can be found together in most theraphosine species, are segregated into distinct areas on the spider's abdomen: type III occurs on the median and posterior areas with either type I or IV surrounding the patch of type III setae. Morphological intermediates between types I and III, as well as between III and IV, were found. We propose that type III urticating setae have evolved through modifications of body setae on specific areas of abdomen dorsum and subsequently gave independent origin to areas having either type I or IV. A parallel evolution seems to have occurred in some aviculariine genera in which type II setae evolved also from body setae from specific areas of abdomen dorsum. Concerning the length of the setae, we observed that towards the median and posterior areas of the abdomen the length of the urticating setae increases. These long setae are cast by the spider as part of an active defensive behavior against vertebrate predators. We propose that spiders use the various types of urticating setae differently and according to their different targets: type I setae, when incorporated either into the molting web or eggsac, is more effective against invertebrates (ants or phorid fly larvae) than type III. The latter seems to be used mainly against vertebrate predators.