Diagnóstico da ocorrência de aranhas do gênero Loxosceles Heineken e Lowe, 1832 (Araneae, Sicariidae) no município de União da Vitória, Paraná

Effects of food, kinship, and density on the longevity of spiderlings

Cannibalism is a behavioral characteristic found in a wide variety of animal groups. Although the rates of cannibalism can vary from one group to another, studies indicate that the main factors contributing to an increase in the frequency of such behavior are the availability of food, population density, the behavior and availability of victims, and environmental stress. We carried out different laboratory experiments to assess whether different factors such as the presence or absence of food among siblings and non-siblings, and at different densities among conspecific and heterospecific individuals, affect longevity of recently emerged Brazilian brown recluse (Loxosceles intermedia Mello-Leitão, 1934) and Chilean recluse (Loxosceles laeta (Nicolet, 1849)) spiderlings during periods of starvation. The results revealed that the survivorship of L. laeta during starvation was significantly higher than that of L. intermedia and that the addition of conspecific individuals increased survival rates by 1.5- and 1.6-fold, respectively. The tolerance of conspecifics differed between the two species, and generally, cannibalism was not observed, probably due to the risk of predation and limited consumption by weakened spiders, which coincided with the continued availability of endogenous vitelline reserves, thus indicating that the use of these spiderlings as a food resource may act to regulate starvation in more resistant spiders. The greater longevity and conspecific tolerance of L. laeta may be important factors contributing to the establishment of large populations of this spider in restricted areas, whereas for L. intermedia, hunger probably functions as a trigger for dispersal.

Do agonistic interactions underlie the segregation and relative abundances between two Loxosceles species (Araneae: Sicariidae)?

The medically important spiders Loxosceles intermedia Mello-Leitão and Loxosceles laeta (Nicolet) are segregated in Curitiba, southern Brazil, where L. intermedia is more abundant and widespread than L. laeta. Because they share similar microhabitat preferences and wander in search of web sites, agonistic encounters are likely to occur. The purposes of this study were to describe agonistic interactions and interpret their consequences for the relative abundances and spatial segregation of L. intermedia and L. laeta. Experimental contests were performed between residents and intruders. Asymmetries between contestants included sex, age, species, weight, and residence status. Nine behavioral categories were defined. Through discriminant analyses, it was possible to differentiate spider sex, species, and residence based on their agonistic behaviors. Intruders, juveniles, and L. intermedia individuals were better characterized by exploratory behaviors, whereas L. laeta females were differentiated by aggressiveness. By performing a multiple logistic regression, with winning or defeat as a dependent variable of sex, age, species, size, weight, and residence, it was possible to say that residents and L. intermedia individuals had the highest winning odds in contests, whereas juveniles had lower winning odds than adults. Advantages of the prior residence may help to explain the predominance of L. laeta in old colonization sites, whereas the higher winning odds of L. intermedia and less aggressive behavior toward conspecifics may lead to a successful establishment of dense populations in new sites. A better understanding of agonistic interactions as a mechanism of spacing, segregation, and species replacement among spiders may be helpful for control purposes.