Silk use and spiderling behavior in the tarantula Brachypelma vagans (Araneae: Theraphosidae)

Abstract

Brachypelma vagas belongs to a genus of tarantulas protected against illegal trading. Its distribution range is in Mesoamerica. The spiderlings remain together in the maternal burrow after hatching and after several weeks, they disperse forming a column of about hundred individuals leading away from the burrow. In several spider species, during the gregarious phase and dispersion, spiderlingsdo not show aggressive behavior toward siblings. Silk is an important element in dispersion. However, knowledge of the first stage of B. vagans life is scarce. In the present study, therefore, we describe the natural history of the first stage of life of this species, with particular attention to the development, silk production, interactions between sibling and dispersion. Two egg sacs were collected in the field and maintained in laboratory conditions, without the mother. After hatching, we counted 200 individuals peregg sac; they are cream colored and 2-3 mm long. The first molt occurred at emergence, and the second, five or six days after hatching. After that, a black point appeared on the dorsal part of the abdomen of each spiderling. Production of silk by the spiderlings began from hatching. Dispersion occurred in one group of siblings on the 8th day after emergence, with the production of a thread of silk stretching for at least one meter out of the box. The silk thread produced by the spiderlings during dispersal, could act as a physical guide to indicate the route, or as the support of chemical signs for the orientation of the spiderlings. The absence of silk thread reported in field observations of dispersal could be explained by thefact that it is difficult to see, by lack of production or by production that is environment-dependent. In the group where dispersion was not permitted, no aggressive behavior was observed between siblings; however, a drastic diminution of the individual number occurred in the course of the two-month observation. Mortality could be explained by diseases, unsuitable environmental conditions related to the mother’s absence or not. On the other hand, the remaining spiderlings showed an enlargement of their abdomenin spite of refusal to eat the diary food proposed. It is likely they ate spiderling cadavers or silk. This work offers new knowledge about the first life stage in tarantulas and could be used as a basis for more detailed studies on spiderlings of the tarantula group.