OF Acromyrmex subterraneus brunneus ( HYMENOPTERA : FORMICIDAE ) YOUNG NESTS IN A FRAGMENT OF THE NEOTROPICAL FOREST

Young nests of Acromyrmex subterraneus brunneus are characterized by refuse soil in the exterior of the nest, a single fungus chamber 11 to 20 cm deep in relation to soil surface and internal volume ranging from 0.3 to 1.5 liters. These nidification patterns are important characteristics for identifying and understanding the interactions between species and their habitats.

In the Neotropics, ants of the genera Atta and Acromyrmex are capable of consuming a larger amount of vegetation than any other local herbivore (Hölldobler & Wilson, 1990).These plants are used for symbiotic fungus cultivation of Leucoagaricus gongylophorus (Fisher et al., 1994), the main food source for larvae in colonies (Weber, 1972;Bass & Cherrett, 1995).
The notable social structures combined with the elaborate architecture of nests promote efficient nutrient cycling, essential for forest equilibrium (Haines, 1983).However, the internal nest architecture of many leaf-cutting ant species is still unknown, especially the characterization of nests in the initial phase of development.Thus, the objective of this paper is to describe and characterize young nests of Acromyrmex subterraneus brunneus Forel, 1911.
The study was carried out in a mesophyll semideciduous forest fragment in Piracicaba, São Paulo, Brazil.Ten nests of A. s. brunneus, approximately 1 year old, were excavated with the aim of studying the following characteristics: length (l), width (w), internal depth of fungus chamber (id) and depth of fungus chamber in relation to soil surface (Table 1).These data were used to calculate the internal volume of the fungus chamber, through the sphere formula, V=p/6.l.w.id (Moreira & Forti, 1999).For photographic documentation, chambers were dusted with neutral talc with manual dust equipment.
The nests of A. s. brunneus were observed to be totally subterranean with refuse soil in the exterior and the absence of straw as covering.Adult nests may generally be covered by a conical hill of straw formed from pieces of dry leaves and interlaced wood chips (Gonçalves, 1961;Delabie, 1989) or without covering (Andrade & Forti, 1999).
Most nests of A. s. brunneus presented a volume smaller than 0.60 liters, although nests with more than 1.5 liters of volume may be found (Figure 1).The fungus chambers were found with depths ranging from 11 cm up to 20 cm in relation to the soil surface (Table 1), similar to the young nests of Atta spp.(Autuori, 1949;Pereira-da-Silva, 1975).Differentiated chambers for the depositing of refuse material were not encountered (Figure 2).A large quantity of roots was found inside the fungus chamber, promoting symbiotic fungus culture (Figure 2).Specifics regarding the interaction between the roots of the plants and leaf-cutting ants remain largely unknown.
During excavation, a great number of arthropods associated with the nests were observed, such as collembolans and spiders (personal observation), showing their importance in maintaining biodiversity and also the dilution and genesis of soils in this fragment of Neotropical forest.
Furthermore, nests are of extreme importance for many animals because they facilitate procreation and food storage, as well as protection against natural enemies  and weather.These functions promote nidification patterns that are adjusted to the needs of each species.
Thus, the knowledge of nest architecture contributes to the establishment of species nidification patterns.These characteristics are fundamental to identify and understand the interactions of various species with their habitats.

om yr m ex su bter r a n eu s br u n n eu s,
Piracicaba, S ão Paulo, 2002