This paper presents the first assessment of the diversity of tropical saproxylic Diptera, done on the Osa Peninsula of Costa Rica. Forty-one trees representing nine species and six plant families were sampled in a rearing experiment. In total, 272 individuals attributable to 18 families of Diptera were reared. Low abundance and species richness of flies was observed overall. A majority of taxa were in saprophagous and mycophagous larval feeding guilds, followed by predators. Host associations, successional stage, and stratum preferences are estimated for the more abundant species. The results support previous findings regarding saproxylic Diptera and their association with moist wood and secondary forests. 

Bait branches were prepared during the transition from rainy to dry season, March to April 2013. Host tree families were selected to complement data obtained in French Guiana in a similar study (Tavakilian et al., 1997) and included nine species in the following plant families: Fabaceae, Lecythidaceae, Malvaceae, Moraceae, Rubiaceae, and Sapotaceae. In total, 41 trees were sampled. Tree species available were located within old-growth or secondary forest. A bait branch was severed from each tree. The basal section (approximately 8 cm x 75 cm) was suspended in the canopy, while the remainder of each branch was exposed on the ground. These were exposed to insect activity for three months. The ground branch was then cut into three thick sections (8 cm x 75 cm) and six thin sections (3 cm x 75 cm). Canopy, ground thick, and ground thin wood sections were placed into rearing cages made of No-seeum insect netting. Daily monitoring was conducted for mature insects. Upon emergence, specimens were preserved in EtOH (100%). Specimens were identified to family using external characters. Some specimens were identified to genus using Manual of Central Ameican Diptera (2009, 2010). Feeding guilds were determined for every morphospecies usually with information from Manual of Central American Diptera (2009, 2010).

The column Plant IND refers to individual plant IDs that the wood was taken from. For partition: grande = thick ground bait, peq = thin ground bait, copa. N IND refers to the number of individuals reared.