Background/Question/Methods Spiders (Araneae) are morphologically diverse and employ varied foraging strategies. However, while nectar and pollen are occasional dietary supplements, virtually all of the 40,000 described species are considered obligate predators. Here we describe unique features of the ecology of Bagheera kiplingi Peckham, a widespread Mesoamerican jumping spider (Salticidae) that nests on swollen-thorn acacias and consumes the plant's specialized leaf tips (Beltian bodies) and petiolar nectar as predominant components of its diet. We observed Bagheera on ant-acacias between 2001 and 2008 in dry forests of coastal Quintana Roo, Mexico (Acacia collinsii tenanted by Pseudomyrmex ferruginea); Guanacaste, Costa Rica (A. collinsii and A. cornigera with P. spinicola, P. flavicornis, and P. nigrocinctus); and Belize (A. cookii with P. ferruginea). Bagheera and its nests occurred exclusively on or adjacent to acacias occupied by Pseudomyrmex. Short-term translocation experiments in Costa Rica showed an elevated tendency for spiders to remain on acacias versus non-acacia shrubs. The spiders were disproportionately frequent in mainland Mexico, where they occupied 55% of acacia plants examined, whereas spiders occupied only 13% of acacias on Cozumel Island, and < 5% of those in Costa Rica; spider occupancy was higher in wet season than dry season sampling. Results/Conclusions Focal spider observations, aided by use of high-definition video in Mexico, revealed that Beltian bodies accounted for > 90% of B. kiplingi food items; the spiders exhibited active avoidance of ant guards while carrying out repeated foraging forays directed toward concentrations of fresh leaf tips. Spiders less frequently consumed nectar, rarely preyed on invertebrates other than ants, and even more rarely stole ant larvae being carried by Pseudomyrmex workers; spiders in Mexico occasionally cannibalized other B. kiplingi, especially during dry-season observations. Although Bagheera appeared to be territorial and foraged solitarily, populations on some individual acacias in Mexico comprised several hundred spiders. Females outnumbered males (> 2:1) among adult spiders in local populations sampled intensively; causes of this sex-ratio skew are unclear. Both summer and winter spider populations included developing eggs and individuals at all other life-stages, indicating year-round breeding and overlap among generations. Cohabitation by several individuals of brood-containing nests, along with guarding by adult females of clutches and hatchlings, suggest that Bagheera may be quasi-social. This study provides the first evidence of a spider exploiting or 'cheating' any obligate ant-plant mutualism. That such exploitation is being performed in a taxon never before known to feed on solid plant material is extraordinary.