Phytotelmata (plant-held waters) are useful ecological models for studying predator–prey interactions. However, the ability of terrestrial predators to influence organism abundance within phytotelmata remains poorly studied. We investigated the predation of two pitcher-dwelling spiders, the red crab spider Misumenops nepenthicola and the yellow crab spider Thomisus nepenthiphilus (Araneae: Thomisidae) on dipteran larval abundance by manipulating their presence in the pitcher Nepenthes gracilis. Lower abundance in the larvae of the mosquito Tripteriodes spp. and increased spider mass were recorded after M. nepenthicola was introduced into laboratory-maintained pitchers (n = 10); T. nepenthiphilus did not affect larval abundance and a decrease in spider mass was recorded. Further investigations on two other dipteran larval species, the scuttle fly Endonepenthia schuitemakeri and gall midges Lestodiplosis spp., reported reduced numbers with the introduction of M. nepenthicola. We further tested this predation on dipteran larval abundance by its introduction, removal, and re-introduction to pitchers in the field (n = 42) over 1 mo. The spider's absence and presence significantly influenced larval numbers: all four dipteran species reported a significant decrease in numbers after M. nepenthicola was introduced. These results are one of the first to demonstrate the influence of a terrestrial phytotelm forager on the abundance of pitcher organisms via direct predation, reiterating the ecological importance of terrestrial phytotelm predators on phytotelm community structure and dynamics.