Greenhouses can provide a suitable environment for diverse groups of invertebrates. Each tropical greenhouse has its own characteristics caused by various factors, e.g. climate control, management, composition and origins of plant and soil material. Depending on these factors, complex artificial ecosystems can be formed to a certain extent. Greenhouses often import a large number of (tropical) plant species from remote sources, almost always with accompanying soil, resulting in the introduction of species living in or on these substrates. Some of these are able to settle in the greenhouse. Certain alien species that thrive in greenhouses might even become indoor pests or expansive if they manage to establish themselves outdoors (Wang et al. 2015). Additionally, synanthropic species may colonise or be introduced by transport from (nearby) manmade biotopes. Some native species living in natural habitats occasionally enter greenhouses as well (Kielhorn 2008).A survey of arachnids was carried out in three tropical greenhouses in Rotterdam Zoo. This marks as the first extensive arachnid survey in Dutch tropical greenhouses since 1949 (van der Hammen 1949(van der Hammen , 1969a. At that time, Leendert van der Hammen and fourteen other collectors surveyed eleven greenhouses in the midwest and centre of the Netherlands, including Rotterdam Zoo, by hand, searching over a seven-year period. Most greenhouses were, however, visited only once. Thirty-two arachnid species were collected, with only one species, Zygiella x-notata (Clerck, 1757), sampled at Rotterdam Zoo (van der Hammen 1949). Currently, only a handful of studies on arachnid faunas of European tropical greenhouses exist, e.g. the Eden Project in England (Snazell & Smithers 2007), several botanical and zoological gardens in the German states Berlin and Brandenburg (Kielhorn 2008, 2009, 2016), the Botanical Garden of the University of Debrecen in Hungary (Pfliegler 2014), the Botanical Garden of the PJ Šafárik University in Slovakia (Šestáková et al. 2017) and multiple zoological gardens in the Czech Republic (Hula & Pešan 2018).The aim of the present paper is to give insight into the arachnid fauna of the tropical greenhouses at Rotterdam Zoo and document species found in the Netherlands for the first time. Additionally, a graph is given to illustrate which method proved most effective in collecting arachnids in tropical greenhouses. Material and methods LocationsThis study was carried out in three tropical greenhouses located at Rotterdam Zoo: Amazonica, Victoria Serre and the Kwekerij. Amazonica (51.9280°N, 4.4473°E, -1 m alt., Fig. 1) is a large circular greenhouse designed as a butterfly garden, opened in 2013. It has a diameter of 60 m and its highest point is 12 m. A total of 318 plant species are growing in open soil in Amazonica, mostly of South American origin. The average temperature is 25-28°C during daytime and goes down to 18-23°C at night. During warm summer days, the average temperature can be as high as 32-34°C.The Victoria Serre (51.9260°N, 4.452...