A keen entomologist, Hulstaert was interested in the Lepidoptera and before his posting to the Congo studied insects from the Dutch East Indies (including Dutch New Guinea) sent to him by other missionaries.
His collection is held by Naturalis Biodiversity Center in Leiden.
His contributions also extend to anthropologic fields, such as ethnography and linguistics of peoples living in the Democratic Republic of the Congo, especially those pertaining to Mongo people.
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