Our instinctual fears are discussed in, Kellert, Stephen and E. O. Wilson, Eds. 1995. The Biophilia Hypothesis. Island Press.

"The human inclination to fear and avoid threatening aspects of nature has been particularly associated with reptiles such as snakes and arthropods such as spiders and various biting and stinging invertebrates... This potential has been described by Ulrich et al. in a review of the scientific literature (1991:206): 'Conditioning studies have shown that nature settings containing snakes or spiders can elicit pronounced autonomic responses... even when presented subliminally.' Schneirla (1965) further notes that the occurrence of 'ugly, slimy, erratic' moving animals, such as certain snakes and invertebrates, provokes withdrawal responses among vertebrate neonates in the absence of overt or obvious threat." --p. 57

"Ohman and Dimberg, using classical conditioning paradigms, were able to elicit and maintain fear responses to spiders, snakes, and hostile human faces but not to "neutral" stimuli such as flowers or to such modern hazards as guns." – p. 167

(http://www.meangenes.org/notes/notes.html#c1)