The domed front of a spitting spider is not decoration. It houses a pair of enlarged glands divided into two parts, one producing venom and the other a silk-like glue, and both empty through the fangs.
When prey comes within roughly a centimetre or two, the spider fires. Two streams leave the fang tips, oscillating from side to side so that the glue lands as a pair of zigzags across the target and the surface beneath it. The whole discharge takes on the order of a thousandth of a second. The glue contracts as it sets, tightening the prey down, and the venom mixed into it begins to act before the spider walks in to bite and wrap.
The mechanism is still argued over. Whether the side-to-side oscillation comes from rapid muscular movement of the fangs, from hydraulic instability in the jet itself, or from both, has not been settled.