Thanks to responses from two spider experts, the “spider milk mystery” has been solved! Here are my initial questions and their answers:
“What the heck is going on???” The “milking” behavior is in fact…get ready for it… SPERM REMOVAL. Yes, as strange as that may sound, male spiders practice the behavior of extracting other males’ sperm from females before mating with them. Why??? Because… evolution selects for traits that maximize an organism’s reproductive potential, and sometimes this goes to extreme measures. In this case, removing sperm from a female increases the chances that a male’s genes will be passed on to future generations. This may seem like an anomaly of nature, but many insects, including fruit flies, grasshoppers, dragonflies, and beetles, and even some birds practice sperm removal.
“What is the orange goop? How could the male extract it for five hours without sucking the female dry?” The orange goop isn’t a liquid at all! It’s actually a membrane (called the hematodochae) used to transfer sperm into and out of females. That’s right, male spiders transfer packets of their sperm into females with their pedipalps.
As for why the male was drinking the extracted sperm, one expert suggested that he’s simply recovering valuable calories. Yup. That spider must be pretty darn hungry…