I have just recieved news that there is at least onenspecies of spider that produces milk. Kinda-sorta milk. Its got proteins, sugars and fats in it.
Im just stealing this from a national geographic article but;"In one illustrative experiment, the researchers glued shut the spider mothers' epigastric furrow, the egg-laying organ that also secretes milk. The hatchlings all died within their first 11 days."
So literally, take the "milk" out of the equation, and the spiderlings die, theyre dependent on this.
"Even after hatchlings are old enough to find food on their own, the researchers discovered that spiderlings will continue to take advantage of their mom's milk for an additional 20 days. And daughters (but not sons) were allowed to continue nursing even after reaching sexual maturity."
Seems odd at first. But maybe theres a risk of inbreeding if now-sexually-mature males are allowed to stick around too? I recall research has shown that mice specifically avoid mating with other mice that smell too closely related to them (proved by breaking the mouses' sense of smell; they immediately started fucking their siblings from the sound of things.)
Theres several other articles talking about it, the sciencemag.org one says "The “milk” may be liquified eggs that are passed out of the birth canal prematurely, Quan says. Some amphibians and other invertebrates lay similar “trophic eggs” for offspring to eat, he notes, although only when those offspring are really young."
So thats nice. Either milk or premature eggs.