How aware is the queen, and the hivemind, of the PCs?
At the edges of the hive, the PCs might not come to the attention of the queen. If the population of spiders drops suddenly in an outer area, does the queen notice that? Does the queen then direct more attention, in the form of scout spiders which explore autonomously then return with a report? Does the queen direct more attention, in the form of more directly controlling spiders in the area infested with PCs?
Is the hivemind aware that it's not the only intelligence in the world? Does it worship a deity? Does it trade with other hiveminds, or with Drow, or with anything else? Has it ever had a conversation with any intelligent entity?
Does it have any goals beyond keeping the queen well fed? Does it *want* to spread out into more territory? Does it want to produce a second queen which can take some spiders in search of another lair and a separate hive?
One possible set of clues: tactical evolution. If the PCs have used fire to destroy webs in cave A, then the spiders in cave B take appropriate counter measures, such as diverting a stream to keep the webs damp. If the PCs investigate, they see that spiders spun webbing into tubes, from a spring or stream, then radiating like an irrigation system. This is evidence of intelligent opposition. They may wonder whether the spiders in cave B observed the events of cave A.
The players reach cave C. They see a few spiders, and attack. Those spiders flee. If the PCs pursue, then they will follow the fleeing spiders into a chokepoint, with LOTS of webs ready to drop onto them from above, and spiders with Prepared Actions who attack from hiding (sometimes with Advantage) as the PCs move into adjacent squares. Why did the spiders of cave C set up an ambush? How did they know the PCs were coming?
The PCs approach cave D more cautiously. Good, because now the spiders have recruited allies. As the PCs enter, a spider pulls a web so that it no longer blocks the connection to cave E... which is full of bats. Thousands of bats. The bats do no harm, but the cloud of bats obscures vision and counts as difficult terrain.
Perhaps the hive learns from the PCs. Perhaps it notices that armor is useful, and the spiders of cave F spin extra layers of webbing around their carapaces, adding 1 to their AC, also causing a chance that a melee weapon which hits will then stick to the target. The spiders of cave G then do a layer of webbing, followed by bits of carapace from dead spiders, then more webbing to hold it all in place: they have invented studded (composite) armor, and thus heavy infantry!
Perhaps the hive learns to make and use fire.
Perhaps the hive observes the party's spellcasters, and learns to cast spells! If the party includes a bard, then spiders learn that music can influence the Weave! The PCs have killed many spiders, but they have taught the hive so much!
Perhaps the hive queen learns to Animate Dead. The PCs have, by now, turned many spiders into corpses. Well, the queen can still direct those corpses, after she animates them.
The hive might try to communicate with the PCs. I ran a SF adventure in which there are animals which mostly roam autonomously, except when the hive queen takes direct control. So there are animals acting normally... then two of them walk over to a PC, sit in front of him, and stare, not moving. Then a third one joins, and sits to form an equilateral triangle. Then two more, and they form a pentagon. Then two more. Then four more. Then two more.
At this point, a player realized that the totals were 2, 3, 5, 7, 11, 13 which are prime numbers.
And then the hive queen gave up. She had shown the PCs that she was intelligent, and they had not responded. So she released the 11 animals, and they scatter to hunt, groom, nap, and do other animal behaviors. This baffled the players and the PCs.
But if the PCs had made stacks of rocks, in the same sequence, then the hive would have noticed.