How did Moody use the Imperius Curse to tell the Spider what to do?

Question

So when Harry is put under the Imperius Curse by Moody, he hears Moody telling him what to do. I wonder how Moody communicated to the spiders what he wanted them to do? Can he speak spider? Does the Imperius magically change the instructions into the language that the person being Imperiused can understand? How does it work?

Answer 1

It's unclear but Moody was presumably able to communicate with the spider.

How Moody was able to direct the spider isn't specified. However, he seems confident in his ability to make the spider do pretty much whatever he wants.

“Total control,” said Moody quietly as the spider balled itself up and began to roll over and over. “I could make it jump out of the window, drown itself, throw itself down one of your throats...”
_{(Goblet of Fire, Chapter 14, The Unforgiveable Curses)}

The fact that he makes the spider do a backflip and a tap-dance backs up his confidence. So Moody can effectively control the spider.

The spider's lack of mental capacity may have provided a barrier to communication but it also made its ability to resist the curse very limited. Resisting requires strength of character, which the spider clearly lacked.

“The Imperius Curse can be fought, and I’ll be teaching you how, but it takes real strength of character, and not everyone’s got it.
_{(Goblet of Fire, Chapter 14, The Unforgiveable Curses)}

The only time we see the effects of the Imperius Curse described from the point of view of the person casting the spell (as opposed to the victim) is when Harry uses it on Bogrod and Travers in Gringotts. Even here, it doesn't specify exactly how Harry was able to control them - whether he spoke to them like Moody did in his classes, for instance.

“Act now, act now,” whispered Griphook in Harry’s ear, “the Imperius Curse!”
Harry raised the hawthorn wand beneath the cloak, pointed it at the old goblin, and whispered, for the first time in his life, “Imperio!”
A curious sensation shot down Harry’s arm, a feeling of tingling warmth that seemed to flow from his mind, down the sinews and veins connecting him to the wand and the curse it had just cast. The goblin took Bellatrix’s wand, examined it closely, and then said, “Ah, you have had a new wand made, Madam Lestrange!”
_{(Deathly Hallows, Chapter 26, Gringotts)}

Both of Harry's targets spoke English, which obviously wasn't the case with the spider. Nevertheless, the tingling sensation and the connection between the caster's mind and the target are features of casting the curse. Some level of communication is occurring, facilitated by the magic of the spell.

At the end of the day, we probably don't have enough information on how the Imperius Curse works in specifics to be able to say anything definitively. It's a magic spell that allows a connection between the mind of the person casting the spell and their victim. Allowing that communication to happen is part of the magic. There may well be limitations to the effectiveness of the spell but we'd need a more specific canon example (e.g. someone who only speaks English casting the Imperius Curse on someone who only spoke German) to be able to do more than speculate on this.

Interestingly, all of this assumes that spiders in Harry Potter have the same level of intelligence as real-world spiders (i.e. not much). However, we do have the canon example of Acromantula, giant spiders which are capable of human speech and intelligence.

ACROMANTULA
The Acromantula is a monstrous eight-eyed spider capable of human speech...This beast is believed to be wizard-bred, possibly intended to guard wizard dwellings or treasure, as is often the case with magically created monsters. Despite its near-human intelligence, the Acromantula is untrainable and highly dangerous to wizard and Muggle alike.

The smaller spiders in Harry Potter show no signs of speech or intelligence but they do clearly have some level of affinity with the Acromantula as they gather to their den in the Forbidden Forest in Chamber of Secrets. So it's possible that 'normal' spiders have some element of magic to them that differentiates them from spiders in our universe. Consquently, the Imperius Curse worked on a spider but may not have worked on another creature.