Mdthbs

What's the sf short story where an entomologist discovers a swarm of intelligent ants that collect butterfly and moth eggs for him? But then realizes they are extremely dangerous? (I think they might have some sort of telepathy?)

"Doomsday Deferred", a short story by Will F. Jenkins aka Murray Leinster.

The narrator, an American butterfly hunter in Brazil, is visited by a desperate man:

It happened in Milhao, where José Ribiera came to me. Milhao is in Brazil, but from it the Andes can be seen against the sky at sunset. It is a town the jungle unfortunately did not finish burying when the rubber boom collapsed. It is so far up the Amazon basin that its principal contacts with the outer world are smugglers and fugitives from Peruvian justice who come across the mountains, and nobody at all goes there except for his sins. I don't know what took José Ribiera there. I went because one of the three known specimens of Morpho andiensis was captured nearby by Böhler in 1911, and a lunatic millionaire in Chicago was willing to pay for a try at a fourth for his collection.

[. . . .]

"I am not a businessman," I repeated. "I would not be able to help you." Then at the terrified look in his eyes I explained, "I am here after butterflies."

He couldn't understand that. He began to stammer, pleading. So I explained.

"There is a rich man," I said wryly, "who wishes to possess a certain butterfly. I have pictures of it. I am sent to find it. I can pay one thousand milreis for one butterfly of a certain sort. But I have no authority to do other business, such as the purchase of gold or cattle."

José looked extraordinarily despairing. He looked numbed by the loss of hope. So, merely to say or do something, I showed him a color photograph of the specimen of Morpho andiensis which is in the Goriot collection in Paris. Bug collectors were in despair about it during the war. They were sure the Nazis would manage to seize it. Then José's eyes lighted hopefully.

"Senhor!" he said urgently. "Perhaps my—friends can find you such a butterfly! Would you pay for such a butterfly in cattle sent here from São Pedro, senhor?"

I said rather blankly that I would, but— Then I was talking to myself. José had bolted out of my room, leaving maybe five pounds of gravelly gold nuggets in my hands. That was not usual.

José's "friends" are intelligent telepathic army ants, who use him as an agent to trade gold for cattle. The narrator goes to visit José at his home in the jungle:

But José sat still, his throat working convulsively. I had seen soldados on him. But there were no soldados. After a moment José got to his feet and came stumbling toward me. He looked like a dead man. He could not speak.

"But look!" I cried. My voice was high-pitched. "I saw soldado ants! I saw them!

José gulped by pure effort of will. I put down the child. He ran back to his mother.

"Si-i. Yes," said José, as if his lips were very stiff and his throat without moisture. "But they are—special soldados. They are—pets. Yes. They are tame. They are my—friends. They—do tricks, senhor. I will show you!"

He held out his hand and made sucking noises with his mouth. What followed is not to be believed. An ant—a large ant, an inch or more long—walked calmly out of his sleeve and onto his outstretched hand. It perched there passively while the hand quivered like an aspen leaf.

"But yes!" said José hysterically. "He does tricks, senhor! Observe! He will stand on his head."

Now, this I saw, but I do not believe it. The ant did something so that it seemed to stand on its head. Then it turned and crawled tranquilly over his hand and wrist and up his sleeve again.

The narrator is terrified:

I believed that an army-ant army was as much a single creature as a sponge. I believed that the Something in José's jungle clearing—its body cells were soldado ants—had discovered that other creatures perceived and thought as it did. Nothing more was needed to explain everything. An army-ant creature, without physical linkages, could know what its own members saw and knew and felt. It should need only to open its mind to perceive what other creatures saw and knew and felt.

The frightening thing was that when it could interpret such unantish sensations, it could find its prey with a terrible infallibility. It could flow through the jungle in a streaming, crawling tide of billions of tiny stridulating bodies. It could know the whereabouts and thoughts of every living thing around it. Nothing could avoid it, as nothing could withstand it. And if it came upon a man, it could know his thoughts too. It could perceive in his mind vast horizons beyond its former ken. It could know of food—animal food—in quantities never before imagined. It could, intelligently, try to arrange to secure that food.

It had.

But if so much was true, there was something else it could do. The thought made the blood seem to cake in my veins. I began frantically to thrust away the idea. The Something in José's clearing hadn't discovered it yet. But pure terror of the discovery had me drenched in sweat when I got back to Milhao.