The best sequence in the movie!

Here's a more detailed summary of the movie:

The Mexican desert. Two state-policemen find a young girl who was reported to walk on her own along the road. The girl appears to be in a state of shock. They also find a car  and a severely damaged abandoned trailer.  In the sand they find an unknown track. An ambulance and an investigating team arrive. During the investigation we hear  a chirping sound. Nobody notices that the girl, who appears to be sleeping in the ambulance, sits up straight and opens her eyes. After the sound has stopped, she lies back on the bed and closes here eyes. The ambulance leaves and the policemen decide to go to the nearby store. Whatever attacked the trailer must have attacked the store too. The store is also severely damage, its owner is dead, and his money is still in the cash-register. Sergeant Ben Peterson leaves to get the investigating team. When the other man hears the strange chirping again, he goes outside. We hear a few gunshots and a loud screaming. At the police-station the men have absolutely no clue as to what happened with the trailer and the store. As the owner of the trailer was an FBI-agent, another FBI-agent, Robert Graham, arrives. A print of the track is sent to Washington for further investigation. The autopsy-report says that the bodies of all victims are loaded with formic acid.  A few days later father and daughter Medford, who both are doctors, arrive. First they pay a visit to the girl in the hospital. After they let her smell some formic acid, the girl comes around and starts to scream: "THEM!!! THEM!!!" (hence the title of this film). At the site of the trailer they find another track. During their investigation the chirping starts again. Pat, the daughter, is being approached by a huge monster which has two antennas and pincers. After they kill the creature, the doctors finally tell the others what they are dealing with: Camponotus Vicinus, or better known as ants of which mutations are probably caused  by the lingering radiation from a nuclear test which took place nine years before. Suddenly everything makes sense. Now it is imperative that they find the nest as soon as possible. The next day they find the entrance of the nest. After they kill all the inhabitants with cyanide-gas, they enter the nest. Unfortunately they arrive too late. Two new queens and some males have already left the nest. Now they have to locate the queens, in order to prevent a major national disaster.

In Washington  the army is informed and all news-agencies are ordered to report all news about kidnapping, migration of wildlife, large theft of sugar and other strange phenomena. A report comes in of a pilot crashing due to seeing flying saucers shaped like ants. The pilot has been taken to a psychopathic ward of a hospital. The Peterson, Graham and Pat pay him a visit. They believe his story and ask the doctors to keep the pilot in the ward until further orders. After their return to Washington they receive news that the first new nest hatched aboard a ship at sea. The queen entered the ship three days earlier in Acapulco. When it is discovered that the entire crew has been killed, the navy destroys and sinks the ship.Another report comes in. Forty tons of sugar has been stolen near Los Angeles. Graham and Peterson go to Los Angeles to investigate. A train wagon has been broken into, but nobody heard nor saw anything. At the same time a man dies because he crashed into a tree with his car. The strange things is that one of his arms was ripped off before he crashed. The mother has no idea where the man and his two children, who are now missing, spent the day. To find out who might have seen something, Graham and Peterson decide to question anyone who have been arrested by the police. Two drunks claim to have seen nothing important. A third man questioned, is on the alcoholic ward of a hospital. He tells them that in the morning, when he looked outside his windows, he saw a few small planes, but they were not as big as the ants. He has been seeing them for about five months now, mostly at night near the water-outlets of the storm-drain in a riverbed which can be seen from his windows. The Medfords are asked to come to Los Angeles. In the area indicated, Graham and Peterson search the area. On the ground they find a model-plane. In the sand near one of the water-outlets they find a familiar track of one of the ants . They find the location of the second nest. When they hear that the plane belonged to the two missing children, they think that the children probably have entered the water-outlets, and are therefore in danger.

Some time later a press-conference is held by the city counsel. At last everything is explained to the citizens of Los Angeles. Very large and dangerous ants have built their nest in the storm-drains beneath the city. The city has been proclaimed to be under Marshall Law. All water-outlets are guarded by the army. Army-jeeps enter all water-outlets to find the children and the nest. Near an area which is still under construction, Peterson stops as he thinks he hears something. When, at his request, the lights which the construction workers use for their work are turned on, the chirping of the ants start again. They also smell the same scent which they smelled while being in the nest in the desert. They indeed have found the nest. Nearby, at the end of a small tube in the wall, Peterson finds the two children who are being approached by the ants.  He pushes the children through the tube, but the ants kill him before he can escape. The army arrives and they use heavy guns and armory to kill the ants.  Doctor Medford tells them it is crucial that they find the egg-chamber, as to find out if new queens have left the nest. A large hole in the floors puts them right above the chamber. They are just in time. New queens are ready to leave the nest. They are destroyed by the flame-throwers. Their destruction is the beginning of the end of the dangerous species.