Questions 20-29

　　Many ants forage across the countryside in large numbers and undertake mass

　　migrations; these activities proceed because one ant lays a trail on the ground for the others

　　to follow. As a worker ant returns home after finding a source of food, it marks the route

　　by intermittently touching its stinger to the ground and depositing a tiny amount of trail

　　(5 )pheromone—a mixture of chemicals that delivers diverse messages as the context changes. These trails incorporate no directional information and may be followed by other ants in

　　either direction.

　　Unlike some other messages, such as the one arising from a dead ant, a food trail has to

　　be kept secret from members of other species. It is not surprising then that ant species use

　　(10)a wide variety of compounds as trail pheromones. Ants can be extremely sensitive to these signals. Investigators working with the trail pheromone of the leafcutter ant Atta texana calculated that one milligram of this substance would suffice to lead a column of ants three times around Earth.

　　The vapor of the evaporating pheromone over the trail guides an ant along the way,

　　(15)and the ant detects this signal with receptors in its antennae. A trail pheromone will

　　evaporate to furnish the highest concentration of vapor right over the trail, in what is called a vapor space. In following the trail, the ant moves to the right and left, oscillating from side

　　to side across the line of the trail itself, bringing first one and then the other antenna into

　　the vapor space. As the ant moves to the right, its left antenna arrives in the vapor space.

　　(20)The signal it receives causes it to swing to the left, and the ant then pursues this new course

　　until its right antenna reaches the vapor space. It then swings back to the right, and so

　　weaves back and forth down the trail.

　　20. What does the passage mainly discuss?

　　(A) The mass migration of ants

　　(B) How ants mark and follow a chemical trail

　　(C) Different species of ants around the world

　　(D) The information contained in pheromones

　　21. The word “forage” in line 1 is closest in meaning to

　　(A) look up

　　(B) walk toward

　　(C) revolve around

　　(D) search for food

　　22. The word “intermittently” in live 4 is closest in meaning to

　　(A) periodically

　　(B) incorrectly

　　(C) rapidly

　　(D) roughly

　　23. The phrase “the one” in line 8 refers to a single

　　(A) message

　　(B) dead ant

　　(C) food trail

　　(D) species

(实习编辑：武勐勐)