Questions 30-40

　　The smooth operation of an ant colony depends on ten to twenty different signals, most of which are pheromones (chemical signals triggering behavioral responses). It is estimated that red fire ants employ at least twelve different chemical signals. The simples Line of these is the carbon dioxide from the respiration of an ant cluster, a chemical that acts as (5) a pheromone to promote aggregation. Workers move toward a source of carbon dioxide, resulting in solitary ants moving to join a group. At the other extreme, the most complex of the fire ants' signals is probably colony odor, by which the workers of a particular colony or nest identify another worker as local or foreign. Each ant nest has its own odor as a result of its location, history, and local food supply. The resident ants pick up this (10) odor on their bodies, so that ants of the same species, but from different nests, have different colony odors. This allows ants to identify intruders and maintain colony integrity.

　　Fire ants also make use of an alarm pheromone to alert workers to an emergency, and their scouts lay down a trail pheromone as a guide during mass migrations. A fire ant (15) queen emits a chemical signal that identifies her to the colony's workers. They respond by scurrying to gather around her. The decomposing corpse of a dead ant also generates a signal, to which workers respond by eliminating the corpse from the nest.

　　Ants provide examples of both public (accessible to other species) and private messages. One of their most important private messages concerns food, for a food source (20) is worth keeping secret. Each species marks its trails with signals that are meaningless to others, so that an ant crossing a trail left by another ant species typically notices nothing.

　　On the other hand, a secret signal to mark a dead body is unnecessary. Many kinds of ants perceive a natural decomposition product of dead insects as a signal to remove a corpse. If an outsider recognizes this message and moves the body, no harm is done.

　　30. What aspect of ants does the passage mainly discuss?

　　(A) The relationship between the queen and the worker ants

　　(B) Ways in which ants use chemical signals

　　(C) Methods ants use to identify food sources

　　(D) The importance of respiration in the production of ant pheromones

　　31. The phrase "smooth operation" in line 1 is closest in meaning to

　　(A) daily activity

　　(B) effective functioning

　　(C) delicate balance

　　(D) permanent location

　　32. According to the passage, carbon dioxide serves which of the following functions for fire ants?

　　(A) It protects the queen.

　　(B) It attracts other ant species.

　　(C) It informs workers of possible danger.

　　(D) It encourages the ants to gather together.

　　33. The word "cluster" in line 4 is closest in meaning to

　　(A) organ

　　(B) activity

　　(C) group

　　(D) cycle

　　34.According to the passage, each nest has a distinct odor that allows its inhabitants to

　　(A) find the location of the nest in the dark

　　(B) distinguish worker ants from other ants

　　(C) distinguish foreign ants from resident ants

　　(D) signal other inhabitants when foreign ants attack