Pheromones are substances that serve as chemical signals between members of the

　　same species. They are secreted to the outside of the body and cause other individuals

　　of the species to have specific reactions. Pheromones, which are sometimes called

　　Line "social hormones," affect a group of individuals somewhat like hormones do an individual

　　(5) animal. Pheromones are the predominant medium of communication among insects

　　(but rarely the sole method). Some species have simple pheromone systems and produce

　　only a few pheromones, but others produce many with various functions. Pheromone

　　systems are the most complex in some of the so-called social insects, insects that live

　　in organized groups.

　　(10) Chemical communication differs from that by sight or sound in several ways.

　　Transmission is relatively slow (the chemical signals are usually airborne), but the

　　signal can be persistent, depending upon the volatility of the chemical, and is sometimes

　　effective over a very long range. Localization of the signal is generally poorer than

　　localization of a sound or visual stimulus and is usually effected by the animal's moving

　　(15) upwind in response to the stimulus. The ability to modulate a chemical signal is limited,

　　compared with communication by visual or acoustic means, but some pheromones may

　　convey different meanings and consequently result in different behavioral or physiological

　　responses, depending on their concentration or when presented in combination. The

　　modulation of chemical signals occurs via the elaboration of the number of exocrine

　　(20) glands that produce pheromones. Some species, such as ants, seem to be very articulate

　　creatures, but their medium of communication is difficult for humans to study and

　　appreciate because of our own olfactory, insensitivity and the technological difficulties

　　in detecting and analyzing these pheromones.

　　Pheromones play numerous roles in the activities of insects. They may act as alarm

　　(25) substances, play a role in individual and group recognition, serve as attractants between

　　sexes, mediate the formation of aggregations, identify foraging trails, and be involved in

　　caste determination. For example, pheromones involved in caste determination include

　　the "queen substance" produced by queen honey bees. Aphids, which are particularly

　　vulnerable to predators because of their gregarious habits and sedentary nature, secrete

　　an alarm pheromone when attacked that causes nearby aphids to respond by moving away.

　　40. What does the passage mainly discuss?

　　(A) How insects use pheromones to communicate

　　(B) How pheromones are produced by insects

　　(C) Why analyzing insect pheromones is difficult

　　(D) The different uses of pheromones among various insect species

　　41. The word "serve" in line I is closest in meaning to

　　(A) improve

　　(B) function