题目内容
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.
What does the passage mainly discuss？
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No two comets ever look identical, but they have basic features in common, one of the
most obvious of which is a coma. A coma looks like a misty, patch of light with one or more
tails often streaming from it in the direction away from the Sun.
At the heart of a comet's coma lies a nucleus of solid material, typically no more than
(5) 10 kilometers across. The visible coma is a huge cloud of gas and dust that has escaped
from the nucleus, which it then surrounds like an extended atmosphere. The coma can extend
as far as a million kilometers outward from the nucleus. Around the coma there is often an
even larger invisible envelope of hydrogen gas.
The most graphic proof that the grand spectacle of a comet develops from a relatively
(10) small and inconspicuous chunk of ice and dust was the close-up image obtained in 1986 by
the European Giotto probe of the nucleus of Halley's Comet. It turned out to be a bit like a
very dark asteroid, measuring 16 by 8 kilometers. Ices have evaporated from its outer layers
to leave a crust of nearly black dust all over the surface. Bright jets of gas from evaporating
ice burst out on the side facing the Sun, where the surface gets heated up, carrying dust
(15) with them. This is how the coma and the tails are created.
Comets grow tails only when they get warm enough for ice and dust to boil off. As a
comet's orbit brings it closer to the Sun, first the coma grows, then two distinct tails usually
form. One, the less common kind, contains electrically charged (i.e., ionized) atoms of gas,
which are blown off directly in the direction away from the Sun by the magnetic field of
(20) the solar wind. The other tail is made of neutral dust particles, which get gently pushed back
by the pressure of the sunlight itself. Unlike the ion tail, which is straight, the dust tail
becomes curved as the particles follow their own orbits around the Sun.
The passage focuses on comets primarily in terms of their
The author mentions tulip growing in New Netherland. Pennsylvania. and Michigan in order to illustrate how
All of the following statements about the tails of comets are true EXCEPT:
According to the passage, when was the Olmsted Report officially accepted as the master plan .for the Seattle public park system？
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