Sensing Airflow
Paragraph 1: Sensing the flow of air over the body can be useful to an animal that flies. On a large scale, if an animal can sense the speed and direction of the relative wind, it can keep itself aligned with its intended flight direction by correcting for the transient effects of wind gusts. On a much finer scale, if an animal could detect the fine details of airflows, particularly on its wings, it could, in principle, adjust its movements to optimize its wing stroke. Biologists have demonstrated a variety of large-scale wind sensors in flying animals, but they have not yet found any fine-scale mechanisms.
1. According to paragraph 1, large-scale wind sensors help a flying animal to
A. determine how fast it needs to fly to make use of the relative wind
B. avoid being caught in wind gusts
C. maintain its course when hit by passing wind gusts
D. adjust its movements for effective wing stroke
Paragraph 2: We know much more about insect wind sensors than about those of vertebrates. This surprising situation is largely because of the constraints of the insect exoskeleton (outer casing) and the relative simplicity of the insect nervous system. The exoskeleton forms a rigid body covering, so that every sensory receptor must have some structural specialization to allow it to detect anything outside the body. Often, these modifications take the form of a hair. Some biologists have become quite adept at determining what type of stimulus — wind, sound, odors — triggers a given hair, and how that hair’s sensation are used by the insect’s brain.
2. Which of the following can be inferred from paragraph 2 about the insect exoskeleton?
A. It is less rigid in insects with simple nervous systems than in insects with more complex nervous systems.
B. It prevents the insect nervous system from detecting external stimuli except through special structures.
C. Its rigidity determines how effective insect sensory receptors are at detecting external stimuli.
D. It controls where in the insect brain sensory information is sent.
3. According to paragraph 2, scientists have sometimes been able to tell which of the following about insects?
A. Exactly how many structural specializations a specific insect has
B. How often a stimulus must occur before it triggers a given hair
C. Why sensory modifications often take the form of a hair
D. How a hair’s sensations are used by the insect brain
Paragraph 3: Most flying insects use either their antennae or wind-sensitive hairs on their heads as large-scale wind detectors. Desert locusts (large grasshoppers) have been favorite subjects of researchers interested in wind sensing. In locusts, wind-sensitive hairs on the head trigger reflex steering responses, which is probably their function in other insects as well. Antennae are alternate wind sensors in many insects. Insects use their antennae for all sorts of sensing — smell, hearing, touch — and aphids, flies, bees, grasshoppers, and moths use them for wind sensing as well. When insects use their antennae to detect wind, they typically hold their antennae in a characteristic orientation during flight. If wind strikes them from some orientation other than head-on, the relative wind deflects the antennae slightly. A nerve at the base of the antenna senses this deflection, and the insect uses this sensation to trigger a steering reflex.
4. According to paragraph 3, what indicates that a flying insect is using its antennae for wind sensing?
A. The specific location of the antennae on the insect’s head
B. The way the insect holds its antennae while flying
C. The size and shape of the insect’s antennae
D. The increased sensitivity of the nerves at the base of the antennae
5. According to paragraph 3, what happens when the antennae of a flying insect are deflected by wind?
A. The deflection is detected by sensitive hairs on the insect’s head.
B. The antennae stop responding to smell, sounds, and touch and focus completely on wind sensing.
C. The deflection produces a sensation that triggers a steering reflex.
D. The insect adjusts its antennae so that they point in the direction of the relative wind.
Paragraph 4: The insects studied so far seem to use both antennae and head hairs for wind sensing. But why bother? Having more than one system may simply be redundant, so that if a predator happened to bite off the antennae, the head hairs could take over their functions, but this kind of redundancy is rare in biology. The two systems may detect slightly different aspects of the relative wind: for example, the antennae might be more sensitive to speed, while the hairs detect direction. Or the antennae might respond more to horizontal gusts and the head hairs to vertical gusts. They also probably operate on different scales: the hairs may respond to smaller, quicker disturbances, while the larger antennae respond to larger, more prolonged changes.
6. Why does the author point out that having “both antennae and head hairs for wind sensing” is a “kind of redundancy” that is “rare in biology”?
A. To challenge the idea that insect antennae detect exactly the same aspects of the wind that insect head hairs do
B. To point out a specific way in which the insects studied so far differ from most other insects
C. To explain why it is thought that the insects studied so far are likely to have predators that bite off their antennae
D. To argue that it is unnecessary for the insects studied so far to have two different mechanisms for sensing wind
7. According to paragraph 4, head hairs may be better than antennae at performing all of the following functions EXCEPT
A. sensing wind speed
B. responding to brief changes in the wind
C. sensing the direction of the wind
D. responding to vertical gusts of wind
Paragraph 5: Birds and bats surely have sensing abilities similar to those of insects, but these abilities are more difficult to study in birds and bats. Their tactile sensors (sensors for detecting touch) are much denser and more numerous than those of insects, and they are not necessarily associated with obvious structural modifications — birds and bats have no obstructing exoskeleton. Moreover, the complexity of vertebrate nervous systems means that researchers have much more difficulty demonstrating a physiological connection between a stimulus (like a change in wind direction) and a particular steering response. These animals may not even have specialized wind sensors (sensors that only detect wind). For instance, although humans are not fliers, a person can easily determine the direction of a brisk wind just from the sensation of the wind on his or her skin. Because birds and bats actually have an important use for such sensations, they ought to be at least as good as humans at detecting the speed and direction of wind on their faces. In addition, fur or feathers could make very sensitive small-scale flow detectors if coupled with a sensitive tactile nerve ending (as are many vertebrate hairs). As yet, however, biologists have done little or no work trying to connect wind sensing with flight steering behavior in birds and bats, and have not searched for small-scale flow sensors.
8. According to paragraph 5, wind-sensing abilities are more difficult to study in birds and bats than in insects for each of the following reasons EXCEPT:
A. Birds and bats have far more tactile sensors that are capable of detecting wind than insects have.
B. The nervous systems of birds and bats are much more complex than those of insects.
C. Birds and bats are less affected by wind than insects are.
D. The tactile sensors of birds and bats are not always connected to body structures that have a clear sensory function.
Paragraph 4: The insects studied so far seem to use both antennae and head hairs for wind sensing. But why bother? Having more than one system may simply be redundant, so that if a predator happened to bite off the antennae, the head hairs could take over their functions, but this kind of redundancy is rare in biology. ■The two systems may detect slightly different aspects of the relative wind, for example, the antennae might be more sensitive to speed, while the hairs detect direction. ■Or the antennae might respond more to horizontal gusts and the head hairs to vertical gusts. ■They also probably operate on different scales; the hairs may respond to smaller, quicker disturbances, while the larger antennae respond to larger, more prolonged changes. ■
9. Look at the four squares [■] that indicate where the following sentence could be added to the passage.
A more likely explanation is that each one has its own function.
Where would the sentence best fit?
10. Directions: An introductory sentence for a brief summary of the passage is provided below. Complete the summary by selected THREE answer choices that express the most important ideas in the passage. Some sentences do not belong in the summary express ideas that are not presented in the passage or are minor ideas in the passage. This question is worth 2 points.
Sensing airflow can be very useful to an animal that flies, particularly in terms of triggering flight-steering behavior.
Answer Choices
A. Insects detect outside stimuli through specialized structures — often hairs — that transmit signals to the nervous system, thereby triggering behavioral responses.
B. Animals that can detect the fine details of airflow on their wings are better at steering than animals that sense wind speed only, but animals that can do both have the best steering response.
C. In addition to using wind-sensitive hairs, insects use antennae as large-scale wind detectors, with a nerve at the base of the antenna triggering a steering reflex.
D. If an insect’s antennae are bitten off by predators, insects can then grow new head hairs that develop the function of wind sensing.
E. Flying animals have primary as well as secondary wind sensors, using the primary sensors in moderate wind conditions and the secondary sensors in extreme wind conditions.
F. Flight-steering behavior has not yet been linked to wind sensing in birds and bats, whose complex nervous systems and tactile sensors make study difficult.
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