Water Bug Adaptation

Paragraph 1

Three basic adaptive responses—regulatory, acclimatory, and developmental—may occur in organisms as they react to changing environmental conditions. In all three, adjustment of biological features (morphological adjustment) or of their use (functional adjustment) may occur. Regulatory responses involve rapid changes in the organism’s use of its physiological apparatus—increasing or decreasing the rates of various processes, for example. Acclimation involves morphological change—thickening of fur or red blood cell proliferation—which alters physiology itself. Such structural changes require more time than regulatory response changes. Regulatory and acclimatory responses are both reversible.

Paragraph 2

Developmental responses, however, are usually permanent and irreversible; they become fixed in the course of the individual’s development in response to environmental conditions at the time the response occurs. One such response occurs in many kinds of water bugs.Most water-bug species inhabiting small lakes and ponds have two generations per year. The first hatches during the spring, reproduces during the summer, then dies. The eggs laid in the summer hatch and develop into adults in late summer. They live over the winter before breeding in early spring. Individuals in the second (overwintering) generation have fully developed wings and leave the water in autumn to overwinter in forests, returning in spring to small bodies of water to lay eggs. Their wings are absolutely necessary for this seasonal dispersal. The summer (early) generation, in contrast, is usually dimorphic—some individuals have normal functional (macropterous) wings; others have much-reduced (micropterous) wings of no use for flight. The summer generation’s dimorphism is a compromise strategy, for these individuals usually do not leave the ponds and thus generally have no use for fully developed wings. But small ponds occasionally dry up during the summer, forcing the water bugs to search for new habitats, an eventuality that macropterous individuals are well adapted to meet.

Paragraph 3

The dimorphism of micropterous and macropterous individuals in the summer generation expresses developmental flexibility; it is not genetically determined. The individual’s wing form is environmentally determined by the temperature to which developing eggs are exposed prior to their being laid. Eggs maintained in a warm environment always produce bugs with normal wings, but exposure to cold produces micropterous individuals. Eggs producing the overwintering brood are all formed during the late summer’s warm temperatures. Hence, all individuals in the overwintering brood have normal wings. Eggs laid by the overwintering adults in the spring, which develop into the summer generation of adults, are formed in early autumn and early spring. Those eggs formed in autumn are exposed to cold winter temperatures, and thus produce micropterous adults in the summer generation. Those formed during the spring are never exposed to cold temperatures, and thus yield individuals with normal wings. Adult water bugs of the overwintering generation, brought into the laboratory during the cold months and kept warm, produce only macropterous offspring.

Topic and Scope:

Developmental responses; specifically, water bugs as an illustration of how organisms can adapt permanently and irreversibly to changing environmental conditions.

Purpose and Main Idea:

The author’s purpose is to describe three types of adaptations that organisms may have in response to changing environmental conditions, and to discuss one of those responses in depth through an extended example. Since this is a descriptive (rather than argumentative) passage, the author doesn’t set forth any particular “main idea.”

Paragraph Structure:

Paragraph1 names three types of adaptive responses to changing environmental conditions—regulatory, acclimatory, and developmental—but only discusses the first two. Paragraphs 2 and 3 pick up the author’s real concern, by discussing in great detail how developmental responses work in the case of water bugs. Specifically, Paragraph2 describes the two different water bug generations and the differences in their wing spans, and Paragraph3 explains how environment accounts for those differences.

The Big Picture:

  • This science passage is about as complex as they get on the  CAT. In the unlikely event that you get a passage as intricate as this one, it’s probably best to leave it for last, unless you’re confident that you can get through it quickly and accurately.
  • Despite their complexity, passages like this one generally boil down to a few simple relationships, and those are all you really need to grasp in order to cope with the lion’s share of the questions. Don’t be overly concerned about assimilating all of the jargon and scientific facts. If you need to know that stuff to answer a question or two, you can always look it up.
  • One useful way to get scientific relationships straight in your mind is to make a mental picture of them. Here, you might have tried to picture the relationship between water-bug wing size and season—for example, mentally seeing the need for some long-winged bugs to fly away to water when a summer pond dries up.

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