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This excerpt is from The Life-Story of Insects by Geo H. Carpenter.

Insects as a whole are preeminently creatures of the land and the air. This is shown not only by the possession of wings by a vast majority of the class, but by the mode of breathing to which reference has already been made, a system of branching air-tubes carrying atmospheric air with its combustion-supporting oxygen to all the insect's tissues. The air gains access to these tubes through a number of paired air-holes or spiracles, arranged segmentally in series.

It is of great interest to find that, nevertheless, a number of insects spend much of their time under water. This is true of not a few in the perfect winged state, as for example aquatic beetles and water-bugs ('boatmen' and 'scorpions') which have some way of protecting their spiracles when submerged, and, possessing usually the power of flight, can pass on occasion from pond or stream to upper air. But it is advisable in connection with our present subject to dwell especially on some insects that remain continually under water till they are ready to undergo their final molt and attain the winged state, which they pass entirely in the air.

The preparatory instars of such insects are aquatic; the adult instar is aerial. All may-flies, dragon-flies, and caddis-flies, many beetles and two-winged flies, and a few moths thus divide their life-story between the water and the air. For the present we confine attention to the Stoneflies, the May-flies, and the Dragon-flies, three well-known orders of insects respectively called by systematists the Plecopteran, the Ephemeroptera, and the Odonata.

In the case of many insects that have aquatic larvae, the latter are provided with some arrangement for enabling them to reach atmospheric air through the surface-film of the water. But the larva of a stone-fly, a dragon-fly, or a may-fly is adapted more completely than these for aquatic life; it can, by means of gills of some kind, breathe the air dissolved in water.

What role do the spiracles play in the respiration of the insects described in the passage?

  • A. They allow insects to breathe atmospheric air even when submerged.
  • B. They protect the insect's internal organs from water pressure.
  • C. They filter oxygen from the water for underwater breathing.
  • D. They are used to expel carbon dioxide from the body.
Correct Answer: A

Rationale: The spiracles in insects serve the crucial function of allowing them to breathe atmospheric air even when submerged. The passage explains that insects have a system of branching air-tubes that carry atmospheric air with oxygen to all tissues. The air enters these tubes through paired air-holes or spiracles, enabling insects to respire even when they are underwater. Choices B, C, and D are incorrect because the spiracles do not protect internal organs from water pressure, filter oxygen from water for underwater breathing, or expel carbon dioxide from the body. The primary role of spiracles is to facilitate respiration by allowing insects to access atmospheric air.