PrepTest 147, Section 4, Question 23
The diet of Heliothis subflexa caterpillars consists entirely(62 more words not shown)
Which one of the following(8 more words not shown)
- AH. subflexa caterpillars(6 more words not shown)
- BMost species of(9 more words not shown)
- CAny caterpillar that(11 more words not shown)
- DA food source(10 more words not shown)
- ENo caterpillars other(12 more words not shown)
Reading the argument
Other insects need this acid. Does that tell us these caterpillars need it? No. The passage specifically distinguishes H. subflexa from the insects whose growth and maturation require linolenic acid. We shouldn't turn its acid-free diet into a claim that it must be unhealthy.
What we’re looking for
In fact, there isn't an argument making that claim. We're given a collection of facts and asked what follows. The useful connection is that producing volicitin requires linolenic acid in the diet, and most caterpillar species have volicitin in their saliva. Those species must be eating something that supplies the acid. Since Physalis fruit doesn't contain it, a diet consisting only of that fruit couldn't supply what they need to produce volicitin.
Working the answers
- A
We aren't told that H. subflexa makes linolenic acid inside its body. The fact that many other insects need the acid doesn't establish that this species needs it and therefore must manufacture it. We can't infer an internal source merely because its food lacks the substance.
- BCorrect
Most caterpillar species have volicitin in their saliva. Producing volicitin requires linolenic acid in the diet. So those species must have a dietary source of linolenic acid. We don't have to identify the particular food; we only have to follow the stated requirement from the chemical they have to the dietary ingredient needed to produce it.
- C
Having a required ingredient doesn't guarantee the finished chemical. The passage says an insect needs dietary linolenic acid to produce volicitin. It doesn't say eating the acid automatically produces volicitin, let alone puts it in the insect's saliva. Other requirements or processes could also be needed. B follows from having volicitin back to its requirement; C incorrectly starts with the requirement and treats the result as certain.
- D
Not eating a substance doesn't establish that it is poisonous. The passage describes H. subflexa's diet without saying that foods containing linolenic acid would harm it. We would need separate evidence of toxicity to draw that conclusion.
- E
Other caterpillars could eat Physalis fruit along with something else that contains linolenic acid. Needing another dietary source doesn't mean they cannot also eat this fruit. The passage gives an exclusive diet for H. subflexa, not exclusive access to the fruit.
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