Richard Ebright raised a flock of butterflies because the butterfly-collecting season around Reading, Pennsylvania, lasted only six weeks, and chasing butterflies one by one meant he could not catch many. To tag more monarchs for Dr Urquhart's migration research, he caught a female monarch, took her eggs, and raised them in his basement through their full life cycle — from egg to caterpillar to pupa to adult butterfly. He would then tag the butterflies' wings and release them.
Source: "The Making of a Scientist," Tagging Butterflies and Early Science Fairs section
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The examiner expects three clear points: (1) the short collecting season limited how many butterflies he could catch, (2) he needed large numbers for Dr Urquhart's tagging programme, and (3) he raised them through the complete life cycle in his basement. Avoid padding — one tight paragraph covering all three points earns full marks. Always link the reason to the tagging/migration research context.