Hey Yuuichi,
"If" and "when" clauses are only used in certain situations.
"If" is used to show potential, improbability, or obligation.
"When" is used to refer to a specific time (either real or unreal).
In the example you provided, "There isn't an entomologist in the whole world who wouldn't give all he has to be in [my] shoe[s] today," is referring to an actual state of being (in his shoes). Since it is a real physical state (and not a time), neither "if" nor "when" are necessary.
Also, "to be in someone's shoes" is an idiomatic expression meaning that the entomologist feels lucky. He's insinuating that other entomologists should envy him. To be in someone shoes means to be someone. "They wish they were me."
I hope this helps :-)
お疲れ様でした!
Greg