As background to this joke, know that there is a simply enormous number of distinct species of beetle: well over 300,000; and you need to know of the famous quip by J.B.S. Haldane. When asked "What has the study of biology taught you about the Creator, Dr. Haldane?", he replied
"I'm not sure, but He seems to be inordinately fond of beetles."
This inspired the following mediation on Adam naming the animals...
God: And here's the next species, one I'm particularly proud of... Adam: Beetle. God: Excellent. Now here's another... Adam: Beetle. God: No, you just named the last one "beetle". This one is quite different -- look at the pattern on the wing cases, and the shape of the antennae... Adam: Beetle. God: Well, OK, though they certainly look different to Me. Now, the next species is -- Adam: Beetle.
-- Excerpt from "The *Real* Reason for the Fall" (credits: Ken Cox)