Frank looked at the beetles, and then moved his thumb nail along the rule.
“About an inch and a half in length,” he said.
“That is about right,” said his uncle. “Are these two beetles exactly alike?”
“They are both about the same shape and colour. One is slightly bigger than the other. They are both greenish-black above and brown below. There is a curious broad border of yellow all round the edge of their bodies,” said Tom.
“But they are not both exactly alike,” said Frank, “I notice that one is smooth on the back, while the other’s back is all grooved.”
“You are right, Frank,” said Uncle George. “The one with the grooved wing-cases is the female. The one with the smooth cases is the male.
Fore Leg of Male, showing Sucker Organ.
“Notice the long legs they have for swimming, and how they seem to oar themselves through the water. Notice also that the male has a large flat disc upon each of his fore legs. These are suckers, by which he can cling to things.”
“Why do they come up to the surface so often?” Frank asked.