Cockroaches are incredibly successful at coexisting with humans, even though we are less than thrilled with the prospect of sharing our homes with them. Amongst their many remarkable survival tactics, Don Mullins describes their ability to reproduce, by laying eggs in a protective case called an ootheca, as `spectacular'. Some species abandon the ootheca soon after laying their eggs, but female German cockroaches carry the ootheca until the young hatch almost one month later. How liquid penetrates the apparently solid surface of the ootheca has puzzled scientists for well over a century, but Mullins has finally discovered the microscopic pore structures that keep oothecae hydrated (p. 2987).
Female German cockroaches fall somewhere between the two extremes on the cockroach parental-care spectrum. Having laid their eggs in the protective ootheca, they carry the case close to their abdomens, until the young are ready to hatch. However, without a constant supply of fluid, the young cockroaches may die before hatching. Mullins knew that during gestation,oothecae absorb fluid and become heavier. More then a century ago, William Wheeler had analysed the structure in detail. He described the appearance of the ootheca, and identified the oothecal structure that rests close to the female's vagina, which he called the escutcheon. But the limitations of nineteenth century optics prevented him from finding out how fluid penetrates the solid case. Mullins decided to use two 21st century techniques to solve this 19th century problem.
Mullins and his team turned up the magnification with scanning electron microscopy, searching for structures on the surface that might allow transport of fluid to the inside of the ootheca. Most of the surface appeared quite smooth, but when they began looking at the surface near to the escutcheon, the team found dimple structures. Could the dimples be pores that transported fluid to the ootheca's interior? Mullins' team switched to confocal microscopy to get the three dimensional perspective. They focused down through the case's wall and found that the dimples continued through, inside the case; they were pores!
But could these tiny pores keep the ootheca case hydrated? Mullins tested the membrane's porosity. He divided a tiny glass chamber in two with the porous end of the ootheca, and then he filled one half of the chamber with a radioactive solution, and waited to see if it diffused through into the second chamber. Testing solutes that ranged in size from radiolabelled water up to glucose, Mullins found that small molecules flowed easily across the pore field of the oothecal case. Although larger molecules passed through the membrane more slowly, they were still able to pass through the tiny pores.
Although the offspring in the ootheca don't need nourishment from their mother, Mullins explains that the young may have some way of communicating with their mother as they prepare to hatch. He believes that the pores could provide a direct communication link between the mother and the next generation, as well as providing the inlet and outlet for the ootheca's water distribution system. And he adds that understanding how these pests reproduce could eventually help man gain the upper hand in the eviction battle with our unwelcome housemates.