Why Water Bugs Practice Paternal Care of Eggs

Although paternal care of young (fathers caring for their young) is common among fishes, the trait is rare among other animals, vertebrates and invertebrates alike. Among the few paternal insects are the giant water bugs. In Lethocerus water bugs, males guard and moisten clutches of eggs(eggs deposited during a single episode of laying) that females glue onto the stems of aquatic vegetation located above the waterline. Other kinds of male water bugs(e.g., Abedus and Belostoma)permit their mates to lay eggs directly on their backs, after which the male assumes responsibility for their welfare. A male Abedus caring for its eggs spends hours perched near the water’s surface, pumping his body up and down to keep well-aerated water moving over the eggs. Clutches of Abedus eggs that are experimentally separated from a male attendant do not develop, demonstrating that male parental care is essential for offspring survival in this case.

Bob Smith has explored both the history and the adaptive value of these unusual paternal behaviors. Since the closest relatives of the family that contains the paternal water bugs are the Nepidae, a family of insects without male parental care, we can be confident that the brooding species(species caring for eggs) evolved from nonpaternal ancestors. Whether out-of-water brooding and back brooding evolved independently, or whether one preceded the other, is not known, although some evidence suggests that back brooding came later. In particular, female Lethocerus sometimes lay their eggs on the backs of other individuals, male or female, when they cannot find suitable vegetation for that purpose. This unusual behavior indicates how the transition from out-of-water brooding to back brooding might have occurred. Females with the tendency to lay their eggs on the backs of their mates could have reproduced in temporary ponds and pools where aquatic vegetation sticking out of the water was scarce or absent.

But why do the eggs of water bugs require brooding? Huge numbers of aquatic insects lay eggs that do perfectly well without a caretaker of either sex. However, Smith notes that the eggs of giant water bugs are much larger than the standard aquatic insect egg, with correspondingly large requirement for oxygen, which is needed to sustain the high metabolic rates underlying embryonic development. But the relatively low surface-to-volume ratio of large aquatic egg leads to an oxygen deficit inside the egg. Since oxygen diffuses through air much more easily than through water, laying eggs out of water can solve that problem. But this solution creates another problem, which is the risk of drying out that the eggs face when they are high and dry. The solution, brooding by males that moisten the eggs repeatedly, sets the stage for the evolutionary transition to back brooding at the air-water interface.

Wouldn’t things be simpler if water bugs providing parental care simply laid small eggs with large surface-to-volume ratios? To explain why some water bugs produce eggs so large that they need to be brooded, Smith points out that water bugs are among the world’s largest insects almost certainly because they specialize in grasping and stabbing large vertebrates, including fish, frogs, and tadpoles. Water bugs, like all other insects, grow in size only during the immature stages. After the final molt (loss of outer covering layer) to adulthood, no additional growth occurs. As an immature insect loses its outer covering as it grows from one stage to the next, it acquires a new flexible outer covering that permits an expansion of size, but no immature insect grows more than 50 or 60 percent per molt. One way for an insect to grow large, therefore, would be to increase the number of molts before making the final transition to adulthood. However, no giant water bug molts more than six times. This observation suggests that these insects are locked into a five-or six-molt sequence, just as the bird species the spotted sandpiper evidently cannot lay more than four eggs per clutch. If a water bug is to grow large enough to kill a frog in just five or six molts, then the immature insect that hatches from the egg must be large, because it will get to undergo only five or so 50-percent expansions.?

1.Although paternal care of young (fathers caring for their young) is common among fishes, the trait is rare among other animals, vertebrates and invertebrates alike. Among the few paternal insects are the giant water bugs. In Lethocerus water bugs, males?guard?and moisten clutches of eggs(eggs deposited during a single episode of laying) that females glue onto the stems of aquatic vegetation located above the waterline. Other kinds of male water bugs(e.g., Abedus and Belostoma)permit their mates to lay eggs directly on their backs, after which the male assumes responsibility for their welfare. A male Abedus caring for its eggs spends hours perched near the water’s surface, pumping his body up and down to keep well-aerated water moving over the eggs. Clutches of Abedus eggs that are experimentally separated from a male attendant do not develop, demonstrating that male parental care is essential for offspring survival in this case.?