Jurassic-era water bugs used one leg to lay their eggs until they hatched. (Image source: Courtesy of Huang Diying)
Insects that lived 160 million years ago had clusters of eggs hanging from their legs, like grapes hanging from a vine. Scientists recently discovered evidence of this parental behavior in well-preserved fossils, which may be the earliest examples of brood care among insect species — parents protect their eggs or young by carrying them.
Researchers unearthed insect fossils from the Haiphonggou Formation, a fossil-rich rock formation near the village of Daohugou in northeastern China. A variety of fossils have been recovered from the site in the past, including the well-preserved remains of feathered dinosaurs, ancient mammals, giant fleas and long-nosed scorpions.
In a study published Wednesday (July 13) in the journal Proceedings of the Royal Society B: Biological Sciences, researchers analyzed nearly 160 fossils of Karataviella popovi, an extinct water bug. , the hind legs are like paddles. The fossils - which the study authors call "unique" - are 163.5 million years old, meaning they date to the middle of the Jurassic period (201.3 million to 145.5 million years ago).
Among the fossils, the team identified 30 adult female specimens with a cluster of eggs anchored to their left "midtibia," the middle of the three left-sided legs. The densely packed eggs are arranged in five or six staggered rows, with six to seven eggs in each row, and each egg is attached by a short "egg stalk". Each egg is about 0.04 to 0.05 inches (1.14 to 1.20 mm) in diameter—a sizeable size considering that K. popovi adults are only about 0.5 inches (12.7 mm) long.
A cluster of eggs is visible on one of the extended legs of this specimen of Karataviella popovi. (Image credit: Courtesy of Huang Diying)
The study authors speculate that K. popovi females may expel eggs onto the appropriate limb by first secreting thick mucus and then performing "specific bending movements of the abdomen," depositing the eggs directly on the legs. "During swimming and feeding, the unoccupied right tibia may be used for balance," they write in the report.
The authors note that the water bug's giant eggs may provide adequate nutrition for their offspring - but the production of Big eggs also come with a price. Large eggs are more difficult to inflate with oxygen than smaller eggs because of their lower surface area to volume ratio. Possibly because, by holding the eggs on their legs and letting them dangle gently on their stems, K. popovi maximizes the flow of oxygen from the surrounding water body to the developing offspring.
"To our knowledge, carrying a cluster of eggs on [one] leg is a unique strategy among insects but not uncommon among aquatic arthropods," meaning crustaceans, the study authors wrote. "Our findings push back evidence of explicit brood-rearing behavior in insects by nearly 38 million years, helping to understand the evolutionary and adaptive significance of brood care in insects."
