If you're the kind of person who enjoys watching the perfectly choreographed singing of the von Trapp children in The Sound of Music, then the sight of translucent stink bugs crawling forth simultaneously from their eggs might be the stuff of nightmares. Other people, such as scientists interested in biological timing, are genuinely fascinated by synchronized hatching, since it is quite common throughout the animal kingdom (think of all the egg-laying species, from insects to spiders to turtles). Yet, the mechanisms initiating synchronized hatching are not well studied. Jun Endo and colleagues from Kyoto University, Japan, wanted to find out more about one of these mechanisms, after observing the synchronized birth of brown marmorated stink bugs (Halyomorpha halys). Stink bugs lay clumps of about 28 sesame-seed-sized eggs all at once, which hatch later in virtual unison. It all starts with a single embryo cracking open its shell (there is a mesmerizing video of this online), which somehow signals to its other siblings to crack open their own eggs, too.
The researchers first sought to identify the cue that initiates hatching. To do this, they took pairs of eggs from the same clutch and placed them on paper. They then allocated the paired eggs to a group of naturally attached eggs and another four groups of separated eggs that were either reattached by the researchers; kept physically apart, but connected by a thin piece of carbon; kept physically apart; or isolated by different pieces of paper. Nearly all of the naturally attached eggs and the eggs that had been reunited hatched together. However, only half of the eggs connected by the carbon hatched at the same time, followed by 20–25% of the eggs separated on the same sheet of paper, while even fewer of the isolated eggs successfully hatched in sync. This meant the hatching cue was vibrational – ruling out sound or chemical signals – as it required a physical connection for transmission of the signal. The ability of the naturally attached and reattached eggs to hatch together also demonstrated that embryos transmit the hatching vibration to an adjacent sibling's egg.
Endo and colleagues then investigated the source of the vibrations. In a scene reminiscent of Alien, they saw that the first embryo to hatch wielded an egg burster (a T-shaped structure protruding from the forehead of all embryos) to crack open its shell. This cracking generated the precise vibrations needed to transmit the hatching cue to neighbouring eggs, which the researchers recorded using a sensitive laser. Next, the researchers placed individual eggs on a vibration-producing platform and played the laser recordings back to the eggs. Excitingly, the eggs began to crack immediately. Now that they knew that the egg-cracking vibrations trigger hatching, the researchers wanted to know how efficiently they travel. They noted that as many as three inactive eggs can lie between two live embryos, before the eggs fail to hatch simultaneously, indicating that the vibrations can travel through the eggs of immature embryos.
A synchronized birth provides stink bugs with a survival advantage, as it ensures that only fully developed embryos hatch. Hatchlings will then cannibalize any unhatched, underdeveloped siblings. This gruesome, but essential, behaviour prevents the killing of viable embryos and provides a readily available source of food. Furthermore, although you might believe stink bug hatching to be horrifying and smelly (it is), studying this behaviour has far-reaching implications for biotremology (the study of vibrations). Research like this informs us more about the widespread use and transmission of vibrations by animals to communicate social information.