To facilitate growth and development, insects regularly shed off their old inelastic envelope and replace it with a newly synthesized, larger one. The timing of this molting process is tightly controlled by hormones, which ensures that the insects have built up sufficient energy reserves during their larval stages to fuel molting and metamorphosis (the transformation from juvenile into adult forms). Hormonal regulation of insect development has been extensively studied in the past and many details are known. However, in a recent study published in PNAS, Lyn Riddiford, James Truman and a team of Japanese scientists demonstrate that insects possess a previously unrecognized molting timer.
Because of their relatively large size and ease of rearing, the tobacco hornworm (Manduca sexta) is a widely used model organism for studying the hormonal control of insect development. The larvae of the last (fifth) instar have to reach a critical mass of about 5 g before specific hormones trigger the onset of metamorphosis, which involves a larval-to-pupal (metamorphic) molt. This metamorphic checkpoint is negatively controlled by juvenile hormone (JH), which – as indicated by its name – prevents the larvae from developing into pupal and finally adult stages.
Knowing that the larvae can undergo additional larval-to-larval molts when they don't reach the critical mass for their metamorphic moult as a result of an insufficient supply of food, the team hypothesized that a positive regulator that drives molting regardless of body size must exist. To test this, they designed a set of illuminating experiments. First, they monitored growth rates and the onset of metamorphosis in fifth instar larvae that were starved for various periods of time before being re-fed to allow the metamorphic molt. They observed that starvation is compensated for by the extension of the subsequent feeding period to reach the minimum mass necessary for metamorphosis, so the longer the larvae were starved, the longer they ate before moulting.
However, the situation was different when the negative effects of JH on metamorphosis were suppressed. For this purpose, the scientists surgically removed the glands that secrete JH from larval brains, and starved these insects as before. In line with their hypothesis, they observed that the larvae no longer compensated by increasing the subsequent feeding period, but began metamorphosis after a fixed period of 4 days regardless of whether they had reached the critical mass, as long they had fed for a minimum of 12–24 h during those 4 days. Thus, a ‘molt timer’, independent of body size, appears to exist and is initiated by a feeding stimulus. Performing similar experiments with younger, fourth instar larvae further showed that this timer controls not only metamorphic molts but also larval-to-larval molts.
Next, the team went on to manipulate the dietary composition, to identify which type of nutrient is needed to initiate this timer. It turned out that food quality matters when triggering this response, and that the most important component in the diet is protein. Finally, the team asked where the timer resides in the insect's body. They ligated the neck of larvae to block signals released from the brain and compared normal with JH-deficient larvae. As the timer was functional in these neck-ligated insects, they concluded that a significant part of the timer is localized outside the head region, possibly in the prothoracic gland, which is known to secrete ecdysone, the major insect molting hormone.
In summary, the study of the Riddiford/Truman team represents a fundamental breakthrough in understanding insect development. They have discovered a previously unrecognized developmental clock, which may be ubiquitously found in all molting invertebrates.