A recent study showed that the expression of the neuronal-specific ecdysone receptor isoform EcRB1 is temporally regulated in the neuroblasts; it is expressed from the mid third instar, when Chinmo is no longer expressed27.
This adult ovary phenotype difference becomes established during the final larval instar, when massive programmed cell death leads to the degeneration of 95 99% of the ovariole anlagen in workers.
On the sixth day of the fifth instar large electron-lucent areas become apparent beneath the basement membrane; however, the glands remain intact until the seventh (last) day of the instar when a dramatic fragmentation of the cytoplasm, and condensation and fragmentation of the nucleus are observed.
Wing disks are very small until the last larval instar, when they increase dramatically in size, are invaded by branching tracheae from the wing base that precede the formation of the wing veins, and begin to develop patterns associated with several landmarks of the wing.
M. sexta larvae cause the most damage to N. attenuata plants during the 5th instar, when they consume >98% of their total leaf tissue intake; by comparison, early-instar caterpillars are relatively benign, causing only minor amounts of damage.
Length of the 4th instar was also measured because JH is a key hormone in determining larval molting; larvae molt into each successive instar when JH has dropped below a critical threshold [20].
Parasitized caterpillars continue developing and feeding until the 4th or 5th instar, when up to c. 80 full-grown parasitoid larvae egress from the host to pupate (A.H. Grosman and A. Janssen, pers. obs).
Importantly, the observed trend of gene expression changes, which mostly initiate at the 2nd instar when the maternally contributed wt U6atac snRNA runs out (Fig. 1C), is consistent with the scenario in which the loss of wt U6atac snRNA indeed triggers the observed changes in gene expression.
The critical weight is the size during the final instar when starvation no longer delays metamorphosis.
The enhancer has to function before the larvae have reached the early third instar when cell fate determination occurs.
