Control of Development
The titre of JH found in the haemolymph of the developing insect controls the stage of development that the insect is in. During ecdysis the form of the old cuticle laid down before the next moult is controlled by the JH level in the insect. JH maintains a juvenile state. The level of it gradually decreases during the development of the insect, allowing it to proceed to successive instars with each molt.
This has been demonstrated in various studies, most prominently that by V. B. Wigglesworth in 1960s. In this study, two adult Rhodnius had their blood systems linked, ensuring that the JH titre in both would be equal. One was a third instar Rhodnius, the other was a fourth instar. When the corpora allata of the third instar insect were removed, the level of JH was equal in both insects to that in the fourth instar animal, and hence both proceeded to the fifth instar at the next moult. When the fourth instar Rhodnius had its corpora allata removed, both contained a third instar level of JH and hence one proceeded to instar four, and the other remained at this instar.
Generally, the removal of the corpora allata from juveniles will result in a diminutive adult at the next moult. Implantation of corpora allata into last larval instars will boost JH levels and hence produce a supernumary (extra) juvenile instar etc.
Read more about this topic: Juvenile Hormone
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