3.3.1. Endocrine centers
The hormones of the insect body are produced by neuronal, neuroglandular, or glandular centers (Fig. 3.8). Hormonal production by some organs, such as the ovaries, is secondary to their main function, but several tissues and organs are specialized for an endocrine role.
Neurosecretory cells (NSC) (also called neuroendocrine cells) are modified neurons found throughout the nervous system (within the CNS, peripheral nervous system, and the stomodeal nervous system), but they occur in major groups in the brain. These cells produce most of the known insect hormones, the notable exceptions being the production by non-neural tissues of ecdysteroids and juvenile hormones. However, the synthesis and release of the latter hormones are regulated by neurohormones from NSC.
The corpora cardiaca (singular: corpus cardiacum) are a pair of neuroglandular bodies located on either side of the aorta and behind the brain. As well as producing their own neurohormones, they store and release neurohormones, including prothoracicotropic hormone (PTTH, formerly called brain hormone or ecdysiotropin), originating from the NSC of the brain. PTTH stimulates the secretory activity of the prothoracic glands.
The prothoracic glands are diffuse, paired glands generally located in the thorax or the back of the head. In cyclorrhaphous Diptera they are part of the ring gland, which also contains the corpora cardiaca and corpora allata. The prothoracic glands secrete an ecdysteroid, usually ecdysone (sometimes called molting hormone), which, after hydroxylation, elicits the molting process of the epidermis (section 6.3).
The corpora allata (singular: corpus allatum) are small, discrete, paired glandular bodies derived from the epithelium and located on either side of the foregut. In some insects they fuse to form a single gland. Their function is to secrete juvenile hormone (JH), which has regulatory roles in both metamorphosis and reproduction.