The endocrine system controls insect metamorphosis and molting. These processes are under the control of two developmental hormones, ecdysteroids, which promote development and regulates oocyte maturation in the ovary, and juvenile hormone (JH), which modulate ecdysteroid actions, female reproduction including the development of the genital gland and vitellogenesis (Parthasarathy et al., 2010). The concentration of JH in the hemolymph is regulated not only by its biosynthesis in the corpora allata (CA) but also by its degradation, sequestration or excretion. The JH levels remain high during 1-5 days after post adult emergence (PAE), while ecdysteroid levels decreases (Arakane et al., 2008; Tan and Palli, 2008a). Catabolism of JH can be inhibited or stimulated by neuropeptides termed allatotropins (AT) or allatostatins (AS). Allatostatins of the B- and C-type and the T. castaneum allatotropin are essential compounds during the pupal stage. Moreover, they are sine qua on for proper ovarian maturation and egg laying in young adults. In the case of Tc-AT, the isoform AT3 is the most effective preprohormone. Although, the distinct mechanisms of action of the peptides in the pupa remain to be elucidated (Abdel-latief and Hoffmann, 2014). Endocrine regulation of insect vitellogenesis has been investigated for many years. It is known that during vitellogenesis, female-specific proteins, vitellogenins (Vg), are synthesized in the fat body, secreted into the hemolymph and taken up by the developing oocyte. The previtellogenic phase covers 0-3 days PAE and the vitellogenin cycle starts after day 3 PAE. Ecdysteroid levels decrease from day 0 to day 5 PAE, it appears that the presence of ecdysteroid is not a requisite for Vg synthesis in the fat body (Parthasarathy et al., 2010).
Metamorphosis is a marked change between larval and adult forms that occur in all insects. In insect holometabolous, the larvae metamorphose into adults by an intermediate stage termed the pupa. In larvae, JH prevents the steroid molting hormones (ecdysteroids) from initiating metamorphosis, so that after a molt another larval stage follows (Konopova and Jindra, 2007). Subsequently, the larvae move to the pupal stage, and adult ecdysis behavior begins five to six days after pupation and consists of three behavioral intervals, namely preecdysis, ecdysis, and postecdysis. Preecdysis, the first recognizable patterned behavior starts approximately 10 h before the onset of ecdysis. This behavior is characterized by dorsal-ventral (D–V) contractions observable by the movement of the posterior end in a ventral direction. These contractions become stronger and their frequencies increase the time close to ecdysis. For most individuals, ecdysis begins with 3–8 strong bouts of reverse-bending behavior. This motion leads eventually to a shedding of the pupal cuticle. This event coincides with the freeing of the antennae and the filling of wing tracheae with air, followed by sequential leg and wing stretching. After completion of wing stretching, the motor pattern changes to anterior– posterior (A–P) contractions, this facilitates the actual shedding of the pupal cuticle starting at the anterior (head) end and proceeding to the posterior. Ecdysis behavior, beginning with strong reverse-bending and culminating in complete emergence of the adult from the pupal cuticle, requires about 22 ± 2 min (Arakane et al., 2008). Postecdysis behavior occurs after the shedding of the pupal cuticle. During most of this interval, the beetle is sluggish and often at rest. The exuvium remains attached to the posterior end immediately after ecdysis, but it is eventually detached by a strong body-shaking motion. Gradually, the elongated hindwings and abdomen retract under the elytra. Tanning of the exoskeleton takes place gradually over a period of 5 days after ecdysis (Arakane et al., 2008). The above-mentioned process is controlled by the prothoracicotropic hormone, which also controls the release of ecdysteroid hormone from thoracic glands (Amare and Sweedler, 2007). Ecdysteroids regulate insect growth and development through a heterodimeric complex of nuclear receptors consisting of ecdysone receptor (EcR) and ultraspiracle (USP). The most active form of ecdysteroids, 20-hydroxyecdysone (20E), transduces its signal through a heterodimeric complex of two nuclear receptors, the EcR and USP, a homolog of the vertebrate retinoid X receptor (RXR). Receptors of EcR and USP are essential for metamorphosis in T. castaneum. The EcRA isoform initiates ecdysteroid action by regulating the expression of the EcRB isoform and other early genes involved in ecdysteroid signal transduction (Tan and Palli, 2008a). In T. castaneum, have been identified 34 prohormones and three defensin precursors based on similarity searches from known insect precursors (Amare and Sweedler, 2007). Other compounds, such as isoprenoids, fatty acid derivatives, and amino acid derivatives have all been found to mediate intraspecific behavior in the Coleoptera (Tillman et al., 1999).
Several genes that code for protein synthesis involved in molting and metamorphosis have been identified. Among these are JHAMT, Kr-h1, JHE, JHEH, and JHIPs involved in the JH biosynthesis, metabolism and signaling (Parthasarathy et al., 2010). Eclosion hormone (EH), apart from Aedes aegypti, Tribolium is the only other insect for which multiple EH genes are reported (Amare and Sweedler, 2007). Nuclear receptors (NRs), TcE75, TcHR3, TcHR4, TcEcR, TcUSP, TcFTZ-F1, TcHR51, SVP, TcHR38, TcHR39 are important for metamorphosis (Tan & Palli, 2008). Seven NRs, TcE75, TcHR3, TcEcR, TcUSP, TcFTZ-F1, TcHR4 and TcHR51 are critical for larval-pupal metamorphosis. Additional three NRs TcHR38, TcHR39, and TcSVP are important for both larval-pupal and pupal-adult metamorphosis. EcR–USP heterodimer of NRs mediates 20E action (Tan and Palli, 2008b).