(a) What is metamorphosis? Describe the hormonal regulation of metamorphosis in insects. (20 marks) (b) What is alternation of…

Metamorphosis and Hormonal Regulation in Insects

Metamorphosis is the post-embryonic developmental process involving abrupt morphological, anatomical, and physiological transformation from a juvenile form to an adult. It occurs as incomplete metamorphosis (hemimetaboly, characterized by gradual nymphal stages lacking a pupa) or complete metamorphosis (holometaboly, proceeding through egg, feeding larva, quiescent pupa, and winged adult).

The process is strictly regulated by a tripartite neuroendocrine axis comprising neurosecretory cells of the brain, the corpora allata, and the prothoracic glands. Environmental or developmental cues stimulate the median neurosecretory cells of the protocerebrum to synthesize Prothoracicotropic Hormone (PTTH), which is transported to and released from the corpora cardiaca into the hemolymph. PTTH acts on the thoracic prothoracic glands, stimulating the secretion of the steroid prohormone ecdysone. Peripheral target tissues convert ecdysone into the active hormone 20-hydroxyecdysone (20E), which triggers apolysis and ecdysis (moulting).

The nature of the moult is determined by Juvenile Hormone (JH, a sesquiterpenoid) secreted by the corpora allata. Functioning as a "status quo" hormone, high titers of JH in the hemolymph ensure that 20E pulses induce a larva-to-larva moult, preserving juvenile characteristics and repressing adult gene expression. In holometabolous insects, a critical drop in JH levels during the final larval instar allows 20E to trigger pupation. In the subsequent pupal stage, the complete absence of JH permits 20E to direct imaginal disc differentiation and histolysis of larval tissues, culminating in adult emergence (imago).

**Alternation of Generations in *Obelia***

Alternation of generations in cnidarians, termed metagenesis, describes the cyclic alternation between a sessile, asexually reproducing polypoid generation and a free-swimming, sexually reproducing medusoid generation. Because both the polyp and medusa phases are diploid, this differs from the true alternation of generations (haplodiplontic cycle) in plants; only the gametes are haploid.

Obelia exhibits a polymorphic colonial hydroid stage anchored to the substratum by a horizontal, root-like hydrorhiza, which gives rise to erect, branching hydrocauli. The colony displays two main types of zooids: feeding hydranths (gastrozooids) possessing tentacles and a hypostome for nutrient capture, and reproductive blastostyles (gonozooids) enclosed within a gonotheca, forming a gonangium.

The blastostyle asexually produces miniature, bell-shaped, dioecious medusae by lateral budding. The medusae detach, become free-swimming, and produce haploid gametes (sperm and ova) from four gonads situated on the radial canals. External fertilization in seawater yields a diploid zygote that undergoes holoblastic cleavage to form a ciliated, free-swimming planula larva. The planula settles onto a suitable substratum, loses its cilia, attaches at its anterior end, and undergoes morphogenesis into a simple hydrula polyp, which subsequently buds to establish a new branched colonial hydroid.

Structure and Functions of the Internal Ear in Mammals

The mammalian internal ear (labyrinth) is embedded within the petrous temporal bone and comprises two concentric systems: an outer bony labyrinth filled with perilymph and an inner membranous labyrinth filled with endolymph. It is anatomically differentiated into the cochlea for auditory perception and the vestibular apparatus for balance.

The cochlea is a coiled, snail-like structure partitioned into three longitudinal fluid-filled canals: the upper scala vestibuli and lower scala tympani (containing perilymph), and the intermediate scala media (containing endolymph). Resting on the basilar membrane within the scala media is the organ of Corti, composed of rows of inner and outer sensory hair cells whose stereocilia project beneath the acellular tectorial membrane. Sound waves striking the tympanum vibrate the stapes against the oval window, initiating fluid waves in the perilymph. These waves displace the basilar membrane, shearing the hair cell stereocilia against the tectorial membrane. This mechanical deformation triggers mechanotransduction via the opening of mechanically-gated potassium channels, depolarizing the hair cells and firing action potentials along the cochlear nerve. Frequency discrimination is achieved via tonotopy: high frequencies resonate with the stiff, narrow base of the basilar membrane, whereas low frequencies displace the wide, flexible apex near the helicotrema.

The vestibular apparatus comprises the utricle, saccule, and three orthogonal semicircular canals. The maculae of the utricle and saccule contain sensory hair cells covered by a gelatinous otolithic membrane embedded with calcium carbonate otoconia, detecting gravity, linear acceleration, and static equilibrium. The dilated ampulla of each semicircular canal contains a crista ampullaris with hair cells embedded in a gelatinous cupula; rotational head movements deflect the cupula via endolymph inertia, detecting angular acceleration and maintaining dynamic equilibrium.

Together, endocrine titration in insect metamorphosis, life-cycle alternation in hydrozoans, and mechanoreceptive specialization in the mammalian labyrinth represent evolutionary solutions across animal phyla for growth transitions, reproductive dispersal, and precise physiological homeostasis.