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

(a) Metamorphosis and its hormonal regulation in insects

Metamorphosis is the post-embryonic change from a juvenile form to the adult through marked changes in form, structure and habit. It is incomplete (hemimetabolous: egg, nymph, adult, as in a grasshopper or cockroach) or complete (holometabolous: egg, larva, pupa, adult, as in a butterfly or housefly).

Three endocrine centres control it: the neurosecretory cells of the brain, the prothoracic glands and the corpora allata.

• Brain hormone (PTTH). Developmental and environmental cues make the median neurosecretory cells of the protocerebrum secrete prothoracicotropic hormone. It passes along axons to the corpora cardiaca (or corpora allata in some groups) and is released into the haemolymph.
• Ecdysone. PTTH stimulates the prothoracic glands to secrete ecdysone, which peripheral tissues convert to the active 20-hydroxyecdysone. Each pulse initiates a moult (apolysis and ecdysis).
• Juvenile hormone (JH). The corpora allata secrete JH, which decides the kind of moult. With high JH, ecdysone causes a larva-to-larva moult, so JH is the "status quo" hormone. When JH falls in the last larval instar, the next ecdysone pulse causes pupation. With no JH in the pupa, ecdysone drives the differentiation of imaginal discs, breakdown of larval tissues (histolysis) and emergence of the adult.

So ecdysone causes moulting, while JH determines whether the result is another larva, a pupa or an adult. Removing the corpora allata from a young larva causes early precocious metamorphosis, which shows this.

(b) Alternation of generations in Obelia

Alternation of generations (metagenesis) in cnidarians is the regular alternation of an asexual, sessile polyp generation with a sexual, free-swimming medusa generation. It is not the plant-type alternation of haploid and diploid generations, since both polyp and medusa are diploid and only the gametes are haploid.

The Obelia colony is a polymorphic hydroid fixed to the substratum by a root-like hydrorhiza from which erect, branched hydrocauli arise, covered by a transparent perisarc. It has two kinds of zooids:

• hydranths (gastrozooids), feeding polyps with a hypostome and tentacles, protected by a hydrotheca;
• blastostyles (gonozooids), reproductive zooids enclosed in a gonotheca, which together form the gonangium.

The blastostyle buds off small, bell-shaped, dioecious medusae asexually. They break free and swim, and each develops four gonads on its radial canals. Sperm and ova are shed into the sea water, where fertilisation gives a diploid zygote. It cleaves into a ciliated planula larva that swims, settles, attaches at its anterior end and develops into a hydrula-like polyp, which grows and buds into a new colony. The cycle is therefore polyp, then medusa (by budding), gametes, zygote, planula and polyp again.

(c) Structure and functions of the internal ear in mammals

The internal ear (membranous labyrinth, filled with endolymph) lies inside the bony labyrinth of the petrous temporal bone, separated from it by perilymph. It has two parts, the cochlea for hearing and the vestibular apparatus for balance.

The cochlea is a coiled tube of about 2.5 turns with three canals: the scala vestibuli (above) and scala tympani (below), both with perilymph, and the scala media (middle) with endolymph. The organ of Corti rests on the basilar membrane in the scala media and has rows of inner and outer hair cells whose stereocilia touch the overlying tectorial membrane. Sound makes the stapes push on the oval window and set up waves in the perilymph, which move the basilar membrane. The hair cells bend against the tectorial membrane, open potassium channels, depolarise and excite the cochlear nerve. High pitches act near the narrow, stiff base and low pitches near the wide apex.

The vestibular apparatus has the utricle and saccule and three semicircular canals at right angles. The maculae of the utricle and saccule have hair cells under a membrane with otoliths and sense gravity and linear acceleration (static balance). The crista in the ampulla of each canal has hair cells in a gelatinous cupula, which is bent by endolymph during rotation, and so sense angular acceleration (dynamic balance).