Animal Physiology and Development Insects – 1 JCS Lecture Outcome: At the end of this lecture students should have: Understood the three different types of insect life cycles and the nomenclature of different developmental stages. Appreciated the importance, structure and properties of the insect cuticle Been reminded that as arthropods insect growth requires a series of moults. Seen the details of the moulting process and learnt how it is controlled by hormones. JCS 30/10/02

Insect life cycles A B C A: Ametabolous – primitive (Apterygota) B: Hemimetabolous C: Holometabolous

Holometabolous - Diptera Egg – embryo(genesis) Larva – 3 instars Puparium Prepupa  Pupa Adult - IMAGO

Insect cuticle Waxy layer Epicuticle Exocuticle ] Procuticle Endocuticle ] Subcuticular space Epidermis Basement membrane

Keratin Chitin Chitin myofobril

Resilin – elastic polymer relaxed stretched polyphenols

Cuticle apolysis, deposition and ecdysis

Kopec’s experiments on gypsy moth caterpillars Conclusions: A hormone secreted by the brain is necessary for pupation The brain hormone acts between the seventh and tenth day of the last instar.

Brain control of moulting Brain target is the prothoracic gland Hormone - PTTH prothoracotropic hormone PTTH is made in the neurosecretory cells and transported to the corpora cardiaca and corpora allata by the axons (where it is stored before release into the blood). c. cardiaca and c. allata are neurohemal organs

Ecdysone the moulting hormone

Ecdysone titres and moulting (ecdysis) Schistocerca gregaria (locust) – hemimetabolous Manduca sexta (tobacco hornworm) – holometabolous E = ecdysis, W = wandering larva, PP = prepupa Radioimmunoassay; NB log scale

Juvenile hormone Moult can lead to: larva pupa adult (imago) JH titre, secreted from the corpora allata determines the nature of the moult. JH is neither a sterol or a peptide (typical hormone molecules). It is a terpene