Download Insect Development I: General Aspects and the Mol ng Cycle

Insect Development I: General Aspects and the Mol6ng Cycle General ques+ons regarding insect development •  How is insect development controlled physiologically, especially during the major transi+onal stages? •  How did the pupal stage in holometabolous insects evolve? •  What is the evolu+onary significance of polyphenisms? Are they evolu+onarily stable? Ametabolous Development Hemimetabolous Development Paurometabolous Development Holometabolous Development Growth and Insect Development •  Because the exoskeleton is inextensible aGer tanning, insect growth must proceed by a series of stages (instars) separated by a series of molts. •  The number of larval instars (and molts) varies among species. •  Number of larval molts is usually high in basal groups (20 or more in some Ephemenoptera) and low in more derived groups (3 or 4 in many holometabolous groups). •  Number larval may be constant within an en+re order, or highly variable even within species. Changes during Growth •  Mass increases geometrically between egg and adult stage (by 10,000 +mes or more in some insects. •  Growth of soG body parts, as reflected in changes in mass, may appear to be con+nuous. •  Growth of scleri+zed body parts is discon+nuous with linear dimensions frequently increasing by 1.4x between instars. This constant ra+o is called Dyar’s Law. Differences in Wing Development •  Wings in exopterygotes develop in external sheaths. •  Wings in endopterygotes develop in internal pockets (imaginal discs) in the integument and are everted aGer the larval-­‐pupal apolysis. The Mol+ng Cycle •  Mol+ng cycle is ini+ated by the release of the hormone ecdysone from the prothoracic glands. •  Epidermis secretes new epicu+cle and mol+ng fluid, which dissolves old endocu+cle. •  Epidermis thickens. •  Diges+ve products of the old endocu+cule and mol+ng fluid are resorbed and replaced by air. •  DNA synthesis commences in the epidermis, leading to mitosis. •  Epidermis retracts from the old cu+cle (apolysis). •  Old cu+cle is sloughed off (ecdysis). •  Epidermis con+nues to secrete new endocu+cle aGer ecdysis. When does the mol+ng cycle begin? •  Old tradi+onal view has the mol+ng cycle start and end with ecdysis, the sloughing off of the old exoskeleton (the exuviae). •  Modern view (advocated by Snodgrass and Hinton) has the mol+ng cycle start and end with apolysis, the separa+on and retrac+on of the epidermis from the old exoskeleton. This view recognizes that the new instar typically begins before ecdysis. An insect in the new instar but s+ll enclosed in the old exocu+cle is called a pharate. The old exocu+cle serves two func+ons for the new instar: 1) it protects the new, s+ll tender instar, and 2) it acts as mold for the exoskeleton of the new instar Changes that Occur during Metamorphosis •  Exopterygote insects carry over a large number of larval organs into the adult stage, where as endopterygote insects recons+tute larval organs to varying degrees. •  Epidermis growth in basal insects groups is by cell division, whereas it is by cell enlargement or a combina+on of cell enlargement and cell division is more derived groups. In exopterygotes embryonic replacement cells for many adult +ssues are stored in larvae as imaginal discs. •  In exopterygotes the great majority of larval muscles systems are carried over into the adult stage, whereas in endopterygotes nearly are adult muscles are formed from myoblasts or imaginal discs.