Electronic Reserve Reading from Evans re: Development, Hormones, Molting.

Metamorphosis and Holometabolism Stages of Development Unusual Variations in Holometaboly Molting and Its Control Polymorphism Longevity, Aging, & Senescence OUTLINE

Embryology The initial action following fertilization is multiplication of the zygote nucleus and proliferation of protoplasm at the egg periphery without cell division, the forming of a syncytium. This is peculiar to insects and has to do with dense and voluminous yolk within the egg. Cell membranes form shortly thereafter, making the blastoderm. Insect embryos reveal some aspects of early evolution, including formation of the mouthparts from limb segments. In holometabolous species, adult features form from imaginal discs within the larval body.

Typical insect embryo at different stages.

Insect Development & Life Histories Main Points: Metamorphosis is a transition in form. With wings, most important factor in insect evolutionary progression & diversity. Growth in arthropods requires molting. The intervals between molts are “stadia”; the form at each interval is the “instar”. There are 3 basic variations of development (metamorphosis) in insects: ametaboly, hemimetaboly, and holometaboly. Holometaboly involves distinct larval, pupal (transitional), and adult stages. Advantages to holometabolous life history include: reduced larval-adult competition, better timing of activities with resources, greater efficiency in both larval and adult phases. Disadvantages include vulnerability of the pupal stage and complications in larval-adult transition.

Human development, one type of metamorphosis.

Major types of insect metamorphosis Unusual intermediate types (single orders)

Ametabolous ~.1% Hemimetabolous ~9.9% Holometabolous ~90% whiteflies thrips Relative species diversity by development type mayflies

Hemimetabolous development in a bug ( HEMIPTERA, Heteroptera ). Each stage shows progression toward the adult form, best tracked in the external development of the wings. Names of hemimetabolous immatures General: nymph Terrestrial: nymph Aquatic: nymph or naiad nonepartial “pads”complete

First instar cricket - its main function is to emerge from the subtrranean-laid egg and squirm to the surface, after which it quickly molts into a 2nd (feeding) instar.

Growth and development (molting & metamorphosis) in a chironomid midge, a holometabolous insect. from Gullen & Cranston, 2000

Growth & holometaboly in Danaus plexippus, the monarch butterfy 1st instar and egg chorion last larval instar

Chrysalis, outer cuticle is skin of last larval instar

imago, or adult

From Borror, Triplehorn, & Johnson, 1989 Some insect larval types. In terms of numbers and biomass, most insect life at any one time consists of larvae. DIPTERA HYMENOPTERA COLEOPTERA

An example: development of wing buds in a caterpillar larva (LEPIDOPTERA). from Evans 1984 Distribution of imaginal discs in a Drosophila larva (DIPTERA). Many of the adult features are preformed and packaged in the larval stage. This simplifies pupal transition. from Evans 1984

Imaginal discs in Drosophila courtesy of Dr. James Truman

Growth and morphogenesis of Drosophila leg imaginal discs L1L2L3 wander pupariation embryo size courtesy of Dr. James Truman

The formation of late-forming discs requires feeding during the last larval stage Starvation suppresses the release of insulin-like growth factors and causes elevated levels of JH. courtesy of Dr. James Truman

Major insect pupal types. The insect pupa represents a stage of tissue reorganization. A, B: obtect, limbs appressed butterflymoth parasitic wasp beetle muscoid fly C, D, E: exaerate, limbs loose, movable in some spp. F: coarctate, enclosed in last larval skin

Pupal eclosion of a muscoid fly showing inflated ptilinum. Normal adult face with ptilinum withdrawn.

in a Rhipiphorid beetle, COLEOPTERA in Poropoea, a chalcidoid parasite of beettle larvae, HYMENOPTERA Hypermetamorphosis from Evans, 1984

Hormones and Molting Main Points Molting is necessary in all arthropods in order for growth to occur; the “instar” is the particular stage, the “stadium” is the interval between molts. Molting is a complicated, delicate, and precarious act. Molting can be divided into 7 steps, as per Evans, The new cuticle is formed before the old is shed; part of the old cuticle is recycled; the new instar stretches into the new exoskeleton. Major endocrine centers are the brain, corpora allata, corpora cardiaca, & prothoracic gland. Major hormone groups that affect molting include juvenile hormone (JH), ecdysial hormones (“ecdysone”), & prothroacicotropic hormone (PTTH).

from Evans 1984 Neurosecretory aspects of the insect brain. Neurosecretory cell clusters may contain as few as a single cell each.

Basic hormones, pathways, & control of molting in holometabolous insects. = PTTH = ecdysone = JH

from Gullen & Cranston 2000 REVIEW: basic insect cuticle structure

from Evans 1984 The seven basic phases of insect molting.

from Gullen & Cranston 2000 In early instars, ecdysteroid initiates apolysis and formation of new larval cuticle to allow for growth. Following the last larval instar the homone mix & timing changes and allows for the formation of pupal cuticle, which forms beneath the (retained) last larval cuticle. The imago forms within the pupal cuticle and finally emerges from the last larval skin. Details of the process differ significantly between different groups (orders) of insects and its orchestration is one of the major elements of evolutionary change between them. Molting phases and hormonal influences in a caterpillar.

post-eclosion insects in teneral phase (= “callows”) Cricket, ORTHOPTERA True bug, HEMIPTERA Cockroach, BLATTODEA Molting (ecdysis): Usually takes place in early morning because of peak humidity. Precarious because of helplessness of molting insect. Faulty molting is a major cause of mortality.

Polymorphism Def.: Marked differences in appearance or behavior within the same species. Terms & Determinants: Polymorphism per se, genetic, e.g. butterfly mimicry clines, rings. Also the general term (refers to all 3 types). Polyphenism, environmental: a. climate, nutrition, e.g. aphids (HEMIPTERA) b. pollution, e.g. lady beetles (COLEOPTERA) c. colony-influenced (social/eusocial insects), e.g. ants, bees (HYMENOPTERA), termites (ISOPTERA) d. parasite-influenced, e.g. stylopization (HYMENOPTERA) Polyethism, behavioral, hormones, developmental stage, colony conditions & feedback especially social insects, e.g. caste polyethism in honey bees. [Wigglesworth: developmental stages “another form of polymorphism”, ref. especially hypermetamorphosis]

Polyphenism in aphids. Determined by season, food quality, crowding, & predator pressure. Mediated by hormones. In many spp., involves assexual & sexual reproductive phase, apterous and winged phases. Sunflower aphid a), b) ovoviviparous, apterous forms Summer, plentiful, rich food c) sexual alate (lays eggs) Fall, decreasing food quality, crowding

Polymorphism in social insects: ants It involves several axes of differentiation: 1) sexual [(male vs. queen (female)] h vs g, 2) reproductive (vs. non reproductive) h+g vs. a-f, 3) worker castes (grades of morphology & behavior) a vs. c vs. f vs. d.

Temporal polyethism in the honey bee, Apis mellifera (HYMENOPTERA). housekeeping, nursing signaling foraging Some discrete age-related worker tasks:

Age-related polyethism in the honey bee, Apis mellifera. Responsive (to colony & environment), structured (by age), but flexible (contingent on colony needs). from Winston 1987

Parasite-caused polyphenism in a solitary bee, Andrena sp. (HYMENOPTERA) Parasite:

Insect Longevity Determining Factors Genetic Environment Mortality Factors (season, life stage) Physical Factors (temperature, humidity) Timing (especially season) Life cycle duration, (egg to egg) may be dependent on season. Adult form may be short-lived seldom survives beyond reproduction. Immature phase almost always longer duration. One stage may diapause, extending life duration with no activity.

Age Determination Usually relative age more meaningful, i.e. “what instar” vs. “how many days”. Correlation with size is tenuous Difficult in larvae (few rigid body parts to measure) Factoids: Longest-lived Insects Cicadas: 17 years (mostly as nymphs) Some wood-boring beetles: many years Queen honey bees: ~12 years Queen termites: > 20 years

Long-lived beetle. emergence holeimported sculpture

Why “age-grade” insects? Some practical examples: 1. Pest population outbreak prediction Agriculture/phytophage, e.g. Caterpillar or weevil infestations in alfalfa require timing of management program (spray or harvest). Medical/disease vector, e.g. mosquito control relies on assessment of stage of growth, which determines state of population relative to potential for disease spread. 2. Forensic Entomology Indicator species, e.g. blowflies. Stage of development of larvae on corpse indicates approximate time of death.

from Gullen & Cranston 2000 The most reliable age grading of larvae depends on rigid body parts, e.g. head width &/or mandible dimensions.

Predator-inflicted wing “strikes”, an element of adult wear. Dispensable wing edges is a common survival strategy.

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long-lived wood-boring beetle