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SCI 355B: Lecture 7Size, Success, & Build-A-Bug

Size

• Small size of insects has both advantages &disadvantages.

• The advantages are numerous, but there isone major disadvantage = water loss

• Water loss (or prevention of..) is especiallycritical in organisms with a high surfacearea to volume ratio.

Surface Area and Volume

• Surface area (SA) = the “exposed” area ofthe body expressed in squared2 units.

• Volume (V) = the three dimensional spaceoccupied expressed in mL or cm3

• The problems insects face can be tracedback to one relationship:

• The surface area/volume ratio (SA/V)

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SA/V Ratio

• Transpiration (water loss) increases as afunction of an increased SA/V ratio.

• The smaller the animal, the less its volumeand the greater the SA to V ratio

• In other words, small animals can maintainlittle water reserves, yet the evaporativesurface is high.

EXAMPLE

• Consider two cubes: one is 2 cm and theother is 4 cm

• SA=height x width x number of sides• SA of the 2-cm cube = 2cm x 2cm = 4cm2 x 6

sides = 24cm2.• SA of the 4-cm cube = 4cm x 4cm = 16cm2 x

6 sides = 96cm2

EXAMPLE

• Consider two cubes: one is 2 cm and theother is 4 cm

• V = height x width x depth• V of 2-cm cube = 2cm x 2cm x 2cm = 8cm3

• V of 4cm cube = 4cm x 4cm x 4cm = 64cm3

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EXAMPLE

• 2-cm Cube• SA = 24cm2

• V= 8cm3

• SA/V= 24/8 = 3:1

• 4-cm Cube• SA= 96cm2

• V= 64cm3

• SA/V= 1.5:1

Transpiration increases as a function of an increased SA/V ratio. There is a much larger difference between SA and V in smaller animals compared to larger animals

Why are insects so successful?

• Small size (huh?) - can exploit a greaternumber/variety of habitats and muscle strength isproportional to cross-sectional area. Insectmuscles are very powerful since they are moving avolume that is relatively small.

• Exoskeleton + jointed appendages• Wings• Reproductive capacity (numbers + speed)• Anatomical diversity - (e.g., mouthparts)

InsectBasic Body Plan

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Head6 segments fused into a solid capsulethat houses the brain & mouthparts

PrognathousHypognathous

Opisthognathous

Cicada

Two antennae can be used for touch, smell,taste and hearing

Scape and pedicel alone have intrinsic muscles

Filliform

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Setaceous & Aristate

Dragonfly

Horsefly

plumose geniculate

capitate

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dragonfly

whirligig beetle

March fly

thrips

Mayfly

blow fly

Insects have 2 compound eyes composed ofmany individual photoreceptors (ommatidia);dragonflies have up to 30,000

Compound eyes can be present or absent

Most insects see in ultraviolet rangeEach ommatidia can “see” a portionof the field of view

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Compound Eyesin UV range: greens + reds not detected

Human visual spectrum “Bee purple” = yellow + UVNectar Guides

Ocelli - light and dark perception

Caterpillar Stink bug Cicada

Simple Eyes

Insect Mouthparts

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Back of Grasshopper head

Insects have modified mouthparts thatreflect their food sources

Chewing Beetles & caterpillarsPiercing-sucking Aphids, true bugs, mosquitoesSponging Flies (some)Siphoning Moths & ButterfliesRasping-sucking ThripsCutting-sponging Horse fliesChewing-lapping WaspsVestigial Some flies

Thorax - primary function islocomotion via legs & wings

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Legs

1-5 ‘segments’

Tarsus (singular) Tarsi (plural)

Leg Modifications usuallyoccur in fore or hind legs

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Leg Modifications

Cursorial Fossorial

RaptorialSaltatorial

Natatorial

Water Boatman (Hemipteran)Swimming/natatorial Legs

Raptorial Legs

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Special Leg Structures

Antennal comb

Wings• Lateral outgrowths of the body walls

with no muscles attached inside them

How do insects move wings = IFM

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Halteres

Flight stabilizers – swing in out, vibrate in a plane at right angle to long axis of body;changes in body attitude create torque at base of halteres

AbdomenHouses digestive organs, excretory organs &

reproductive structures

• 9-11 segments• Spiracles along the sides of each section• Cerci, sensory structures of crickets & cockroaches, etc

Ovipositor = egg laying device of females

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Ovipositor - used for defense in some insects

Central Nervous System

•Brain - proto-, deuto- & tritocerebrum (3 fused ganglia)•Circumesophageal connectives - “around” esophagus•Subesophageal ganglion - controls mouthparts

(Ocelli)(CE’s)

(antenna)(Labrum + SEG)

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Alimentary Canal

• The alimentary canal extends from the mouth to the anus• Food is acquired & processed in the foregut, midgut & hindgut to

breakdown organic material into absorbable molecules and excretenon-usable products of digestion

Salivary Glands

• in addition to digestive enzymes, salivary glands may secrete silk(caterpillars & Hymenoptera) and anticoagulants (blood feeders)

Foregut (Stomodeum)

• The foregut extends from the mouth to the end of the proventriculus &is lined with a layer of cuticle, the intima, which is shed with each molt& serves as mechanical protection

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Foregut (Stomodeum)

•Food taken into the cibarium (preoral cavity), ground, pushed into themouth by maxillae, bathed & lubricated with saliva, & moves down theesophagus via peristalsis, a series of muscular contractions, then maybe stored in a crop.

• midgut - - main site of digestion & absorption of food, where gastriccaecae increase surface area for digestion.

(cardiac)

Midgut(Mesenteron)

Proctodeum (Hindgut)

•from pyloric valve to anus, resorption of H2O, salts & amino acids•lined with cuticle = little absorption•Malpighian tubules, anterior intestine, and rectum

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•Open system consisting of:•dorsal vessel – elongate tubeextending length of the body•hemocoel – body cavity where thehemolymph flows

• Dorsal Vessel•Aorta – anterior•Heart- posterior w/ segmentedchambers•Ostia – pair of openings/segment

• No direct involvement in gas exchange