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SCI 355B: Lecture 7 Size, Success, & Build-A-Bug Size • Small size of insects has both advantages & disadvantages. • The advantages are numerous, but there is one major disadvantage = water loss • Water loss (or prevention of..) is especially critical in organisms with a high surface area to volume ratio. Surface Area and Volume • Surface area (SA) = the “exposed” area of the body expressed in squared2 units. • Volume (V) = the three dimensional space occupied expressed in mL or cm3 • The problems insects face can be traced back to one relationship: • The surface area/volume ratio (SA/V) 1 SA/V Ratio • Transpiration (water loss) increases as a function of an increased SA/V ratio. • The smaller the animal, the less its volume and the greater the SA to V ratio • In other words, small animals can maintain little water reserves, yet the evaporative surface is high. EXAMPLE • Consider two cubes: one is 2 cm and the other 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 the other 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 2 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 greater number/variety of habitats and muscle strength is proportional to cross-sectional area. Insect muscles are very powerful since they are moving a volume that is relatively small. • Exoskeleton + jointed appendages • Wings • Reproductive capacity (numbers + speed) • Anatomical diversity - (e.g., mouthparts) Insect Basic Body Plan 3 Head 6 segments fused into a solid capsule that houses the brain & mouthparts Prognathous Hypognathous Cicada Opisthognathous Two antennae can be used for touch, smell, taste and hearing Scape and pedicel alone have intrinsic muscles Filliform 4 Setaceous & Aristate Dragonfly Horsefly plumose geniculate capitate 5 thrips dragonfly whirligig beetle Mayfly blow fly March fly Insects have 2 compound eyes composed of many individual photoreceptors (ommatidia); dragonflies have up to 30,000 Compound eyes can be present or absent Most insects see in ultraviolet range Each ommatidia can “see” a portion of the field of view 6 Compound Eyes in UV range: greens + reds not detected Human visual spectrum “Bee purple” = yellow + UV Nectar Guides Ocelli - light and dark perception Simple Eyes Caterpillar Stink bug Cicada Insect Mouthparts 7 Back of Grasshopper head Insects have modified mouthparts that reflect their food sources Chewing Piercing-sucking Sponging Siphoning Rasping-sucking Cutting-sponging Chewing-lapping Vestigial Beetles & caterpillars Aphids, true bugs, mosquitoes Flies (some) Moths & Butterflies Thrips Horse flies Wasps Some flies Thorax - primary function is locomotion via legs & wings 8 Legs 1-5 ‘segments’ Tarsus (singular) Tarsi (plural) Leg Modifications usually occur in fore or hind legs 9 Leg Modifications Cursorial Raptorial Fossorial Saltatorial Natatorial Water Boatman (Hemipteran) Swimming/natatorial Legs Raptorial Legs 10 Special Leg Structures Antennal comb Wings • Lateral outgrowths of the body walls with no muscles attached inside them How do insects move wings = IFM 11 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 Abdomen Houses 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 12 Ovipositor - used for defense in some insects Central Nervous System (Ocelli) (CE’s) (antenna) (Labrum + SEG) •Brain - proto-, deuto- & tritocerebrum (3 fused ganglia) •Circumesophageal connectives - “around” esophagus •Subesophageal ganglion - controls mouthparts 13 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 excrete non-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 14 Foregut (Stomodeum) •Food taken into the cibarium (preoral cavity), ground, pushed into the mouth by maxillae, bathed & lubricated with saliva, & moves down the esophagus via peristalsis, a series of muscular contractions, then may be stored in a crop. (cardiac) Midgut (Mesenteron) • midgut - - main site of digestion & absorption of food, where gastric caecae increase surface area for digestion. Proctodeum (Hindgut) •from pyloric valve to anus, resorption of H2 O, salts & amino acids •lined with cuticle = little absorption •Malpighian tubules, anterior intestine, and rectum 15 •Open system consisting of: •dorsal vessel – elongate tube extending length of the body •hemocoel – body cavity where the hemolymph flows • Dorsal Vessel •Aorta – anterior •Heart- posterior w/ segmented chambers •Ostia – pair of openings/segment • No direct involvement in gas exchange 16