TODAY: Structure & Function Part I

• Muscles & Movement

• Blood & Circulation

• Tracheal System & Gas Exchange

Muscles & Movement

IMPORTANT CONCEPTS:

• All striated (no smooth)

• Major Types: 1) Synchronous 2) Asynchronous

• Motion often driven by both muscles and cuticular flexure + energy storage

Diagrammatic structure of a striated muscle fibril, the basic muscle unit.

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X-sec

A muscle unit comprised of several fibers each made up of many fibrils

from Chapman

Typical ennervation of insect muscle; slow & fast axons in parallel (vs. graded response of vertebrates).

from Chapman

from Gullen & Cranston

Muscle attachments to exoskeleton.

the tentorium

from Snodgrass 1935

an internal framework for muscle attachment in some insects

Muscles of honey bee abdomen.from Snodgrass

Caterpillar body wall musculature; functions: undulatory movement & hydrostatic skeleton.

Caterpillar gut musculature.

Types of Movement

LarvaeSinuous motion, lateral muscular waves, some primitive fly larvae e.g midgesUndulatory movement, anterior + posterior waves, typical of moth & butterfly caterpillarsWhip-like, posterior + anterior waves, used with turgor muscles some caterpillars such as inch worms

Adults Walking, leg strokes Jumping, aided by cuticular flexion Swimming, aided by hairs, special appendages Flying, aided by cuticular flexion at wing base & whole thorax

Typical “tripod gate” of an insect, maximum center-of-gravity stability with simplest mechanics and control.

Action:Thoracic muscles pull on leg bases; fine control by extension & flexion of internal leg muscles

Rhythm: slight offset between legs: 1-2-3 & 1-2-3 … it’s a waltz!

The main power source comes from the release of energy in the cuticle, which has been “cocked” by the muscles. Super-flexible resilin allows extreme bending of the joint.

Visible “chevrons” = muscle attachments to cuticle.

Jumping

distortion, cuticular energy storage

Resilin

SwimmingOften assisted by paddle-like appendages &/or hairs that fold backward on protraction, reducing drag.

In aquatic beetles, the different syncopation of swimming legs is characteristic of some families.

Predaceous diving beetle swimming adult (top), walking larva (bottom).

Thoracic musculature of honey bee.

Indirect flight muscle action within thorax

from Snodgrass 1935

Circulatory System

Main Points:

• Blood = “haemolymph”

• Generally not pressurized

• Does not distribute oxygen

• Heart (“aorta”) is dorsal

Generalized insect circulatory system. (Gullen & Cranston, 2000, Fig. 3.9)

Haemolymph, insect blood

Body compositionLarvae 20-40%Adults <20%

ConstituentsH2O (~90%)Plasma

amino acids organic acids phosphates sugars, trehalose (energy rich disaccharide characteristic of insect blood) Haemocytes, diverse cells with numerous functions

Haemolymph Functions

• Chemical exchange (e.g. ion exchange in excretory system)• Nutrient distribution• Waste removal• Hormone transport• Pressure changes

support: hydrostatic skeletonmolting turgorventilation

• Thermoregulation (heat distribution, protection against freezing)• H2O reserve• Defense

wound healingtoxinshaemocytic action

Haemocytes

Cell Type Major Function(s) Location

Plasmatocytes, Granulocytes, Prohaemocytes

Defense (e.g. phago-cytosis, encapsulation, coagulation), storage & distribution of nutrients

throughout hoemocoel

Cystocytes coagulation throughout hoemocoel

Nephrocyes haemolymph filtering, metabolize wastes for excretion

localized: near dorsal vessel

Oenocytes lipid synthsis (haemoglobin synthesis, rare)

localized: fat body, epidermis

Origin: embryonic mesoderm, singular generation (no blood-making organs in adult insects)

Defense Functions of HaemolymphCoagulationPhagocytosisAntibacterial protein reactionsImmune response signalingNoxious/toxic compound reservoir & delivery:Encapsulation:

from Gullen & Cranston 2000

from Gullen & Cranston 2000from Chapman ca. 1970

Tracheal System

Main Points:

• Oxygenation of tissues is accomplished mostly by

passive diffusion

• Double diffusion gradients: O2 (in) & CO2 (out)

• Basic structure: spiracles =>tracheal system=>

ending in tracheoles

• Insect size partially determined by physical limits to

diffusion & tracheal system

Tracheal System

tracheole-tissue interface

taenidia

Tracheoles

• Interface with O2-demanding tissue

• Beginning of CO2 diffusion gradient

• Microscopic blind-end• Liquid-filled• May penetrate tissue

• Most numerous at highly active tissue

• Can proliferate in response to long-term O2 deprivation

The end terminals of the tracheal system.

Spiracles

• Interface with environment

• Beginning of O2 diffusion gradient

• Generally one pair per segment (up to 10 seg.) but varies between species; position, shape, number may be characteristic of taxon

from Gullen & Cranston 2000

Physical Basis of Tracheal system: Diffusion

Longest known modern insect (body):Megaphasma denticris (PHASMATODEA)~ 30 cm long, native of SE Asian tropics. Long, slender body => shorter diffusion distance

• Limits: diffusion only works over thin layers of tissue; increased requirement for tracheation with increase in size.

• Implication: Insect size limited by air supply. Most “large” insects are long and slender, or have low metabolism, or display short durations of activity&/or are highly tracheated. Tiny insects have reduced tracheae because they can breathe through their outer cuticle.

Amongst tropical slow-moving rhinocerous beetles are the most massive modern insect species. Surprizingly, most can fly. Internally they are filled with a dense mass of tracheae.

Modification & Control of the Tracheal System

Basic Division into sections: spiracle => (trunk) => trachea => tracheole

Subdivision & specialization trunks & air sacs tracheole proliferation gills aeriferous tracheae

Control of flow spiracular valves water-conserving matrices, filters atrial chambers

Movement-assisted air flow (“breathing”) Thoracic/Abdominal pumping (trunks as “bellows”) Tracheal contracting

Air Sacs

• Adaptations for more effective air supply during flight, i.e. high oxygen expendature.• Expansion of lateral tracheal trunks.• Present in many flying insects.• May take up large proportion of body cavity.• “Bellows” or quasi-lung function.• Depends on adaptation of abdomen for “pumping” action.

Air sacs in the honey bee.

from Snodgrass ca. 1935

Gullen & Cranston, 2000, Fig. 3.11

Open vs. ClosedTracheal Systems

a) cockroach, lateral trunks

b) honey bee, air sacs

c) mosquito larva, siphon

d) small fly larva, cutaneous gas exchange

e) mayfly nymph, external gills

f) dragon fly nymph, “internal” gills

Gills

Closed system

Thin membrane allowing diffusion of oxygen

Configuration, extent, location may be diagnostic of taxon

Some types:

abdominal(TRICHOPTERA)

leaf-like abdominal lobes(ODONATA: ZYGOPTERA) c.f. siphon (open, NOT gills)

(HEMIPERA)

internal chamber(ODONATA:

ANISOPTERA)

CADDISFLY

GIANT WATER BUG

from Gullen & Cranston 2000

from Borror & White

internal gills

The Physical or “Gas Bubble” Gill• Underwater respiration with an open tracheal system• A bubble of atmosphere is captured…• …and serves as a gas-transfer chamber and water-stopper

Modifications of the Cuticle for Aquatic Respiration

Subelytral space in a predaceous diving beetle; it physically traps a temporary sir supply.

The “plastron”, physical gill integrated into the integument; channelized cuticle with hydrophobic hairs; it holds a bubble by physical entrapment and surface tension.

from Gullen & Cranston 2000

from Chapman ca. 1970

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