1. Lecture 19 Circulation: moving fluid internally: hearts and other pumps Course theme revisited

2. Course Theme reminder Don’t take morphology for granted. Puzzle about it. Compare and question the morphology of an animals, plants and their parts and materials. Structures have evolved under certain regimes of selection. Squirrels have tails. Watch them in use. Tail behaviour in squirrels. Organs of balance on a street wire; signalling with flicks to potential mates, keeping warm. The tail is not just the ‘bit left over’. Ask why structures have evolved with certain features of shape, size, stiffness, segmentation, elasticity, branching, colour, etc. And why have these features come to exist in certain taxa while in other taxa they are lacking? Wikki

3. Orb-weaver spider and the food filter/trap it builds. Silk fibres are very very tiny: only 1 micrometre in diameter: is this an adaptation to reduce visibility?

4. Ted McRae Argiope: genus of garden spiders, is an orb-weaver. Many orb-weavers add a ‘stabilimentum’ to their web: thick silk radiated near the web hub and before the viscid spiral zone, quite variable in form. In quotes because this is a functional term which favours a particular hypothesis of stabilimentum function; best to avoid this and leave it an open question. Call it a ‘silk mass’? Don’t prejudge.

5. Eisner T. 1983. Spider web protection through visual advertisement: role of the stabilimentum. Science 219: 185-187. Hypotheses to explain the selective advantage of stabilimenta: 1.Reinforcement 2.Tension adjustment 3.Camouflage 4.Visual marker Notice leg allignment of this spider re stabilimentum: consistent with camouflage Ho.

6. Muhammed Mahdi Karim This picture seems to argue for stabilimentum as a cryptic adaptation: camouflage. Of course different spider species might have evolved a stabilimentum for different selective reasons: more than one hypothesis might be right depending on the species.

7. Observe behaviour: the structure in use, i.e.the structure behaving. Measure the spider’s behaviour in relation to the stabilimentum. Measure/observe the nature of the stabilimentum itself. Spiders position themselves with respect to the stabilimentum. Spiders do stabilimentum push-ups when disturbed. Functioning as an insect trap, web silk is apparently selected to reflect light poorly in the UV, where prey insects see well; but this is not so for stabilimentum silk which returns UV well. So perhaps stabilimenta have never been selected to HIDE from prey? Spiders do push-ups at the hub, responding to the advance of a relatively larger non-prey item [bird, cow, human]; pushups are consistent with the visual marker hypothesis of function of the stabilimentum; the movements would make the stabilimentum more visually effective since vertebrate eyes respond well to movement in the visual field.

8. Eisner T. 1983. Spider web protection through visual advertisement: role of the stabilimentum. Science 219: 185-187. Tom Eisner showed that the presence of visual markers significantly affected the time a web could last without mechanical damage from flying birds. His experiments support the hypothesis that a stabilimentum is a visual marker enabling large animals and birds to avoid blundering through the spider’s web. Some spider species keep their webs up night & day, and some species spin anew each evening, taking their web down again at dawn; this latter group of species don’t make stabilimenta and their webs were used in Eisner’s experiment: the spider residents were captured and removed from their webs.

9. An elegant experiment 30 natural webs without stabilimenta (control). 30 other natural webs adorned with artificial equivalents of stabilimenta: triangular strips of white paper forming an X. Webs with artificial stabilimenta survived “intact through the early morning period when birds are on the wing”.

10. What is the function of moth wing scales? Imagine them with glassy wings like bees or beetles. How would life success be affected? Eisner’s field experiment tested the idea that moth scales are an adaptation to make moths ‘nonadherent’ to spider webs. They are covered with “detachable powder”.

11. Orb-weavers create a ‘viscid spiral’ of silk with beaded sticky droplets (bottom right) as glue for trapped prey. Moth wing scales are shown (bottom left) and adhering to a viscid spiral strand in the middle right picture. Upper right empty scale sockets. Illustration from a book by Thomas Eisner. For Love of Insects. Belknap Press, Harvard Univ. Press. Moth scales have evolved from articulating hairs (mechanoreceptors) and the open sockets (upper right photo) show where once scales lodged.

12. Pumps Circulation and respiration make one think of lungs and hearts. Lungs and hearts are pumps: pumps creating force as pressure to displace body fluids: in animals --Fluid flow comes about by squeezing, dilation, bailing, peristalsis. Water, blood, air even gut contents, may be regarded as pumped fluids. Pumps circulating fluids already mentioned in lectures: Chaetopterus, echinoderm tube feet, sap-feeding insects, filter feeding salps etc. Echinoderm tube feet and ampullae involved pumping fluid from one part of the organ to another. The basis of a hydrostatic skeleton is squeezing incompressible fluid from one spot to another. The basis of heart beating circulation is squeezing incompressible fluid from one spot to another.

13. “...we can’t draw a sharp line between thrust-producing devices of locomotion [think of insect flight, or fish fins] and pumps that push fluid” The same organ, the mesothoracic wing or pectoral fin could suffice for both pumping and locomotion: bees use their wings not just for flight but to circulate air in their hives, ventilating and cooling (an example of a fluid-dynamic pump with the wings acting as impellers). Another example: stickleback males (parental care by males) use their pectoral fins as impellers to circulate freshwater over the clutch of eggs in their nest and aerate the m..

14. Pumping air: recall the system employed by birds in flight, the way the movement of the skeleton draws in and expels air through the agency of air sacs and the inextensible lungs: a costal suction pump Costal suction pump Intercostal muscles run between ribs and contract to move ribs forward and down during inspiration: sternum moves forward and down. Forward and down the volume of thoracic cavity greatly increased with accompanying reduced internal pressure causing air inrush.

15. Gill ventilation in fishes involves pumping both water and blood Oxford Illustrated Science Encyclopedia Buccal force pump combined with an opercular suction pump Unidirectional flow of water in fishes is generated by a buccal force pump combined with an opercular suction pump. These two pumping actions operate out of phase, to achieve a near continuous flow of water. (As the phase differences in the cycles of the three muscular fans of Chaetopterus achieve more even flow.) The water enters the fish’s mouth, passes through the buccal cavity, then is forced out through gill slits in the gut wall into a branchial cavity containing the gills. The branchial cavity is covered with an operculum and the water exits posteriorly between the edges of the operculum and the body. Irrigating gill filaments

18. The heart in a tetrapod embryo begins as a muscular tube formed from mesoderm; the circulation of blood begins very early in the embryo’s development and the heart pumps blood. Vertebrate heart

19. Two muscular chambers are arranged in series, ventricle and atrium: why not just one chamber? Why is the atrium less powerful than the ventricle? The same serial arrangement occurs in a fish but atrium is bent to overlie the ventricle Problem of filling a too muscular chamber: need to bring up low pressure of venous blood in 2 stages.

20. Function of the pericardial cavity The vertebrate heart occupies its own fluid- filled space, the pericardial cavity, a space of the coelom. Makes sense to have an organ like a heart, one that changes its shape to function, situated in its own inviolate space. Don’t want other viscera ‘crowding in’ as the heart contracts and so having to be pushed back out of the way when the heart fills again with blood. There is an adaptive linkage between the atrium and the ventricle: as each of these chambers contracts, the volume of the pericardial cavity is slightly increased and its fluid pressure decreased, i.e., there is reduced external pressure on the heart. This reduced pressure makes it easier to fill whichever is the expanding heart chamber. The contraction of one chamber (e.g, atrium) is coupled to the distension of the other (e.g., ventricle).

21. Closed high-pressure systems contrast with open low-pressure circulatory system of many arthropods. Insect circulatory system

22. Because in insects the tracheal system does the job of moving respiratory gases to and from the tissues, the circulatory system proper can be low pressure, low flow. There is a heart, a long muscular tube, situated within a pericardial sinus, a space defined by a dorsal diaphragm. The dorsal longitudinal vessel (heart posteriorly, aorta anteriorly) draws in haemolymph from this sinus through ostia (with valves) and pumps blood peristaltically anteriorly; blood leaves the vessel in the head to circulate rearward through sinuses. Aliform [wing-shaped] muscles are the antagonists of the circular heart muscles.

23. Metrioptera sphagnorum, bog katydid Insects ventilate their tracheal system using primarily the abdomen (tagma). Intersegmental muscles make the segments of the abdomen of a stridulating katydid telescope in and out; the abdomen pumps like a bellows, supplying oxygen as the insect is stridulating. Air flow is made unidirectional through timing the opening and closing of metameric spiracles. Front to back flow in the locust: anterior spiracles are opened, all others closed and inspiration by expanding abdomen; then spiracle 10 opens while all others close, abdomen volume reduced giving expiration. Dita Klimas

24. Tracheae are branching tubes conducting air inward from spiracular openings. They need to be prevented from collapsing against the force of the surrounding haemolymph and other tissue. They are strengthened by spiral cuticular taenidia.

25. Internally tracheae end blindly in fine intracellular tubes known as tracheoles. “These are partly filled with fluid and the extent to which the air penetrates down them varies with the state of activity of the insects. If it is very active, or if oxygen is witheld, the air extends further down the tubes and comes nearer to the tissues, thus shortening the final path of slow diffusion in water (Ramsay J.A. Physiological Approach to the Lower Animals) “The great advantage of the tracheal system is that a high oxygen tension can be maintained in the tissues without energy being wasted in maintaining a rapid flow of blood... But on the other hand, since the rate of diffusion is inversely proportional to the distance, the tracheal system is not readily adaptable to the needs of larger animals.”