March 14, 2013

1. March 14, 2013 Radula: snails eating algae Fluid feeders: aphids Pollen feeders: bees and bats Weapons and other pointy things

2. Two-way table for comparisons

3. Excluding seeds and fruits herbivory on green tissue of plants is characteristic of three large groups of animals: gastropod molluscs, Orthoptera (grasshoppers and allies) and ruminant mammals. In the gut wall, just behind the mouth of a gastropod, recessed, there is a skeletal plate the odontophore; it acts as backing to a ribbon of keratinous teeth called the radula. Antagonistic protractor and retractor muscles contract alternately to pull the radula to and fro across the surface of the odontophore. This organ is a machine for rasping algae off substrate. The odontophore can be shifted into various positions by its own set of muscles and protruded far out through the mollusc’s mouth. So it can be applied against substratum (rocks, coral heads, aquarium glass) coated with algae.

4. Teeth of the radula of gastropods are species diagnostic: why should the teeth be this specific? Why doesn’t selection converge on the single most efficient form of rasping tooth? You can see a snail’s radula as it rasps off alga from the glass of an aquarium. These teeth not for trituration, they are for accessing food not rendering it particulate. Teeth are recurved, designed to pull one way.

5. Posterior face honeybee metathoracic leg showing basitarsal brush, hint of branching of hairs. Collection, storage and transport of food Hairs are branched; pollen transferred from them by brushes of basitarsus: a tarsal segment enlarged into a rectangular brush, shaped for this purpose. Cross-body use of rake and brush draws pollen up into press of leg of opposite side. Pollen press compresses and squeezes one-way moving pollen into pollen baskets. See detailed account in lab outline.

6. homoplasy • “Many species of bats are highly specialized for feeding on nectar and pollen. Their tongues extend to unusual lengths, often up to one third the length of their bodies. This extension is achieved in part by contracting muscles in the tongue to push blood to the tip, much as the end of a party toy unrolls when you blow into it. The tip of the tongue is covered with conical papillae that increase the surface area and the bats nectar-lapping capacity. Furthermore the scales on their hairs extend from the shafts and act as pollen traps. The scales on the hairs of bats that do not feed on flowers usually adhere closely to the shafts. (Brock Fenton, Just Bats)”. • Tooth reduction has evolved in some of these herbivorous bats. Merlin Tuttle

7. “Three scanning electron micrographs of the hairs of Geoffroy’s Tailless Bat a), Woermann’s Long-tongued Fruit Bat b), and a Mexican Bulldog Bat (c) show that the hairs of different species can be distinctly different. Note that the scales on the shafts of the hairs of GTB and WLF bats protrude more than those of the animal-eating bat. Protruding scales in flower-visiting species have been proposed as pollen traps, permitting the bats to carry pollen more efficiently. The similarity of the hairs of the two flower bats is striking, given that they are from different families (a) New World Leaf-nosed bats (b) Old world fruit bats. The hairs are about a tenth of a millimetre in diameter.” [Brock Fenton]

8. homology, homoplasy • Convergent evolution - homoplasy -- describes the acquisition of the same biological trait in unrelated lineages. The wing is a classic example of convergent evolution in action. Flying insects, birds, and bats have all evolved the capacity of flight independently. They have "converged" on this useful trait. • The ancestors of both bats and birds were terrestrial quadrupeds, and each has independently evolved powered flight via adaptations of their forelimbs. [Wikkipedia] • The branched hairs of bees and the erect scales of bat hairs are another example of functional homoplasy. The concept of homology is fundamental to the field of comparative biology. In 1843, Richard Owen defined homology as "the same organ in different animals under every variety of form and function". Homology is evaluated strictly in an evolutionary context. That is, organs in two species are homologous only if the same structure was present in their last common ancestor. Organs as disparate as a bat's wing, a seal's flipper, a cat's limb and a human arm have a common underlying anatomy which was present in their last common ancestor and so therefore are homologous as forelimbs. [Wikkipedia]

9. Gut diverticulae and storage: sacculate crop of leech: discontinuous feeder The leech has a disproportionately large crop in which it stores blood; this crop is comprised of many blindly ending lateral sacs that increase its capacity. Leeches feed on a food that is ready to digest so no teeth are required to ‘chew’ it. But the leech does have teeth to make the entry wound in its host. Obviously those teeth are not homologous with the teeth of a vertebrate Diagram from sharon-taxonomy on the web McGill Office Science & Society

10. Bird crop • A crop in a leech and a crop in a bird: homoplasy? • ‘acquisition of the same biological trait in unrelated lineages’ • Device for discontinuous feeding arises in birds and annelids by independent evolutionary events. Rocking T Ranch

11. Mosquito (Culicidae) crop; function of insect abdominal tagma • Another discontinuous feeder, only finding its blood meal on occasion, the mosquito must get all its intake in one event. It uses piercing and sucking mouthparts elongated into a ‘straw’, half-grooved appendages adjoined into channels; like membracids and aphids feeding on fluid, but on blood not sap. • This insect takes in a large, once-only, quantity of blood and has to store it for later digestion. No teeth needed – no trituration – but where to put the blood meal?

12. Diverticulae of the gut tube arise in the thorax; but the thorax is a locomotory tagma, its volume mostly occupied by flight muscle; it must remain a sturdy fixed-wall ‘box’ – to give proper anchorage for walking and flying muscles (the muscle can’t work if both ends can move). So there is no room for blood storage in the thorax. On the other hand the abdomen’s telescoped segments are designed for expansion. • The main crop extends into and occupies the mosquito abdomen. It receives the blood ingested (a valve shuts off the midgut) and the abdomen expands greatly. So one of the abdomen’s functions as a tagma is expansion to accomodate food intake. • Since the vertebrate’s blood is under pressure there is no need for a mosquito pump (but there is a need to prevent coagulation – leech and mosquito solve this ‘chemical’ problem). Some mosquitoes feed on frog blood.

13. Cibarial pump of cicadas, spittlebugs: fluid feeders that need to suck Dilator muscles contract dropping internal pressure relative to external: this pressure differential moves fluid up stylet bundle. Where a pump is needed, because the fluid food is not itself under pressure, a plant feeding insect evolves a pump: pharyngeal or cibarial. This is why cicadas, spittle bugs etc. have such a dilated face. (Puzzle) Aphids don’t need a pump to feed on sap; cicadas do need a pump to feed on sap; are there 2 kinds of sap re pressure? Do cicadas feed on xylem rather than phloem and xylem has lower pressure? Sparky’s ice cream, Columbia Mo.

14. Feeding by aphids: piercing and fluid extraction by Homoptera • Mouthparts (mandibles, maxillae, labrum and labium) are greatly lengthened, the first two drawn out into a long slender ‘stylet bundle’, the labrum and labium supporting the bundle base against the head. • Each stylet is moved by retractor and protractor muscles situated in the head. The insect has channels within the adpressed maxillae, one to convey phloem sap up and in and one to carry saliva downward to lubricate bundle penetration with glandular secretion (i.e., aphid spit). • Two grooves on the inner face of each maxillary stylet make up the two halves of a salivary canal and a food canal.

15. transverse section stylet bundle Working its way down through the plant tissue, the stylet bundle gains support from the cellulose cell walls like a ‘worm’ used to clear blocked drains; confined laterally it cannot bend very far before being pushed forward; the insect takes many minutes (up to an hour) to reach the phloem and begin feeding. Rosy Apple Aphid with stylets inserted in plant tissue ghppr head mpts colour coded frons Mandibular stylets (red) do the work of piercing the plant tissue, a right is thrust out a short distance (protracted) and penetrates down; then the left pushed until its tip meets that of the first; then the two maxillary stylets are lowered together into the created space: this is repeated between the walls of adjoining plant cells until the pholoem bundle is reached phloem pressure pushes the sap up the maxillary food canal; no need to pump/suck the fluid: note small head and thorax

16. Aphids can be used to collect phloem sap. Top photograph: a feeding aphid with its stylet embedded in a sieve tube. scl, sclerenchyma; st, stylet; x, xylem; p, phloem. Note the drop of ‘honeydew’ being excreted from the aphid’s body. (a) to (e) show stylet cutting with a microcautery unit at about 3-5 s intervals (a to d) followed by a two-minute interval (d to e) which allowed exudate to accumulate. The stylet has just been cut in (b); droplets of hemolymph (aphid origin) are visible in (b) and (c); once the aphid moves to one side the first exudate appears (d), and within minutes a droplet (e) is available for microanalysis. Plants in action

17. Winged aphids can appear in populations and are the basis of dispersal

18. The aphid has the problem of securing a quiet backwater in its gut for secretion of enzymes and absorption of the products of digestion. The pressure of the fluid on which it feeds is a problem as it pushes the fluid through the gut at a high rate. And the aphid’s food (sap) is very dilute so it must process copious amounts. Its gut loops back on itself and then is turned in ‘switchbacks’ overtop of the anterior midgut. This ‘filter chamber’ also functions a shunt: it allows for certain portions of the fluid to bypass the posterior midgut; through differential absorption t the two overlain gut walls filter the wanted out of the unwanted.

19. Teasel Dipsacus carnivory Generic name of this plant is derived from Greek word for ‘thirst’, referring to perfoliate leaves that collect rainwater. True carnivory is “typically found in perennial plants of acid, nutrient-poor boggy soils” (e.g., bladderwort). Plants in these nutrient deprived conditions can benefit from additional Nitrogen and Phosphorus. But some plant carnivory is more subtle. Walter Muma

20. Dipsacus • The water accumulated in the leaf bases of Dipsacus traps small invertebrates (mainly insects) whose decomposing bodies have now been shown to provide the plant with nutrients. • Dipteran larvae were added to leaf-base pools and produced “a 30% increase in seed set”. First demonstration that this plant can benefit in this way. • Leaves function to prevent aphids (sap feeders) from climbing higher to rapidly growing meristems Walter Muma water accumulates here