April 2, 2013

1. April 2, 2013 Diagnostic genitalia Sperm competition Odonata Calopteryx Nerve and sensory systems: signal generators, signal receivers Giant neurons Cricket sound generation Cricket sound localization

2. Diagnostic animal genitalia • Animal genitalia are typically quite complex and especially useful in diagnosis of species (Eberhard W.G. 1985). • Why should this be so? • This is the approach of a book worth reading: Sexual Selection and Animal Genitalia, Harvard Univ. Press, by William Eberhard. • “Since any structure that is a consistently useful taxonomic character at the species level must have evolved both rapidly and divergently (that is, it acquires a new form in each new species), the question can be rephrased as: why do animal genitalia so often evolve both rapidly and divergently?

3. Eberhard W.G. 1985. Sexual selection and Animal Genitalia. Harvard Univ. Press, Cambridge Mass. Rapid and divergent genitalic evolution is common in animals that fertilize internally using an intromittentorgan. [And internal fertilization is common terrestrially (?) rather than among aquatic organisms.]. And there is a “wider trend toward rapid and divergent evolution of any male structure, genitalic or not, that is specialized to contact females in sexual contexts” (p.181). Why? Several hypotheses brought forward to explain evolution of genitalic diversity are rejected by Eberhard in his book. 1. Hypothesis: ‘lock and key’: elaborate diagnostic genitalia evolve to keep females from being fertilized by males of other species. [mutual structural adjustments don’t occur] 2. Hypothesis: pleiotropism: variations in genitalia are selectively neutral and are the result of selection for other genes. That is, they are effects not functions. [ Ho fails to explain why genitalia are affected like this and not other organs.] 3. Hypothesis: sexual selection by male conflict – intrasexual sexual selection. The hypothesis Eberhardfavours: Hypothesis: sexual selection by female choice: “male genitalia function as ‘internal courtship’ devices to increase the likelihood that females will actually use a given male’s sperm to fertilize her eggs rather than those of another male.” and “…the diversity of male genitalic form is the result of runaway evolutionary processes generated by sexual selection by female choice on otherwise arbitrary aspects of male genitalia.” Males with certain abritrary morphologies (forms) of penis do better at leaving offspring than males lacking these features. So the penis evolves to become more and more elaborate just as with the peacock’s tail. (He applies the same argument to explain the evolution of song-specific diversity among acoustic insects.)

4. Diversity diagnosis from natural selectionSensory capacities of animals are different • So if the diversity of membracid pronota (helmets) is not the product of intersexual selection – what is the functional explanation? [what was the adaptive consequence that produced them?] • Natural selection for crypsis is the accepted answer for most. And if this seems arguably implausible, given the strikingly odd structural diversity of the helmets, keep in mind that you are not viewing these species against their natural environment, or in poor lighting (photographers take care to illuminate their subjects) and that in any case your eye is not the same as the eye of the predator against which such crypsis will have evolved. • The sensory capacities of other animals are vastly different. Visible spectra differ between species. Bats, mice and katydids hear different sound frequencies.

5. Shadows and different lighting or a different plant substrate etc. could increase obscurity or of course the markings may be aposematic Enchenopa binotata where they live

6. Lichens: fractalsself-similar at any magnification strange substrates strange crypsis?

7. Calopteryx maculata and sperm competition: ‘competition among the sperm of different males to fertilize the same female’s eggs’ “The male first grasps the front of the female’s thorax with specialized claspers at the tip of his abdomen. A receptive female swings her abdomen under the male’s body and places her genitalia over the male’s penis equivalent, which occupies a place on the underside of his abdomen near the thorax... The male then rhythmically pumps his abdomen up and down, during which time his [secondary] penis acts as a scrub brush... catching and drawing out any sperm already stored in the female’s sperm storage organ, called a spermatheca (Alcock 1997 Animal Behavior 6th edit.).

8. The male’s terminalia are occupied in grasping the female by her thorax. Thus the place where his primary genitalia are located and where his sperm are produced is ‘otherwise engaged’. But prior to taking ahold of the female (on the wing?) he has transferred sperm to his secondary genitalia. The female must co-operate (choose) to mate and if she places her (terminal) genitalia up against the secondary aedeagus. At this point the male scrubs out her spermatheca and then he introduces his own sperm there. Royal BC Museum Wagge: copulating male removes between 90 and 100 % of his rival’s sperm, then releases his own gametes. She will use these stored sperm to fertilize her eggs as she oviposits – unless she mates with another male in which case “his stored ejaculate will experience the same sad fate as those of his previous rivals” (Alcock)

9. If insects didn’t store sperm this sort of sperm competition wouldn’t be possible.

10. The lateral horns and spines on the aedeagus of the male let him ‘scrub out’ the spermatheca (female sperm storage organ) before he intromits his own sperm. The lower picture shows rival sperm caught in the horn’s spiny hairs. • Another example of the function of ‘pointy things’.

11. Nerve and sensory systems: signal generators, signal receivers • Animals have complex networks of neurons, elongate cells [soma, axon, dendritic arborization] specialized for coordination of activities of body parts. This system detects stimuli in the environment (sounds, light waves, odours) [gathers information]; it transduces (alters energy state) and amplifies: e.g., it transduces the mechanical energy of sound input into the chemical energy of ion fluxes, an action potential; it can make that signal stronger by involving more action potentials; these depolarizations are decoded and transmitted to central decision-making regions such as a brain. • There are giant neurons selected for speed of transmission in many invertebrates their axons :

12. Giant neurons were studied in the earliest days of neurophysiology. Giant neurons are widespread in animals, e.g., Annelids, Arthropods, Molluscs. They are adaptive in coordinating the many metameres for escape: feather duster worms withdraw into the safety of their tubes when attacked by firing giant neurons. The brain is not ‘consulted’. • These cells were used by early neurophysiologists trying to understand depolarization of the nerve cell membrane. In 1936 J.Z. Young (recall earlier mention of his text) discovered that certain long structures present in squids (previously thought to be blood vessels) were actually nerves. They were axons of unusually large diameter: up to 1 mm. • A typical axon is about 40 microns in diameter: these giants are about 700 microns. They run through the length of the chain of ganglia, i.e., they are not metameric. • .

13. Stellate ganglion in squid mantle. The stellate nerve contains the giant axon. This large-diametered motor nerve depolarizes very rapidly, ensuring nearly synchronous activation of mantle muscles in a jet-propelled escape. Time of arrival differences of the motor commands are made synchronized over the mantle because of size and hence speed differences. Pictures taken from a website illustrating the dissecting out of a nervous preparation of the squid giant synapse (USCRC IBRO)

14. Interneurons and shape specificity (From J. Insect Physiology, George Boyan, 1984)Interneurons that receive auditory inputnote topographical similarity of omegas of cricket Gryllus and katydid Tettigonia.Neurons can be identified across taxa based upon their morphology, e.g., making binaural comparisons or as in the case of AN1 conveying activity from the prothoracic ganglion to the brain. Analogous to wiring, the neuron says something about its function by where it originates and where it winds up: a motor message going from the CNS to a certain muscle (efferent), a sensory message returning from an eardrum to the CNS (afferent)

15. Interneurons can show something of their function in their shape Omega interneuron: the name suggested by the shape, located within the prothoracic ganglion of a cricket or katydid. It receives input from the ears out on the forelegs. • Parts of a neuron: cell body (soma), dendriticarborization, axon. • Omega neuron is here filled with green dye via the electrode that once monitored its firing activity; omega neurons occur in the prothoracic ganglion of a cricket as an overlain mirror-image pair and the firing of each feeds back upon the activity of the other. • Thus a difference in perception to one side is enhanced, supporting better localization. Gerry Pollack, Montreal

16. The mirror-image structure of the omega neurons reflects a function in contralateral inhibition: this enhances binaural contrast: input from the left inhibits input from the right and makes right-left localization more dramatic.

17. Interneurons and brain neurons that discriminate song pattern

18. Gryllidae: sound production and reception • Crickets are insects in the order Orthoptera, close relatives of katydids. • Males broadcast an acoustic signal to attract females for mating. • High diversity in species and morphology . • Calling songs are made by males rubbing their forewings together: usually right over left with bilateral wing symmetry. • They listen with eardrums on their knees. Anurogryllus Florida

19. Katydid sound generation file (see photo right) on the left forewing underside, the file teeth pointing down at the insect’s back, are rubbed to and fro by a scraper, the edge of the right tegmen (wing). scraper result is often a nonresonant spectrum

20. Resonant vs non-resonant stridulation B is the ‘time domain’ form of a cricket call, a sine wave, whose ‘frequency domain’ spectrum of 9 kHz shows in C as a single peak. The pulses in A. are each complex waveforms that show as the broad-band spectrum of C.

21. Conocephalus: non-resonant stridulation • Time domain • Frequency domain • Ultrasonic • Fourier analysis • High and low-Q spectra

22. Cricket stridulation produces a single-peak spectrum through the sound-radiating membranes: harp and mirror. Anurogryllus

23. Sound communication in field crickets • Michelsen A. 1998. The tuned cricket. News Physiol. Sci. 32-38 • Michelsen A., Lohe Gudrun 1995. Tuned directionality in cricket ears. Nature 375: 639. Male field crickets call to attract mates, rubbing forewings together. Their calling song spectrum is dominated by a single pure-tone frequency at about 4.5 kHz – a single wave length of about 7 cm. Most field cricket species use this particular wavelength. The adaptive basis for calling to females at this particular wavelength are determined by excess attenuation and body size. Teleogrylluscommodus very similar to Gryllus bimaculatus used by Michelsen.

24. What is adaptive about a carrier frequency of 4500 Hz? [4.5 kHz] • When sound travels in air from a source (a singing male cricket) it naturally drops in intensity through the spreading of its energy over an increasing (roughly spherical) area [this is called the ‘inverse square law’*]. But it also loses intensity through excess attenuation: excess attenuation due to things like other sounds or ground-reflected interference, or absorption by vegetation. For the ‘communication channel near the ground where the field crickets call for their mates’ excess attenuation gets worse and worse for frequencies above 6 kHz. So a cricket can best call in the ground channel below 6 kHz. • At the same time crickets are necessarily small animals. The diaphragm they are using to make the sound waves is quite tiny relative to the wavelength. The smaller a diaphragm is relative to the wavelength (frequency) it generates, the less efficient its generation. So a cricket makes 3 kHz (11.4 cm) more efficiently than 2 kHz (17.2 cm); it makes 4000 Hz (8.6 cm) more efficiently than 3000 Hz. “These factors leave only a narrow frequency range that can be exploited for field cricket calling songs: above 3 kHz and below 6 kHz (5.7 cm): the middle of this window is 4.5 kHz (7.6 cm) and selection for optimal range and optimal minimizing of attenuation has put it there. • {*Inverse Square Law: root mean-square sound pressure level varies inversely as the square of the distance from the source; under ideal conditions (no reflecting surfaces, other background sound or interference), a sound level drops 6 dB for every doubling of the distance from the source.}