Box 44 Biological clocks

Seasonal changes in environmental conditions allow insects to adjust their life histories to optimize the use of suitable conditions and minimize the impact of unsuitable ones (e.g. through diapause; section 6.5). Similar physical fluctuations on a daily scale encourage a diurnal (daily) cycle of activity and quiescence. Nocturnal insects are active at night, diurnal ones by day, and crepuscular insect activity occurs at dusk and dawn when light intensities are transitional. The external physical environment, such as light/dark or temperature, controls some daily activity patterns, called exogenous rhythms. However, many other periodic activities are internally driven endogenous rhythms that have a clock-like or calendar-like frequency irrespective of external conditions. Endogenous periodicity is frequently about 24 hours (circadian), but lunar and tidal periodicities govern the emergence of adult aquatic midges from large lakes and the marine intertidal zones, respectively. This unlearned, once-in-a-lifetime rhythm which allows synchronization of eclosion demonstrates the innate ability of insects to measure passing time.

Experimentation is required to discriminate between exogenous and endogenous rhythms. This involves observing what happens to rhythmic behavior when external environmental cues are altered, removed, or made invariate. Such experiments show that inception (setting) of endogenous rhythms is found to be day length, with the clock then free-running, without daily reinforcement by the light/dark cycle, often for a considerable period. Thus, if nocturnal cockroaches that become active at dusk are kept at constant temperature in constant light or dark, they will maintain the dusk commencement of their activities at a circadian rhythm of 23-25 hours. Rhythmic activities of other insects may require an occasional clock-setting (such as darkness) to prevent the circadian rhythm drifting, either through adaptation to an exogenous rhythm or into arrhythmy.

Biological clocks allow solar orientation - the use of the sun's elevation above the horizon as a compass - provided that there is a means of assessing (and compensating for) the passage of time. Some ants and honey bees use a "light-compass", finding direction from the sun's elevation and using the biological clock to compensate for the sun's movement across the sky. Evidence came from an elegant experiment with honey bees trained to forage in the late afternoon at a feeding table (F) placed 180 m northwest of their hive (H), as depicted in the left figure (after Lindauer 1960). Overnight the hive

Locality 1, Day 1, Afternoon Loca|itv 2 Day 2 Morning

was moved to a new location to prevent use of familiar landmarks in foraging, and a selection of four feeding tables (F1-4) was provided at 180 m, northwest, southwest, southeast, and northeast from the hive. In the morning, despite the sun being at a very different angle to that during the afternoon training, 15 of the 19 bees were able to locate the northwest table (as depicted in the figure on the right). The honey bee "dance language" that communicates direction and distance of food to other workers (Box 12.1) depends upon the capacity to calculate direction from the sun.

The circadian pacemaker (oscillator) that controls the rhythm is located in the brain; it is not an external photoperiod receptor. Experimental evidence shows that in cockroaches, beetles, and crickets a pacemaker lies in the optic lobes, whereas in some silkworms it lies in the cerebral lobes of the brain. In the well-studied Drosophila, a major oscillator site appears to be located between the lateral protocerebellum and the medulla of the optic lobe. However, visualization of the sites of period gene activity is not localized, and there is increasing evidence of multiple pacemaker centers located throughout the tissues. Whether they communicate with each other or run independently is not yet clear.

Sympetrum, some 675 receptor cells converge onto one large neuron, two medium-sized neurons, and a few small ones in the ocellar nerve.

The ocelli thus integrate light over a large visual field, both optically and neurally. They are very sensitive to low light intensities and to subtle changes in light, but they are not designed for high-resolution vision. They appear to function as "horizon detectors" for control of roll and pitch movements in flight and to register cyclical changes in light intensity that correlate with diurnal behavioral rhythms.

4.4.4 Compound eyes

The most sophisticated insect visual organ is the compound eye. Virtually all adult insects and nymphs have a pair of large, prominent compound eyes, which often cover nearly 360° of visual space.

The compound eye is based on repetition of many individual units called ommatidia (Fig. 4.10). Each ommatidium resembles a simple stemma: it has a cuticular lens overlying a crystalline cone, which directs and focuses light onto eight (or maybe 6-10) elongate retinula cells (see transverse section in Fig. 4.10). The retinula cells are clustered around the longitudinal axis of each ommatidium and each contributes a rhabdomere to the rhabdom at the center

Fig. 4.9 (left) Longitudinal sections through simple eyes:

(a) a simple stemma of a lepidopteran larva; (b) a light-adapted median ocellus of a locust. ((a) After Snodgrass 1935;

Fig. 4.10 Details of the compound eye: (a) a cutaway view showing the arrangement of the ommatidia and the facets; (b) a single ommatidium with an enlargement of a transverse section. (After CSIRO 1970; Rossel 1989.)

of the ommatidium. Each cluster of retinula cells is surrounded by a ring of light-absorbing pigment cells, which optically isolates an ommatidium from its neighbors.

The corneal lens and crystalline cone of each omma-tidium focus light onto the distal tip of the rhabdom from a region about 2-5° across. The field of view of each ommatidium differs from that of its neighbors and together the array of all ommatidia provides the insect with a panoramic image of the world. Thus, the actual image formed by the compound eye is of a series of apposed points of light of different intensities, hence the name apposition eye.

The light sensitivity of apposition eyes is limited severely by the small diameter of facet lenses. Crepuscular and nocturnal insects, such as moths and some beetles, overcome this limitation with a modified optical design of compound eyes, called optical superposition eyes. In these, ommatidia are not isolated optically from each other by pigment cells. Instead, the retina is separated by a wide clear zone from the corneal facet lenses, and many lenses co-operate to focus light on an individual rhabdom (light from many lenses super-imposes on the retina). The light sensitivity of these eyes is thus greatly enhanced. In some optical superposition eyes screening pigment moves into the clear zone during light adaptation and by this means the ommatidia become isolated optically as in the apposition eye. At low light levels, the screening pigment moves again towards the outer surface of the eye to open up the clear zone for optical superposition to occur.

Because the light arriving at a rhabdom has passed through many facet lenses, blurring is a problem in optical superposition eyes and resolution generally is not as good as in apposition eyes. However, high light sensitivity is much more important than good resolving power in crepuscular and nocturnal insects whose main concern is to see anything at all. In the eyes of some insects, photon-capture is increased even further by a mirror-like tapetum of small tracheae at the base of the retinula cells; this reflects light that has passed unabsorbed through a rhabdom, allowing it a second pass. Light reflecting from the tapetum produces the bright eye shine seen when an insect with an optical superposition eye is illuminated in the flashlight or car headlight beam at night.

In comparison with a vertebrate eye, the resolving power of insect compound eyes is rather unimpressive. However, for the purpose of flight control, navigation, prey capture, predator avoidance, and mate-finding they obviously do a splendid job. Bees can memorize quite sophisticated shapes and patterns, and flies and odonates hunt down prey insects or mates in extremely fast, aerobatic flight. Insects in general are exquisitely sensitive to image motion, which provides them with useful cues for avoiding obstacles and landing, and for distance judgment. Insects, however, cannot easily use binocular vision for the perception of distance because their eyes are so close together and their resolution is quite poor. A notable exception is the praying mantid, which is the only insect known to make use of binocular disparity to localize prey.

Within one ommatidium, most studied insects possess several classes of retinula cells that differ in their spectral sensitivities; this feature means that each responds best to light of a different wavelength. Variations in the molecular structure of visual pigments are responsible for these differences in spectral sensitivity and are a prerequisite for the color vision of flower visitors such as bees and butterflies. Some insects are pentachromats, with five classes of receptors of differing spectral sensitivities, compared with human di- or trichromats. Most insects can perceive ultraviolet light (which is invisible to us) allowing them to see distinctive alluring flower patterns visible only in the ultraviolet.

Light emanating from the sky and reflected light from water surfaces or shiny leaves is polarized; that is, it has greater vibration in some planes than in others. Many insects can detect the plane of polarization of light and utilize this in navigation, as a compass or as an indicator of water surfaces. The pattern of polarized skylight changes its position as the sun moves across the sky, so that insects can use small patches of clear sky to infer the position of the sun, even when it is not visible. In like manner, an African dung beetle has been shown to orientate using polarized moonlight in the absence of direct sighting of the moon, perhaps representing a more general ability amongst nocturnal insects. The microvillar organization of the insect rhabdomere makes insect photoreceptors inherently sensitive to the plane of polarization of light, unless precautions are taken to scramble the alignment of microvilli. Insects with well-developed navigational abilities often have a specialized region of retina in the dorsal visual field, the dorsal rim, in which retinula cells are highly sensitive to the plane of polarization of light. Ocelli and stemmata also may be involved in the detection of polarized light.

4.4.5 Light production

The most spectacular visual displays of insects involve light production, or bioluminescence. Some insects co-opt symbiotic luminescent bacteria or fungi, but self-luminescence is found in a few Collembola, one hemipteran (the fulgorid lantern bug), a few dipteran fungus gnats, and a diverse group amongst several families of coleopterans. The beetles are members of the Phengodidae, Drilidae, some lesser known families, and notably the Lampyridae, and are commonly given colloquial names including fireflies, glow worms, and lightning bugs. Any or all stages and sexes in the life history may glow, using one to many luminescent organs, which may be located nearly anywhere on the body. Light emitted may be white, yellow, red, or green.

The light-emitting mechanism studied in the lampy-rid firefly Photinus pyralis may be typical of luminescent Coleoptera. The enzyme luciferase oxidizes a substrate, luciferin, in the presence of an energy source of adenosine triphosphate (ATP) and oxygen, to produce oxyluciferin, carbon dioxide, and light. Variation in ATP release controls the rate of flashing, and differences in pH may allow variation in the frequency (color) of light emitted.

The principal role of light emission was argued to be in courtship signaling. This involves species-specific variation in the duration, number, and rate of flashes in a pattern, and the frequency of repetition of the pattern (Fig. 4.11). Generally, a mobile male advertises his presence by instigating the signaling with one or more flashes and a sedentary female indicates her location with a flash in response. As with all communication systems, there is scope for abuse, for example that involving luring of prey by a carnivorous female lampyrid of Photurus (section 13.1.2). Recent phyloge-netic studies have suggested a rather different interpretation of beetle bioluminescence, with it originating only in larvae of a broadly defined lampyrid clade, where it serves as a warning of distastefulness (section 14.4). From this origin in larvae, luminescence appears to have been retained into the adults, serving dual warning and sexual functions. The phylogeny suggests that luminescence was lost in lampyrid relatives and regained subsequently in the Phengodidae, in which it is present in larvae and adults and fulfills a warning function. In this family it is possible that light is used also in illuminating a landing or courtship site, and perhaps red light serves for nocturnal prey detection.

Fig. 4.11 The flash patterns of males of a number of Photinus firefly species (Coleoptera: Lampyridae), each of which generates a distinctive pattern of signals in order to elicit a response from their conspecific females. (After Lloyd 1966.)

Bioluminescence is involved in both luring prey and mate-finding in Australian and New Zealand cave-dwelling Arachnocampa fungus gnats (Diptera: Mycetophilidae). Their luminescent displays in the dark zone of caves have become tourist attractions in some places. All developmental stages of these flies use a reflector to concentrate light that they produce from modified Malpighian tubules. In the dark zone of a cave, the larval light lures prey, particularly small flies, onto a sticky thread suspended by the larva from the cave ceiling. The flying adult male locates the luminescent female while she is still in the pharate state and waits for the opportunity to mate upon her emergence.

Beekeeping for Beginners

Beekeeping for Beginners

The information in this book is useful to anyone wanting to start beekeeping as a hobby or a business. It was written for beginners. Those who have never looked into beekeeping, may not understand the meaning of the terminology used by people in the industry. We have tried to overcome the problem by giving explanations. We want you to be able to use this book as a guide in to beekeeping.

Get My Free Ebook

Post a comment