Summary

Insect sensory cells are almost always primary sense cells; that is, they both receive a stimulus and conduct it to the central nervous system. Type I sense cells are associated with cuticle and accessory cells to form the sense organ (sensillum); Type II cells are not associated with cuticle and always function as proprioceptors.

Mechanosensilla include sensory hairs, hair plates, campaniform sensilla, chordotonal organs, stretch receptors, and nerve nets. Some insects can detect sound by means of fine sensory hairs (sound oflow frequency) or specialized chordotonal structures, such as Johnston's organ and tympanal organs. In both mechanoreception and sound reception the stimulus leads to mechanical deformation of a sense cell's cytoplasm.

Chemosensilla occur as thick-walled (uniporous) hairs, pegs, or papillae that generally have a gustatory function, or as thin-walled (multiporous) hairs, pegs, or plate organs whose usual function is olfactory. Each chemosensillum includes several sensory cells, each of which may respond to a different stimulus. Odor and organic taste molecules reach the sensory cell membrane bound to protein molecules. At the membrane they are transferred to a receptor, an action that induces depolarization. How polar materials reach the sensory cell membrane remains unclear; however, once there, they likely act directly on ion channels to trigger impulse generation.

Generally, humidity and temperature changes are detected by dual-purpose sensilla, in the form of poreless, thin-walled hairs or pegs. Each sensillum has two multidendritic neurons, one for humidity, the other for temperature. Probably, hygroreceptors operate like mechanoreceptors, undergoing a change in form in response to changing humidity. How temperature-sensitive sensilla work remains unclear. A few insects have been shown to be sensitive to infrared heat, using thin-walled, poreless sensilla, each with a single neuron.

Insects detect light energy via compound eyes, ocelli, or stemmata, rarely a dermal light sense. Compound eyes, the chief photosensory structures, are composed of ommatidia, each of which includes a light-focusing system, photosensitive (retinular) cells, and enveloping 401

pigment cells. Two types of ommatidia occur, the photopic characteristic of diurnal insects a+U f A- i ^ 1 Of 1 11 A-ff + ^ SENSORY SYSTEMS

and the scotopic found in crepuscular and nocturnal species. Retinular cells are differentiated along their inner longitudinal axis into a rhabdomere, a series of closely packed microvilli in which are contained visual pigments. Even when an image is formed at the level of the retinular cells it has no functional significance. The primary function of compound eyes appears to be movement perception, though some appreciation of form is gained as a result of the flicker effect. The ability to perceive distance is present in some species and is based on the considerable overlap of the visual fields of the two compound eyes (up to about

20 cm in large insects) and through the use of motion parallax for greater distances. Color vision occurs in some insects and results from the presence of retinular cells with different visual pigments. Compound eyes of some species can detect the plane of polarized light, enabling these insects to carry out complex navigation behavior.

The principal function of ocelli appears to be horizon detection as a component of horizontal flight, though for some insects the ocelli may provide information on sudden changes in light intensity, serve as a general stimulus to the nervous system, initiate specific phototaxes, and detect the plane of polarized light. Stemmata have functions similar to those compound eyes.

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.

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