Box 42 The electroantennogram

Electrophysiology is the study of the electrical properties of biological material, such as all types of nerve cells, including the peripheral sensory receptors of insects. Insect antennae bear a large number of sensilla and are the major site of olfaction in most insects. Electrical recordings can be made from either individual sensilla on the antenna (single cell recordings) or from the whole antenna (electroantennogram) (as explained by Rumbo 1989). The electroantennogram (EAG) technique measures the total response of insect antennal receptor cells to particular stimuli. Recordings can be made using the antenna either excised, or attached to an isolated head or to the whole insect. In the illustrated example, the effects of a particular biologically active compound (a pheromone) blown across the isolated antenna of a male moth are being assessed. The recording electrode, connected to the apex of the antenna, detects the electrical response, which is amplified and visualized as a trace as in the EAG set-up illustrated in the upper drawing. Antennal receptors are very sensitive and specifically perceive particular odors, such as the sex pheromone of potential conspecific partners or volatile chemicals released by the insect's host. Different compounds usually elicit different EAG responses from the same antenna, as depicted in the two traces on the lower right.

This elegant and simple technique has been used extensively in pheromone identification studies as a quick method of bioassaying compounds for activity. For example, the antennal responses of a male moth to the natural sex pheromone obtained from conspecific female moths are compared with responses to synthetic pheromone components or mixtures. Clean air is blown continuously over the antenna at a constant rate and the samples to be tested are introduced into the air stream, and the EAG response is observed. The same samples can be passed through a gas chromatograph (GC) (which can be interfaced with a mass spectrometer to determine molecular structure of the compounds being tested). Thus, the biological response from the antenna can be related directly to the chemical separation (seen as peaks in the GC trace), as illustrated here in the graph on the lower left (after Struble & Arn 1984).

In addition to lepidopteran species, EAG data have been collected for cockroaches, beetles, flies, bees, and other insects, to measure antennal responses to a range of volatile chemicals affecting host attraction, mating, oviposition, and other behaviors. EAG information is of greatest utility when interpreted in conjunction with behavioral studies.

in relation to feeding, there are cells whose activity in response to chemical stimulation either is to enhance or reduce feeding. These receptors are called phago-stimulatory or deterrent.

The major olfactory role comes from multiporous sensilla, which are hair- or peg-like setae, with many round pores or slits in the thin walls, leading into a chamber known as the pore kettle. This is richly endowed with pore tubules, which run inwards to meet multibranched dendrites (Box 4.3). Development of an electroantennogram (Box 4.2) allowed revelation of the specificity of chemoreception by the antenna. Used in conjunction with the scanning electron microscope, micro-electrophysiology and modern molecular techniques have extended our understanding of the ability of insects to detect and respond to very weak chemical signals (Box 4.3). Great sensitivity is achieved by spreading very many receptors over as great an area as possible, and allowing the maximum volume of air to flow across the receptors. Thus, the antennae of many male moths are large and frequently the surface area is enlarged by pectinations that form a sieve-like basket (Fig. 4.6). Each antenna of the male silkworm moth (Bombycidae: Bombyx mori) has some 17,000 sensilla of different sizes and several ultrastructural morphologies. Sensilla respond specifically to sex-signaling chemicals produced by the female (sex pheromones; see below). As each sensillum has up to 3000 pores, each 10-15 nm in diameter, there are some 45 million pores per moth. Calculations concerning the silkworm moth suggest that just a few molecules could stimulate a nerve impulse above the background rate, and behavioral change may be elicited by less than a hundred molecules.

4.3.2 Semiochemicals: pheromones

Many insect behaviors rely on the sense of smell. Chemical odors, termed semiochemicals (from semion -signal), are especially important in both interspecific and intraspecific communication. The latter is particularly highly developed in insects, and involves the use of chemicals called pheromones. When recognized first in the 1950s, pheromones were defined as: substances that are secreted to the outside by one individual and received by a second individual of the same species in which they release a specific reaction, for example a

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