Learning the Language of Insects- and How to Talk Back - Entomology

communication, amplitude modulation involves"chirping" or "trilling" with long orshort periods of silence, rather than emittinga continuous tone (Fig. 2D) Inchemical communication, amplitude modulationusually is provided by the turbulentshearing of the emission as it leaves thesource to create filaments of odor similar towhat we can visualize within smoke plumes(Fig 2F). However, some rhythmic pulsingof chemical emissions can be imparted bythe emitter, such as arctiid or lymantriidmoths emitting pheromone, and these can,in some cases, stand out against the turbulence-inducedmodulation for a half a meteror so downwind (Conner et a1 1980) Suchpulsing thus far has been found to have noextra signal value and also does not increasethe response of males compared to a continuouslyemitted stream of pheromone.No signal from insects has yet beenfound that is frequency modulated and hascommunication value Frequency modulated(FM) signals would involve changingcolor, sweeping through a series of tones, orchanging one's chemical blend However,bats hunting for insect prey are FM emittersthat with each ultrasonic chirp sweepdownward from high to low tones and fromthe particular narrow range of tones of thereflected signal, gather information aboutthe size and wing-beat frequency of theirprey These bats also modulate the amplitudeof their cries by sending discrete chirpswith a silent period that aids in receivingechoes, but there are species that emit AM(amplitude modulated) cries only, with nofrequency sweeps The moths that arehunted have not been shown to beresponsive to the FM modulations of thecries, but rather react defensively by tryingto evade the bats in response to a wide rangeof ultrasonic tones across the frequencyrange of the sweepsBat-moth communication brings up theaspect of reflected versus emitted signals inthe three modalities Bats are providingtheir own sonic lanterns, 01 strobe lights, ifyou will, with which they peer into thedarkness for moving prey items They relyon reflected sound for information to flowto them about prey. Most visually dependentinsect predators and parasitoids relyon ambient light from the sun to reflect offprey items and provide information flow.Likewise, most visual communication systemsuse reflected, ambient light, except inthe notable cases of firefly communication,in which emitted signals are the rule.Emitted, rather than reflected, sounds arethe rule for insects that communicate bymeans of pressure disturbances. However,some species do enhance then resonance byreflecting then emissions off of the sides ofWinter 1 99 3Fig. 1,. Diagram of information flow that results in communication. The signal is depicted asfluctuating in amplitude (wavy purple line), whereas noise in the communication channel is depictedas being unvarying in amplitude (straight lines), but of a different carrier frequency (yellow instead ofpurple). The signal is able to be received and responded to even with the background noise presentdue to the different carrier frequency and its amplitude modulation relative to the noise (afterShannon 6- Weaver 1949)specially constructed burrows, or off ofleaves enlisted to increase the resonance oftheir emissions for the same purpose(Prozesky-Schulze et a1 1975)Semiochemical communication in insectsinvolves both emitted and reflectedcompounds. Obviously "you are what youeat" means that all atoms emitted fromwithin one's body must have at some pointbeen assimilated from an external source,and, therefore, could be considered "reflected."However, some molecules areingested or adsor bed from surroundingsand then reemitted as a signal in more-orlessunaltered form, as shown by TomEisner in his work on the defensive compoundsof many insects (see, for example,Eisner 1970 for review) The results ofmany studies on courtship pheromonesemitted by male moths have shown thatthese pheromones also are comprised ofmainly plant-derived, little-altered molecules.Other semiochemicals are mainlyself-generated, synthesized emissions, asexemplified by the lepidopteran sex pheromonesmanufactured from building-blockacetate molecules, as highlighted by thework of Lou Bjostad and Wendell Roelofs(see Bjostad & Roelofs 1987 for review)Other common properties of communicationin any of these modalities include thefact that sustained reception of intact signalsis affected both by background noise inthe channel and by adaptation of the sensorypathways, whether the system involveslight, sound, 01 chemicals If some ofthe sensory pathways in the receiver havebeen made dysfunctional from adaptationdue to background noise, then the signalthat reaches the brain may have beenseverely altered and rendered ineffective inevoking behaviorAnother major commonality amongcommunication systems in the three mainmodalities is the way in which communicationsystems are dissected by researchersin older to distill the minimal set of signalsimportant in evoking a response in receiversFirst, the natural sequence of behaviorsneeds to be observed and analyzed for theresearcher to get a feeling as to which partsof the sender-r eceiver inter action appear tobe important in evoking a response, or inother words, in successfully communicating,as indicated by the response. Then thenaturally emitted signal needs to be capturedand played back for detailed analysisIn visual communication, relative reflectancesof insect body par ts such as wings canbe gained by obtaining them and placingthem in spectrophotometers to determinethe major hues 01 carrier frequencies thatare preferentially reflected and absorbed bypigments struck by ambient, white sunlightThe emission spectrum of fireflies canbe immediately captured and measured ona spectrophotometer and displayed to findthe major peak emission frequency orwavelength of light Temporal analysis ofthe tempos at which wing reflectances areamplitude modulated 01 firefly flashes arepulsed, as well as the durations of theflashes and pulses can be recor ded on film orvideotape and measured during playback.