The Courtship Call of Chrysopa downesi Banks (Neuroptera: Chrysopidae): Its Evolutionary Significance.
Psyche 86(2-3):291-297, 1979.
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THE COURTSHIP CALL OF CHRYSOPA DOWNESZ BANKS (NEUROPTERA: CHRYSOPIDAE):
ITS EVOLUTIONARY SIGNIFICANCE*
BY CHARLES S. HENRY
Biological Sciences Group, Box U-43
University of Connecticut
Storrs, CT 06268
Tauber and Tauber (1977a) suggest that the conifer-associated green lacewing Chrysopa downesi Banks evolved sympatrically in northeastern North America from its holarctic sibling C. carnea Stephens. The proposed mechanism of speciation follows the gen- eral theoretical model of Maynard Smith (1966) and specifically involves simple allelic changes at three loci, one controlling body color and two altering the insect's response to photoperiod (Tauber and Tauber, 1977b). Gene substitution at the first locus initially induced divergence in habitat association, while later substitutions at the other loci produced complete reproductive isolation of the two populations by causing each to breed at a slightly (but suffi- ciently) different time of year (Tauber and Tauber, 1976). An alternative explanation of C. downesi's origin from C. carnea is proposed which invokes the concept of allopatric speciation (Mayr, 1963): two portions of a previously contiguous C. carnea population became physically isolated from one another for a period of time, perhaps by glaciation events; subsequently, removal or disappearance of the geographic or climatic barrier re-established contact between the two populations, but intervening, independent evolutionary changes prevented much or any gene flow (Hendrick- son, 1978). Tauber and Tauber (1978) argue that allopatric or geo- graphical speciation is a less parsimonious explanation of the existing data than the sympatric model, although they admit that their evidence does not truly discriminate between the two hypo- theses. However, my studies of courtship and mating behavior in the two lacewing species indicate that, in this case, a traditional allopat- ric model may in fact be preferable to the more intriguing sympatric one.
*Manuscript received by the editor February 20, 1980 29 1
292 Psyche [June-September
Implicit in most models of geographical speciation is the streng- thening of reproductive isolation between imperfectly isolated pop- ulations after contact is re-established; in animals, behavioral patterns associated with courtship are thought to be particularly subject to alteration, since premating barriers are presumed to be less wasteful of reproductive effort, gametes, or offspring than those that intervene after copulation (Dobzhansky et al., 1977). In lace- wings of the genus Chrysopa Leach, I have shown that short- distance acoustical communication by substrate transmission of abdominal vibration or jerking is a necessary component of success- ful courtship (Henry, 1979, 1980). A different "calling" pattern char- acterizes each of the eight sympatric lacewing species that I have studied in detail, suggesting that abdominal jerking is or has been important to the reproductive isolation of lacewing species in east- ern North America. This view is strengthened further by the fact that the most closely related species, particularly those of the subge- nus Chrysoperla Steinmann,l predictably show the most distinctive and elaborate patterns. Since C. cornea and C. downesi are members of this latter taxon, the nature of their acoustical signals could indicate how they speciated. If their separation occurred within a single continguous population by the simple genetic changes postulated by Tauber and Tauber, one would predict iden- tity or at least similarity of calling pattern in both species, since (1) barriers to gene flow are presumed complete after the three allelic substitutions and (2) even casual attempts at interspecific matings are precluded by the ~aubers' model. On the other hand, allopatric speciation followed by re-established sympatry should produce unmistakably different patterns of abdominal jerking in the two species, since different calls would terminate heterosexual interac- tions between them before copulation could occur. I propose that the latter alternative is consistent with the extreme calling differen- ces that exist between the two siblings. Lacewing calls consist of discrete bursts or volleys of abdominal strokes (jerks); the call itself is actually a sequence of one or several different kinds of volleys repeated in characteristic temporal pat- ^his taxon has been given full generic status by Y. Semeria (1977).
19791 Henry - Chrysopa downesi 293
terns. The frequency with which the abdomen is jerked or vibrated during each volley is quite low and may be modulated (changed) during the brief course of the volley. A sexually receptive male and female of a given species will establish a duet of abdominal jerking during courtship; in such a duet, partners will alternately trade volleys, or whole sequences, without overlap or interference. I recorded and photographed jerking activity using the equipment and techniques of a previous study (Henry, 1979). A ceramic trans- ducer (crystal phonograph cartridge) picked up substrate vibrations produced in a thin plastic membrane by calling lacewings and fed these signals to a storage oscilloscope. Conclusions regarding the patterns of C. carnea were based on observations of many (more than 30) successful courtships performed by numerous pairs of indi- viduals drawn from populations in Connecticut, New York, Illinois, and California; those concerning C. downesi were based on six successful and 15 unsuccessful courtships by eleven pairs (7 males and 10 females) from the northern Catskill mountains of New York State, observed for 69 hours. The source population of C. downesi was sympatric with that of C. carnea at the Catskill site. I did not find any geographical variation in C. carnea's calling patterns. Chrysopa cornea's call (Fig. 1) consists of a long sequence of 40 or more short (approximately 1 / 2 second) volleys of vibration; volleys are separated by 1-2 second intervals, and the sequences of two insects are interdigitated during duets-i.e., partners alternate vol- leys (Fig. 1A). The frequency of abdominal vibration is modulated during each volley, from around 100 strokes per second at inception to 35 or 40 per second at termination (Fig. 1 B). Additionally, there is a gradual but significant change in the spacing of volleys during each solo or duet sequence (Henry, 1979). The calls of this species are not markedly sexually dimorphic.
In contrast, the call of the C. downesi male or female is more elaborate than and totally unlike that of C. carnea (Fig. 1C and Table 1). A sexually receptive individual will periodically release a 5-8 second (or longer) sequence of closely-spaced jerking volleys, punctuated by a sharp discontinuity where the duration of and interval between volleys abruptly change. Part one of each sequence consists of four to seven identical (except for gradually increasing
294 Psyche [June-September
overall amplitude) volleys at 1 / 2 second intervals; each volley itself lasts about 1 / 3 second and is divided into a short, relatively intense initial section of 5-8 abdominal strokes and a longer, weaker portion of 20 or more strokes, every other one of which is usually empha- sized (Fig. ID). Part two consists of 10-20 distinctly different, shor- ter, simple volleys repeated every 1 / 5-1 /4 second and each made up of 6-8 strokes of the abdomen; overall intensity is high at the onset of part two, but steadily declines almost to a null. Frequency of abdominal vibration is held fairly constant throughout, averaging 68-80 strokes/second. Dueting insects, rather than interdigitating their solo sequences in the manner of C. carnea, alternate entire sequences politely with one another. Again, sexual differences are not profound in C. downesi, although during duets female sequen- ces are nearly always shorter than those of males (99 out of 127 sequences).
In summary, the calls of C. carnea and C. downesi differ greatly in overall complexity, duration, volley amplitude and frequency structure, and manner of exchange in heterosexual duets. Whereas C. carnea adults produce long homogeneous sequences of indeter- minate length and rapidly exchange volleys of jerking when dueting, those of C. downesi produce more structured sequences composed of two distinct classes of volleys and patiently exchange entire sequences when dueting. Additionally, pronounced frequency mod- ulation characterizes single volleys of C. carnea but not of C. dow- nesi. If, as is likely, the divergent calls function primarily to prevent interbreeding2, an allopatric origin of the two species is implicated. An alternative explanation of the observed dissimilarity in their "songs," genetic drift following sympatric speciation, is not as con- vincing. It requires that two species showing grossly different calling patterns also display striking morphological similarity (Tauber, 1974) and complete interfertility in the laboratory (Tauber and Species-specific calls can also originate in response to selective pressures unrelated to reproductive isolation-e.g., one type of call might communicate information more efficiently than another in a particular habitat. However, I am unable to identify any features of the two lacewings' normal habitats that are simultaneously important to the production of sound and sufficiently different to account for the divergent calling patterns.
Henry - Chrysopa downesi
Figure 1. Oscillographs of abdominal vibration patterns produced by courting individuals of Chrysopa cornea (A, B) and C. downesi (C, D). Duets between a sexually receptive male and female are recorded in (A) and (C) at an oscilloscope writing speed of 2 seconds/major division; in C. carnea, each insect exchanges single volleys of abdominal jerking with its partner, while in C. downesi, whole sequences of volleys are exchanged. Individual volleys for each species are shown in B (C. carnea, male) and D (C. downesi, male), recorded at a writing speed of 0.10 second/major division. For C, downesi, volleys typical of part 1 and part 2 of the sequence are indicated.
2 PART 1
Important measurable characteristics of the calls (sequences) of males and females of Chrysopa downesi. Data from heterosexual, ?
homosexual, and solo calls are pooled due to insignificant differences. Mean values and their standard deviations are tabulated; intervals '2
and duration are in seconds, frequency in abdominal strokes per second. Sample sizes are entered parenthetically: the larger figure represents the nun'ber of calls measured, while the smaller one is the number of different individuals producing more than 10 of such calls. ?
Parts 1 and 2 as in Fig. 1.
19791 Henry - Chrysopa downesi 297
Tauber, 1977), whereas one expects random processes to affect all aspects of an organism's genotype more or less equally. This same interfertility and morphological similarity of the two species argue strongly for their close relationship to one another and against their having other closest relatives and other evolutionary histories. This study was supported by N.S.F. award number DEB77- 12443, C. S. Henry, principal investigator. I thank the following colleagues from the University of Connecticut: R. Pupedis, J. O'Donnell and S. Cohen for help in the field collection and labora- tory maintenance of lacewings; and R. J. Schultz, E. Brighty, and G. Clark for constructive comments on the paper and its concepts. C. Tauber, Cornell University, kindly supplied six of the C. downesi adults from a young laboratory colony.
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