Reproductive Behavior and Isolation of Two Sympatric Coenagrionid Damselflies in Florida
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REPRODUCTIVE BEHAVIOR AND ISOLATION OF TWO SYMPATRIC COENAGRIONID DAMSELFLIES IN FLORIDA By KENNETH JOSEPH TENNESSEN A DISSERTATION PRESENTED TO THE GRADUATE COUNCIL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 1975 ACKNOWLEDGEMENTS I sincerely thank the members of my supervisory committee for their time and assistance during this study. Dr. Dale H. Habeck served as committee chairman and provided advice and access to facili- ties. Dr. Philip Callahan stimulated interest in insect communication and sense receptors. Dr. Lewis Berner provided equipment for close-up photography and offered advice and criticism. Dr. F. C. Johnson helped in the choice of the species studied and made many helpful comments. My inspiration and opportunity to study Odonata came through Dr. Minter J. Westfall , Jr., who served as cochairman and unhesitantly provided advice, materials, facilities and help with field work. He has been a teacher and a friend. Special thanks and appreciation are extended to T. C. Carlysle for the scanning electron microscopy, histological preparations, and advice. I thank Dr. James L. Nation for discussions of insect cells and their functions. Discussions with Kenneth W. Knopf and his sug- gestions are sincerely appreciated. I thank Mr. Paul Laessle, staff artist of the Department of Zoology, for equipment and advice in prepa- ration of figures. To my wife, Sandi , I extend sincere appreciation and thanks for encouragement, understanding, and the late dinners. TABLE OF CONTENTS Page ACKNOWLEDGEMENTS ii LIST OF TABLES iv LIST OF FIGURES v ABSTRACT vii INTRODUCTION 1 DESCRIPTION OF STUDY SITES 13 MATERIALS AND METHODS 18 REPRODUCTIVE BEHAVIOR 23 The Pre- reproductive Period 23 The Reproductive Period 26 REPRODUCTIVE ISOLATION 48 SUMMARY AND CONCLUSIONS 70 REFERENCES 73 BIOGRAPHICAL SKETCH 79 LIST OF TABLES Table Page 1 Maturation period (in days) of Enallagma pollutum and E_. signatum , determined by rearing and marking-return experiments 25 2 Number of Enallagma pollutum males attempting tan- dem with simultaneously presented females with straight or decurved abdominal postures 47 3 Response of Enallagm a poll utum and E. signatum males to 900 presentations of conspecific and heterospecific females 52 4 Number of positive responses by males to females of both species when presented simultaneously .... 54 IV LIST OF FIGURES Figure Page 1 Known distributions of Enallagma pollutum and E_. signatum in Florida 4 2 Map of Hatchet Creek drainage northeast of Gainesville; study sites indicated by arrows ... 14 3 Study site on Hatchet Creek, 70 m west of bridge on Hwy. S-225 16 4 Study site on Hatchet Creek, 50 m north of bridge on Hwy. 26 16 5 Lateral view of male apical abdominal append- ages (40X) (E_. pollutum above, E. signatum below) showing superior appendage shape used for field identification 19 6 Seasonal change in onset of daily reproductive activity in males of E_. pollutum and E_. signatum • 28 7 Diurnal fluctuation in number of unpaired males of E_. pollutum and E. signatum in a 10 m stretch of Hatchet Creek, 21 May 1975 30 8 Orientation of a confronting £. signatum male toward a hovering male, and their ensuing flight paths 32 9 Sequence of events in mating behavior of E. pollutum and E. signatum : a) male landing on female; b) male positioning abdominal appendages on thorax; c) tandem flight to a perch; d) "abdo- minal probing" by female; e) transfer of sperm; f) copulation 40 10 Lateral view of E. pol lutum male appendages and corresponding points of contact with female thorax . 41 11 Lateral view of E. signatum male appendages and corresponding points of contact with female thorax . 41 LIST OF FIGURES (Continued) Figure Page 12 Appendages of E. pollutum male clasping thorax of female in tandem (28X) 42 13 Appendages of E_. signatum male clasping thorax of female in tandem (30X) 42 14 Perched unreceptive female of £. pollutum with abdomen decurved and wings spread in response to an approaching male 45 15 Schematic account of mating sequence in E_. pollutum and E_. signatum 49 15 Oblique section through male superior appendage of E_. pollutum ; arrow indicates specialized se- cretory cells (pulled away during fixation) be- neath the thickened, unsclerotized cuticle (500X) . 60 17 Scanning electron micrograph of thorax of an E_. pol 1 utum female showing sensory receptors (100X). 53 18 Scanning electron micrograph of thorax of an E_. signatum female showing sensory receptors (100X). 53 19 Scanning electron micrograph of hind pronotal lobe (left side, posterior view) of an E_. pol lutum female (215X) 64 20 Scanning electron micrograph of hind pronotal lobe (left side, posterior view) of an E_. signatum female (215X) 64 21 Scanning electron micrograph of E. pollutum female mesostigmal plate (170X) 65 22 Scanning electron micrograph of E_. signatum female mesostigmal plate (170X) 65 VI Abstract of Dissertation Presented to the Graduate Council of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy REPRODUCTIVE BEHAVIOR AND ISOLATION OF TWO SYMPATRIC COENAGRIONID DAMSELFLIES IN FLORIDA By Kenneth Joseph Tennessen August, 1975 Chairman: D. H. Habeck Cochairman: M. J. Westfall, Jr. Major Department: Entomology and Nematology The coenagrionid damsel flies Enallagma pollutum (Hagen) and E. signatum (Hagen) are sympatric throughout most of Florida and may be called sibling species. Reproductive activity at water begins early in the afternoon in spring and fall, but the species are semicrepuscular in summer. Males usually appear at water before females, and E. pollu- tum males usually before E_. signatum . Behavior and location of indivi- duals prior to their appearance at water are unknown. Males interact vigorously in order to maintain a position at the water; prey capture at water is uncommon. Males and probably females are capable of sex recognition; the male's orange ninth abdominal segment is an important sex-recognition character. Pairing occurs only at water, without court- ship displays. The male flies directly to the female and grasps her by the thorax with his legs. After clasping her with the abdominal appendages, the pair flies in tandem to a perch. In response to the female's "abdominal probing," the male transfers sperm, followed by copulation which averages 20 minutes. Individuals may mate once in a day, but rarely mate on two successive days. Oviposition usually occurs in tandem shortly after copulation, at or below the water surface. vn Enallagma pollutum and E. signatum are very effectively isolated. Flight seasons and daily mating times are concurrent. Partial isola- tion is gained by differences in microhabitat: E, pol lutum males usually perch whereas E_. signatum males usually hover over open water; E_. pollutum females usually remain on or near vegetation, and although E. signatum females are also found there, they usually fly toward open water. These differences lessen the probability of interspecific meetings. However, many exceptions occur and males attempt tandem with females of either species; 25 percent of the pairings observed were interspecific. Males are incapable of visually distinguishing females. Isolation occurs during tandem, as non-conspecific males release their grasp upon "refusal motions" by the female. Evidence indicates that tactile stimuli, provided by the shape of the male superior appendages, are releasers of mating behavior in females, and may isolate the species; this proposed mechanism is ethological. Males with altered appendage shape were unsuccessful in mating, and while the observations are not as controlled as desired, the results suggest a tandem period of species recognition. The female thoracic parts contacted by the appendages bear numerous trichoid and other sensillae. The function of specialized, secretory cells in the male superior appendages is unknown; chemical isolation is unreported in Odonata, but the possi- bilities should be investigated. I propose that the nongenitalic mechanical isolation theory, in which the male abdominal appendages, because of structure, are sup- posedly unable to securely hold a heterospecific female, is untenable for Coenagrionidae. Clearly, ecological and ethological isolation is operative. viii INTRODUCTION The basic premise of the biological species concept is that groups of natural populations are reproductively isolated. Reproductive iso- lation is maintained by means of isolating mechanisms, defined by Mayr (1970) as biological properties of individuals which prevent the inter- breeding of sympatric populations. Mayr classified isolating mechanisms as premating or postmating. Premating isolation includes 1) seasonal and habitat [ecological] differences, 2) ethological differences, by way of visual, auditory or chemical stimuli, and 3) mechanical differ- ences. Postmating isolation includes gametic and zygotic mortality and hybrid inviability and sterility. Premating isolating mechanisms are more efficient in terms of energy because there is no wastage of gametes; they are highly susceptible to natural selection. Mayr points out that the precise mechanisms preventing interbreeding of closely related species are unknown for the majority of animals. Discovery of these mechanisms requires knowledge of behavior and identification of the stimuli important in mate and species recognition. The early discovery of specific structural differences in insect genitalia led to the idea of a "lock