Swarming Behavior, Sexual Dimorphism, and Female Reproductive Status in the Sex Role-Reversed Dance Fly Species Rhamphomyia marginata

Dance flies are predaceous insects which often form male mating swarms. In many species males prior to swarming catch an insect prey, which is presented to the female at mating. In Rhamphomyia marginata, females in contrast to males gather to swarm, while males carrying a prey visit swarms for mating. Here I describe the swarming and courtship behavior in R. marginata and provide data on sexual dimorphism and swarming female reproductive status. Females swarm in small clearings in the forests. There was no specific swarm-maker. The swarming period lasted for 2–3 h and peaked around sunset. Identical swarm sites were used each evening and for several years. The mean number of females in swarms (swarm sites with at least one female) was 9.9 ± 9.1 (range, 1–40; n = 107) in 1993 and 7.1 ± 7.0 (range, 1–35; n = 68) in 1994. No obvious competition between females in swarms was observed. The operational sex ratio in swarms was extremely female biased (all swarms, 0.04). Less than one-third of male visits to swarms resulted in mating and males were found more often in larger swarms. Nuptial prey consisted of male midges. Females seem to mate more than once. Swarming females had undeveloped eggs, whereas mated females in swarms had further developed eggs than unmated females. Amount of sperm in the spermatheca was correlated with egg size. Amount of sperm and egg size did not correlate with wet weight, wing length, or wing load, except for egg size and weight. The wing coloration pattern and shape in R. marginata females are unique among dance flies, being greatly enlarged (1.6 times larger than that of males) and bicolored (gray part, 60% of wing area). When females, instead of males, possess extravagant secondary sexual characters, it is predicted from sexual selection theory that females should compete for males and that males should be selective in their choice of partner. A sex-role reversal will evolve when assess to males limit female reproductive success. The dance fly species R. marginata, like Empis borealis, another dance fly species studied earlier and discussed here, seems to fit these predictions.     

Male Age,Copulation Duration and Insemination Success in Mystacides azurea (Leptoceridae,Trichoptera)

Time in copula in the swarming caddis fly Mystacides azurea L. (Trichoptera: Leptoceridae) ranged from 0.33 to 44.5 min. It increased with male age (wing wear) and male dry weight, but was independent of male and female size (forewing length), female wing wear and number of eggs in the females. Older males failed to transfer sperm during copulation more frequently than did younger ones. It is suggested that females benefit by interrupting prolonged copulations if sperm is not transferred rapidly, since being in copula might increase the risk of predation. Alternatively, young and old males follow different mating tactics; old males have a lower chance to acquire new mates and they do better by monopolizing a female, once they get one, by prolonging the copulation.     

Swarm Site Fidelity in the Sex Role-Reversed Dance Fly Empis borealis

In the dance fly species Empis borealis (L.), females (1–40) gather to swarm at landmarks (swarm markers, like trees and bushes), and males carrying an insect prey visit these swarms for mating. We noticed earlier that some swarm sites were used for several years and that they appeared to be frequented by a similar number of swarming females in each year, although the numbers of females varied greatly among swarm sites and certain sites attracted more swarming individuals than others. To explore swarm site fidelity in this mating system, in 1993 we monitored the same swarm sites that we studied in 1989, addressing the questions, Would the same swarm sites still attract the same number of females and males after 4 years? and Why do some swarm sites attract more displaying females than others? The number of females swarming at the different markers in 1993 was approximately the same as 4 years earlier. Some of these swarm sites are known to have been used for 18 years. The swarm sites with the largest number of flies had a high sun exposure during the day and were found at coniferous swarm marker trees and in a mixed forest habitat. A swarm site with few females attending and with a low amount of insolation during the day can be predicted to be abandoned as a swarming site soon. Empis borealis swarm sites thus persist over many years and are attended by a similar number of individuals each year. To our knowledge, such site fidelity has not been demonstrated for any swarming insect species earlier.     

Swarm behaviour and mate competition in mayflies (Ephemeroptera)

Although mayfly swarms are frequently cited as an example of lekking by insects, little is known about the behaviour of individuals within a swarm, or how mate-selection takes place. A study of five species of mayfly over a period of 10 consecutive years has revealed species-specific differences in the flight pattern of swarming males and in the ability of males to recognize swarms of their own species. Males of four of the five species jostle other males in the swarm at all times except when mating: mating pairs are not jostled. The pattern of jostling varies with the species. Measurements of the sperm content of the vesicula seminalis and of the wing length of members of individual swarms show that larger wing size is positively correlated with the presence of less sperm. The vesicula seminalis is always filled with sperm at the beginning of the imaginal stage and the testes regress before the beginning of the imaginal stage. If the volume of sperm in the vesicula seminalis is a valid index of mating success then males with larger wings have the highest success. Large wings may bestow an advantage during jostling. The males of Ephemera danica , which do not jostle, glide with outspread wings; these outspread wings may attract females, the largest wings being the most attractive. Females of all five species enter the swarm a few at a time, although many females may be resting beneath the swarm. This phased entry may decrease the attraction of the swarm for predators. The number of females in a swarm is not correlated with swarm size, and the factors which enable females to regulate their entry into a swarm remain obscure.     

Wing wear of adult Choristoneura fumiferana (Lepidoptera: Tortricidae) in relation to age, sex, sex ratio, and presence of host plant

Wing wear of adult butterflies has been used to record age-related demographic parameters in hundreds of studies, but this technique has surprisingly been rarely used in moths and never in the context of pest management. A method for scoring wing wear of eastern spruce budworm, Choristoneura fumiferana (Clemens), the most severe pest in eastern North American boreal forests, is proposed based on the proportion of forewings covered with scales. Studies conducted in the laboratory reveal a higher level of wing damage for males than females, for 4-day-old individuals than 2-day-old individuals, and for adults that are in contact with host plant material. Males provided with mating opportunities had a lower incidence of wing damage than males deprived of mating opportunity, whereas wing wear of females was independent of the presence or absence of males. In combination with other variables, wing wear of adult spruce budworms may help to identify and forecast migration events.     

Sexual dimorphism in relation to swarming and pair formation patterns in leptocerid caddisflies (Trichoptera: Leptoceridae)

North European Leptoceridae (Trichoptera) perform three types of swarming flight patterns: (1) swarming males of Athripsodesand Ceracleafly in horizontal zigzag patterns over the water surface, (2) the Mystacidesspp. perform vertical zigzag movements, and (3) the flight of males of Triaenodes unanimisMcLach. is a mixture of the horizontal and vertical zigzagging. Also three groups of pair formation behavior can be distinguished. In the first group, of Athripsodesand Ceraclea,the females fly into the male swarms, where they are grasped and carried to the riparian vegetation by the flying males with the females hanging upside-down in genitalia coupling. In the second group, a Mystacidesfemale is caught by a male, when approaching a swarm and both use their wings to fly in tandem to the shore where they copulate. In the third group, of Triaenodes bicolor(Curt.) and Oecetis lacustris(Curt.), the males fly searching for females sitting on aquatic plants and when a female is found the male lands and they copulate immediately while clinging to the plant. The different swarming and mating behaviors might have favored selection for three types of sexual dimorphism: (1) longer forewings in males than females in species which fly in copula, (2) larger eyes in males of the vertically zigzagging species, and (3) much smaller males in the group where males search for females sitting on aquatic plants. In the second group approaching females are detected by males before reaching the swarm and in the third group the female almost always mates with the male which is the first to find her. In conclusion, we suggest that females of Athripsodesand Ceracleahave a greater choice among swarming males than do females of Mystacides, T. bicolor,and O. Lacustris.     

Wing length and asymmetry of male Tokunagayusurika akamusi chironomid midges using alternative mating tactics

Male Tokunagayusurika akamusi chironomids have alternative mating tactics.One is to search for females on vegetation (ground mating),and the other is to wait for females in an aerial swarm (swarmmating). Simultaneous sampling of ground-unpaired and ground-pairedmales and of swarm-unpaired and swarm-paired males were performed.The average wing length and right-left wing length difference(wing asymmetry) were compared between males from the four differentcategories. Swarm-unpaired males were larger than ground-unpaired ones,swarm-paired males were larger than swarm-unpaired ones, and ground-pairedmales were not larger than ground-unpaired ones. Thus, large malestended to aggregate in swarms, and larger swarming males matedmore successfully. On the other hand, small males probably enjoyedmating on the ground, especially when large males swarmed. Thewing asymmetry was not significantly different between unpairedand paired males both within and between tactics. There wasa flat or U-shaped relationship between wing length and asymmetry,underpinning the lack of a symmetrical advantage of swarmingto large males. The right-left difference was not normally distributedin four of six samples of unpaired males but, in contrast, wasnot normally distributed in only one of six samples of pairedmales. The non-normal distributions were leptokurtic and includedoutliers. Removal of the outliers improved normality, suggestingthat males with extremely asymmetric wings were not successfulin mating.     

Male size and survival: the effects of male combat and bird predation in Dawson's burrowing bees, Amegilladawsoni

1. Males of Dawson's burrowing bee, Amegilla dawsoni , occur in two size classes, large majors and small minors. Major males compete aggressively for emerging females in completely exposed emergence areas, whereas minors often employ an alternative mating tactic, which involves rapid patrolling in the vegetated periphery of emergence areas.
2. Because major males wrestle violently with rivals on the ground, they experience greater wing wear and a higher risk of wing damage than minor males. In addition, males patrolling emergence areas and waiting on the ground for emerging females are more likely to be killed by predatory birds than are peripheral minors.
3. Measurements of longevity based on mark–recapture data suggest that majors are slightly shorter-lived than minor males. If male–male combat and predators do reduce the lifespan of major males relative to minors, the effect would be to decrease the lifetime mating advantage of majors relative to minors.
4. Differential mortality could therefore be a factor in the maintenance of the two forms of males in this species.     

Swarming and mating of Aedes communis mosquitoes in nature

The date of the beginning of mating behaviour in males and females, the rate of insemination and the increasing of bloodsucking activity of females were studied in natural environments. Over 80% of females mated on the 3-4th day after emergence; after fertilization their behaviour changed from looking for males for coupling to looking for ones for a prey. The male swarming began on the 5th day after emergence and simultaneously the appearance of inseminated females was observed. The places of mating of males and females were investigated. It was established that coupling took place in swarms with swarming males and out of swarms with freely flying males.     

Swarming and mating ofChironomus yoshimatsui (Diptera: Chironomidae): Seasonal change in the timing of swarming and mating

Males ofChironomus yoshimatsui Martin et Sublette swarm at dusk, and copulate with females entering the swarm. It is likely in this species that, by restricting the time and place, swarming has the function of increasing the probability of the encounter between a sexually active male and a receptive female in the air. It is necessary that the timing of females taking wing coincides with that of males swarming. Field observations on swarming and mating from March to November showed that swarms and copulations occurred under lighter conditions at lower temperatures and under darker conditions at higher ones. It was suggested that both sexes may have a similar mechanism, depending on the temperature conditions, regulating the timing of taking wing.     

Non-random Mating in the Dance Fly Empis borealis: The Importance of Male Choice

In the dance-fly Empis borealis (Diptera, Empididae), females form swarms to which males, carrying a nuptial gift, come for mating. We examined whether males or females were choosy and/or competed for mates. First, measurements of the size relationships between copulating males and females, nuptial gifts and the swarming females from different swarms were assessed. Second, male visiting time in differently sized female swarms was recorded. Larger (wing-length) females participated disproportionately in copulations in each swarm, but not for the population at large. Female mating status (virgin/non-virgin) or proximity to oviposition (egg size) did not influence the likelihood of copulation. No assortative mating pattern was found: male size and size of nuptial gift did not correlate with size of the mating female. The time spent by males in swarms increased with the number of females present and it took longer when males left a swarm without copulation than when doing so. Male visiting time per female was negatively correlated with number of females in swarms. Males more often left smaller than larger swarms without mating. We conclude that E. borealis males discriminate among females but find no evidence for male competition or for female choice. It is still a question to what degree females compete for males.     

A selective trade-off for territoriality and non-territoriality in the polymorphic damselfly Mnais costalis

Males of the damselfly Mnais costalis occur as territorial orange-winged 'fighter' males or non-territorial clear-winged 'sneaker' males. Their morph life histories differ considerably but the estimated lifetime reproductive success is the same for the two morphs. In this study we compared the developmental and reproductive costs associated with the two morphs. Orange-winged male and female reproductive costs resulted in a decline in adult fat reserves with increasing age. In contrast, the fat reserves of clear-winged males remained constant with adult age. Body size was positively correlated with mating success in orange-winged males, but had no influence on the mating success of clear-winged males. The orange-winged male flight muscle ratios (FMRs) were significantly higher than the clear-winged male and female FMRs. However, there was no difference in the size-corrected fat reserves of the two morphs; both had higher fat reserves than females. The gain in mass between eclosion and reproduction in orange-winged males and females was almost double the mass gained by clear-winged males, suggesting that clear-winged male development is less costly. An experiment in which pre-reproductive levels of nutrition were manipulated confirmed this.     

Variation in energy reserves and role of body size in the mating system of Anopheles gambiae

Anopheles gambiae mates in flight. Males gather at stationary places at sunset and compete for incoming females. Factors that account for male mating success are not known but are critical for the future of any genetic control strategy. The current study explored variations in nutritional reserves (sugars, glycogen, lipids, and proteins) in wild‐caught swarming and resting males and evaluated the effect of body size and wing symmetry on male mating success. Our results showed that glycogen and sugar reserves are mobilized for flight. Males consume proportionally 5.9‐fold as much energy derived from sugars in swarming activities than when they are at rest. Mated males were on average bigger than unmated ones (P<0.0001). A strong correlation between the left and right wings in both mated and unmated males was found and additional analysis on fluctuating asymmetry did not show any indication of mated males being more symmetrical than unmated ones. The distribution of wing size of mated males was focused around a central value, suggesting that intermediate size of males is advantageous in the An. gambiae mating system. The results are discussed in the context of sexual selection.     

Mate-locating strategies are related to relative body length and wing colour in the speckled wood butterfly Pararge aegeria

1. The mate-locating strategies of Pararge aegeria (L.) males were studied in relation to adult morphology (dorsal wing colour, forewing length, body length and forewing length : body length ratio) and generation.
2. Males locate females either by perching and defending territories, or by patrolling. Individuals were more consistent in their mating strategies than expected by chance.
3. Forewing length : body length ratio was positively correlated with thorax mass : body mass; relatively short-bodied males had relatively heavy thoraxes. Therefore, forewing length : body length ratio was an index of mass allocation.
4. Perching males had higher forewing length : body length ratios and were paler than patrolling males.
5. The higher forewing length : body length ratio was due to the differences in body length and not wing length. Perchers had shorter bodies than patrollers.     

Mating system ofTokunagayusurika akamusi (Diptera: Chironomidae): I. Copulation in the air by swarming and on the ground by searching

Observations on the mating system of the midge,Tokunagayusurika akamusi, revealed mating to occur both in the air by swarming and on the ground by searching. At the shores of Lake Biwa, midges appeared from November to early December. Newly emerged adults arrived at the resting place, lakeside vegetation, in the morning, during which time a number of males also walked about in search of mates. Many copulating pairs were observed at the resting place. Huge swarms occurred chiefly before sunset but the frequency of copulation observed in the swarm was extremely low. It is likely that, in the Lake Biwa population, the proportion of females inseminated by searching males at the resting place was much larger than that by swarming males in the air. Furthermore, by searching, males copulated with younger females than by swarming. The differences between the searching and swarming tactics are discussed.     

Body Mass and not Wing Length Predicts Territorial Success in a Tropical Satyrine Butterfly

The males of numerous butterfly species fight with conspecific rivals to possess mating territories. Although there is little consensus on the nature of fight costs or on what traits favor victory, a recent analysis suggests that size may be of minor importance. However, data are inconsistent, and wing length, the metric that has been most widely used for expressing size in butterfly field studies, may not provide a reliable index of muscle mass or other traits that determine resource holding potential. Here we investigate the influence of wing length, body mass and wing wear on the territorial success of a neotropical satryine butterfly, Paryphthimoides phronius. In a removal experiment, original residents were heavier than replacement males that took over territories in the following minutes. Winners of naturally occurring contests were also heavier than losers after correcting for weight loss since weighing. Although wing length correlated significantly with body mass and body mass decreased with age, neither wing length nor wing wear (a surrogate for age and body condition) contributed to explaining territorial success. As body mass may be related to traits such as muscle mass, fat stores, and age, no conclusion is possible on how mass translates into success in disputes. However, because wing length is unrelated to residency status in P. phronius, whereas mass is both a reliable predictor and performs significantly better than wing length, our results may help explain the frequent failure of wing length to predict territorial success in butterfly field studies.     

Effect of body size on swarming behavior and mating success of maleAnopheles freeborni (Diptera: Culicidae)

We examined whether body size affects the swarming behavior and mating success of male Anopheles freeborninear California rice fields. Swarms formed after dusk and persisted for approximately 30 min. The proportion of males in 33 swarms sampled n=6028 ranged from 100 to 92% but decreased over time (r=0.73, t=6.03, P<0.001).On average, swarming males (n=1058) were larger than males sampled from the resting population (n=735, H=35.6, P<0.0001),indicating that some males never swarm at all. Males swarming early were significantly smaller than those swarming during the peak (H=6.71, P=0.009)or final minutes of the swarm (H=4.86, P=0.002). Mated males returned to the swarm after mating, and larger males enjoyed greater mating success than did smaller ones (n=398, H=16.1, P=0.0005).     

Flight energetics in relation to sexual differences in the mating behaviour of a mayfly,Siphlonurus aestivalis

Summary Mayflies (Ephemeroptera) are known to have short adult life-spans. Adults are unable to feed, and they utilize reserves stored during their aquatic larval stage. Energy reserves (fat, glycogen, and free sugars) of mature larvae, subimagoes and imagoes of both sexes of Siphlonurus aestivalis Eaton were compared. All the stages of both sexes had low glycogen and free sugar contents, and the only significant change occurred during the transformation of the mature larva to subimago when almost all the reserves of free sugars were used up. Glycogen and free sugars may serve as energy sources permitting individuals to swim and fly out of the water during emergence. Fat made up most of the energy reserves of mature larvae and was the main source of energy used during the final development of both sexes. Young adult males had high fat reserves which they used as a source of energy for their swarming flights. In contrast, females did not seem to use a significant amount of fat for flight. This difference is probably related to the different mating strategies of the sexes in this species. Males perform long flights waiting for females, whereas females perform only brief flights to mate and reproduce.     

Differences in size and energy content affect the territorial status and mating success of a neotropical dragonfly

In male odonates, both size and fat content are related to territory defence and mating success. Males that are larger and have higher energy reserves win relatively more disputes for territory and attract more females. Wing colour has also been regarded as a mechanism that influences agonistic behaviour between males, as wing pigmentation might be regarded as a sign of male quality. In this study, we analysed whether a set of male physical (body size and wing colour), physiological (body fat content) and behavioural (disputes between males) characteristics were involved in the territory defence and mating behaviour of the neotropical dragonfly Zenithoptera lanei Santos, 1941 (Anisoptera: Libellulidae). Males were characterised as territorial whenever they warded‐off other males and remained in the same place within the pond for two consecutive days. In general, these territorial males were larger and had more abdominal and thoracic fat, engaged in pursuits more frequently, spent more time on sexual behaviour and female guarding, and mated more in comparison to subordinate males. By evaluating whether the percentage of wing area covered by black ink influenced male behaviour, we found that territorial males tended to act aggressively towards other males whose wings were partially painted, and sexually towards females irrespective of wing area painted. In Z. lanei, both body size and fat content play a role in defining territoriality. By subduing competitors and dominating preferred locations within high‐quality sites, these males are likely to be visited by females and engage in mating.     

Male body size and mating success in swarms of the mayfly Epeorus longimanus

Epeorus longimanus is a widely distributed mayfly in the western United States that forms relatively large mating swarms. The operational sex ratio of swarms is highly male biased and males are potentially polygynous, suggesting that male-male competition over mates may be intense. We investigated whether body size influenced male mating success in E. longimanus , as evidence of sexual selection. Males collected as mating pairs had significantly greater body lengths compared with males collected randomly from the swarm on each of six sampling dates examined, and had significantly greater head widths than males from random collections on two dates. There was no indication that large males occupied preferred positions within the swarm, and we suspect that the large male advantage may be due to greater success in pursuing females. We found no evidence of size-assortative mating in E. longimanus indicating that males attempt to male with every female encountered, consistent with the brief copulatory period in mayflies and overall low parental investment of males.