ANT DISPERSAL OF CALATHEA (MARANTACEAE) SEEDS BY CARNIVOROUS PONERINES (FORMICIDAE) IN A TROPICAL RAIN FOREST

Two tropical herbaceous plant species, Calathea microcephala and Calathea ovandensis, exhibit morphological adaptations for seed dispersal by ants. In the field, 21 ant species are attracted to the arils of the seeds. Previously known as predatory carnivores, the ponerine ants Odontomachus laticeps, O. minutus, Pachycondyla harpax, and P. apicalis carry the seeds toward their nests, behaving as though they were carrying prey. These ants remove the aril in the nest, and seeds without arils germinate more readily than seeds with arils. Ant foraging distances can account for seedling distributions. Myrmecochory may be far more common in tropical rain forests than has been previously suspected.     

Nuptial flights and calling behaviour in the antLeptothomx acervomm (Fabr.)

Summary We describe in detail a very large nuptial flight of the antLeptothorax acervorum at an open hilltop site in Britain. The mating behaviour of these ants involved not only a large mating swarm but also sexual/calling behaviour by the females. The females left the flight to land on vertical objects, where they took up a characteristic calling posture, in which females of closely related species are known to release pheromones that are sexually attractive to males. ThatLeptothorax acervorum has a complex mating behaviour involving both large nuptial flights and sexual calling has important consequences for the interpretation of the evolution of polygyny in this species and social parasitism in its close relatives.     

A Study of the Behaviour of the Australian Field Cricket Teleogryllus commodus (Walker) (Orthoptera: Gryllidae) in the Field and in Habitat Simulations

The behaviour of T. commodus (Walker) in the field and in simulated natural habitats is described. Regular predictable sequences of behaviour are represented by an ethogram. Most behaviour was centred around burrows or sheltered sites. Burrows appeared to be a limiting resource and there may be behavioural dimorphism in ♂ ♂ (burrower and non-burrower strategies). Individuals were found to form gregarious clusters and appeared to be attracted to areas already occupied by crickets. Territorial spacing and other social interactions occurred within these clusters. ♂ ♂ and ♀ ♀ were found to be mobile, and moved continually between sites. The movement of ♂ ♂ and ♀ ♀ between burrows (calling sites) was measured. Adult ♂ ♂ never remained at a burrow for longer than two days. In simulated natural habitats ♂ ♂ moved in response to two factors: first, and most common, as a result of eviction by other ♂ ♂ second, as a response to the absence of ♀ ♀. When cricket density was increased in habitat simulations crickets clustered at calling sites. The adaptive significance of T. commodus behaviour is discussed.     

An unusual,but not unexpected,evolutionary step taken by syrphid flies: the first record of true primary parasitoidism of ants by Microdontinae

Parasitoidism of ants by flies is known only for Phoridae and Tachinidae. We report the first record of a hoverfly (Syrphidae, Microdontinae, Hypselosyrphus) acting as a true primary parasitoid of ants. Previously, larvae of Microdontinae were known only as obligate predators of ant brood. This is also the first host record for any Hypselosyrphus species, the first reliable record of an association between a syrphid fly and a ponerine ant, and the first record of a dipteran parasitoid developing upon the immature stages of ants. We reared adults of Hypselosyrphus trigonus from cocoons of the arboreal ponerine ant, Pachycondyla villosa, nesting in Aechmea bracteata bromeliads in southern Quintana Roo, Mexico, and we succeeded in determining various aspects of the parasitoid's life history. The findings obtained in the present study provide novel insights into the evolutionary transformation and diversification of the feeding habits of microdontine syrphid larvae, from an obligatory, specific predatory association with the ant brood, to parasitoidism of ant prepupae. We also highlight the need for more detailed studies of the interactions of arboreal ants and their parasites. We conclude with an overview of the evolutionary transitions and diversification of larval feeding habits that have taken place within the family Syrphidae. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 111 , 462–472.     

Sexual Interference and Sexual Defense in the Smooth Newt,Triturus vulgaris (Amphibia,Urodela, Salamandridae)

• 1 When a male smooth newt encounters a ♀ who is already engaged in courtship, he may mimic her behaviour during the spermatophore deposition and transfer stages of the courtship. He thereby usurps the courting ♂ and may inseminate the ♀ himself. Such sexual interference depresses the short-term, and perhaps long-term, mating success of the courting ♂.
• 2 In the presence of a potential rival, the courting ♂ alters certain aspects of his sexual behaviour. He displays more intensely to the ♀ and attempts to draw her away from the rival by increasing the duration of his display. He may also “check” that it is the ♀, and not the rival, who will elicit the deposition of a spermatophore from him. These changes in the behaviour of the courting ♂ are interpreted as defense against sexual interference.
• 3 Female smooth newts may be multiply inseminated as a consequence of sexual interference; this may result in sperm competition. However, ♀♀ seem to find competitive interactions between ♂ ♂ “aversive”.
• 4 Sexual interference by ♀-mimicry and associated defensive behaviour patterns are common in the urodele amphibians. Interference can be thought of as a “side-payment” conditional mating strategy.

     

Isolation and characterization of five microsatellite loci in the ponerine ant Pachycondyla inversa (Hymenoptera,Formicidae)

We characterized five polymorphic DNA microsatellite loci for the neotropical ponerine ant Pachycondyla inversa. The variability was initially tested in 19 workers from nine colonies from a Brazilian population. We found five to 12 alleles per locus, with observed heterozygosities between 0.72 and 0.95. The allele size ranged from 73 to 197 bp. The primers also successfully amplified DNA at all five loci in the closely related species P. villosa.     

The unusual social organization of the antPachycondyla tridentata (Formicidae,ponerinae)

Colonies of the ponerine antPachycondyla tridentata from Malaysia occur with and without queens. In a total of 7 colonies we found more than 80% of the workers to be mated, irrespective of the presence or absence of queens. This is a hitherto unknown social organisation in ants. Queens and workers competed equally for reproduction. In the colonies investigated several ants were laying eggs. Behavioral observations revealed persistent dominance interactions between colony members. A few ants, but not necessarily a queen, occupied top positions. Removal of the most dominant ants led to a new hierarchy in which subordinate ants with developed ovaries were attacked significantly more frequently than non-reproductive ants. On the average, callows were more aggressive than older subordinate ants, displacing most of the older laying workers in one colony. Nestmate recognition tests revealed that non-reproductive ants were much more aggressive towards foreign ants than were ants with developed ovaries.     

Hybridization and F1 backcrosses between two closely related tropical species of sea urchins (genus Echinometra) in Okinawa

Summary

Four species of sea urchins of the genus Echinometra, designated species A, B, C, and D, occur along the coast of Okinawa; they are distinguished by color pattern and other characteristics. The two most distinct species, Echinometra sp. A (Ea) with white-tipped spines and Echinometra sp. D (Ed) with nearly black spines (possibly E. oblonga), were examined for potential hybridization through a series of fertilization experiments and rearing of the resulting hybrids. Fertilization was reciprocally asymmetrical; Ed ova were readily fertilized by sperm of Ea, but less than 20% of the ova of Ea were fertilized by sperm of Ed. Nevertheless, hybrids resulting from crosses in both directions developed normally through larval and juvenile stages to produce sexually mature adults. Larvae of Ed♀×Ea♂ and Ed♂×Ed♀ were larger than those of Ea♂ × Ed♀ and Ea♂ × Ea♀, reflecting the larger eggs of Ed. However, Ea♂×Ea♀ juveniles and later stages, up to maturity, were consistently larger than those of equivalent ages of the other crosses; largest to smallest were Ea♂×Ea♀, Ed♂×Ea♀, Ea♂ × Ed♀, and Ed♂×Ed♀, respectively. Ova and sperm of the hybrids were viable and fertilizable in all combination of crosses, although fertilization of those of Ed♂×Ea♀ hybrids was consistently higher than those of Ea♂ × Ed♀ hybrids. These experiments represent the first reported successful production of hybrid sea urchins with viable gametes. They indicate that reproductive isolation is achieved by prezygotic isolating mechanisms in these two species. Of these mechanisms, gametic incompatibility is probably only partly involved, if at all, and differences in spawning times, habitat segregation, or other factors appear to be important for maintaining reproductive isolation in these closely related species.     

How ants find each other; temporal and spatial patterns in nuptial flights

Reproduction is a key factor in understanding population ecology and therefore species occurrence. However, patterns in reproductive behaviour for distinct ant species remain insufficiently known. In this paper strategies in mate finding are studied for six ant species (Lasius niger, Lasius umbratus, Temnothorax nylanderi, Myrmica rubra, Myrmica ruginodis, Stenamma debile) in a forest – forest edge – agricultural field gradient. Using window traps, we studied whether these species had a restricted nuptial flight season, displayed swarming behaviour, and whether the alates aggregated at the forest edge. The flight season was limited to one month or less for L. niger, T. nylanderi, M. rubra, M. ruginodis and S. debile. Swarming behaviour occurred in all but one (L. umbratus) species. Although none of the six species seemed to have highest nest density at the forest edge, three of them, M. rubra, M. ruginodis and S. debile, showed male aggregations there, indicating this to be the main reproduction site. This last finding could be due to a more suitable micro-climate, but most likely, edges are conspicuous land marks which are used by ants to meet mates. The behavioural patterns of ant sexuals at the forest edge can influence dispersal possibilities in fragmented landscapes, reproductive success and nest densities. Received 27 November 2007; revised 27 March; accepted 5 April 2008.     

First Record of Polydomy in a Monogynous Ponerine Ant: A Means to Allow Emigration Between Pachycondyla goeldii Nests

The ponerine ant Pachycondyla goeldii is a monogynous (i.e. one queen per colony) arboreal species that colonizes pioneer areas. Founding queens and first generation workers initiate their own ant garden by building a cardboard-like structure into which epiphyte seeds are integrated. Following the growth of the epiphyte, the colony establishes its nest within the root system. This particular nest-building behavior is crucial in an environment where suitable nest sites are rare. Nevertheless, the slow growing process of ant gardens does not allow this species to readily evacuate and find another refuge in the advent of an attack by a predator or worsening climatic conditions. Previous field studies of P. goeldii were performed after forest destruction and subsequent colonization by P. goeldii. As a result, the colonies studied where relatively young and monodomous (i.e. one nest per colony). Our study of nest composition, worker exchanges between ant gardens in the field, and dyadic encounters shows that mature colonies of P. goeldii are polydomous (i.e. multiple nests per colony). In ants, the association of polydomy with monogyny has infrequently been reported. To our knowledge, P. goeldii represents the first record of a Ponerinae exhibiting both these particular characteristics. Our field and laboratory experiments suggest that polydomy is adaptively advantageous in coping with the microclimatic instability of pioneer areas by providing colonies with easily accessible nests.     

Interaction between ants and fruits of<Emphasis Type="Italic"> Guapira opposita</Emphasis> (Nyctaginaceae) in a Brazilian sandy plain rainforest: ant effects on seeds and seedlings

This study examines the dispersal system of Guapira opposita in a tropical sandy rainforest in southeast Brazil. Guapira trees produce small fruits with a high protein content (28.4%) and low lipid content (0.3%), and the plant is primarily dispersed by birds. Mature fruits of G. opposita can fall spontaneously with the pulp intact, or be dropped by birds with bits of pulp attached. In either case, ground-dwelling ants rapidly remove the fruits to their nest (93% after 12 h). The ponerine ants Odontomachus chelifer and Pachycondyla striata are the main seed vectors among the ants, and together account for 56% (20 of 36) of the ant-fruit interactions recorded on the forest floor. Individual workers of O. chelifer and P. striata transport single fruits to their nests. Bits of pulp are fed to larvae and worker nestmates, and intact seeds are discarded outside the nest. Germination success in Guapira is higher for cleaned seeds (pulp removed) than for seeds coated by pulp. Guapira seedlings and juveniles are more frequent close to Odontomachus nests than at sites without such nests. Soil samples from Odontomachus nests had greater penetrability, and higher concentrations of P, K, and Ca than random soil samples. Field experiments suggest that the association between G. opposita seedlings and O. chelifer nests can potentially render the plant some protection against herbivores. Results indicate that fruit displacement by ponerine ants play an important role in the biology of G. opposita seeds and seedlings in the sandy forest, and illustrate the complex nature of the dispersal ecology of tropical tree species.     

A new ethological test to study nestmate recognition in adult ants

Summary An ethological test was designed to investigate nestmate recognition in adult ants. Contrary to classic dyadic tests, it consisted of a choice situation where a tested-worker was faced with nestmate and non-nestmate workers. These were kept alive, tied down, and thus immobilised in the neutral arena to record the tested-worker's reactions independently of the behaviour of the others. Such tests, applied to the ponerine ant,Ectatomma tuberculatum, enabled us to record various measures of discrimination and to limit aggressive behaviours between alien ants, which often leads to serious injury in dyadic tests. They also demonstrated the influence of age on discrimination behaviour in this species.     

Das Paarungsverhalten von Pycnopogon fasciculatus Loew (Diptera,Asilidae)

• 1 Bei der Raubfliege Pycnopogon fasciculatus vollführt das ♂ vor dem sitzenden ♀ eine Flugbalz, die hauptsächlich anhand von Filmaufnahmen analysiert wurde. Das ♂ schwebt bei der Flugbalz dicht vor dem ♀, schwenkt dann bis 10 cm nach hinten aus, um auf der gleichen Flugbahn stoßartig nach vorne vor das ♀ zu fliegen. Diesen Hin- und Herflug wiederholt das ♂ durchschnittlich 28mal. Zu Beginn der Flugbalz orientiert sich das ♂ durch Ertasten des weiblichen Vorderendes. Das ♂ hält die Beine an den beiden Umkehrpunkten der Flugbahn ganz verschieden (vgl. Abb. 2 c), zeigt aber keine Landehaltung. Flugtechnisch scheint die Veränderung der Beinstellung wenig Bedeutung zu haben. Ob die Beinhaltung vor dem ♀ aus einer ritualisierten Beuteübergabe hervorgegangen ist, bleibt offen. Das ♀ zeigt seine Abwehrbereitschaft bei der Flugbalz durch ritualisiertes Wackeln hochgehaltener Hinterbeine.
• 2 Das ♂ vollführt seine Flugbalz auch vor toten, artgleichen ♀ und ♂♂ und auch vor artfremden, z. T. größeren Fliegen. Die Flugbalz ist kürzer als vor lebenden ♀♀ (durchschnittlich nur 14 Anflüge) und führt wegen des Fehlens von Abwehrsignalen des ♂ doppelt so haufig zu einem unmittelbar anschließenden Kopulationsversuch.
• 3 Sitzende ♂♂ schlagen gegen anfliegende ♂♂ ihre Vorderbeine schnell abwechselnd in die Luft. Dieses Vorderbeinwirbeln tritt auch beim Schweben des ♂ vor dem ♀ während der Balz auf und wird als Abwehrhandlung gedeutet. Durch das Vorderbeinwirbeln gegen andere ♂♂ kommt es unter natürlichen Umständen nicht zur Flugbalz vor sitzenden ♂♂.
• 4 ♀♀ vertreiben arteigene ♀♀ und artfremde Fliegen häufig von ihrem Sitzast durch Anlaufen mit schräg angestellten, seitlich abgespreizten Flügeln.
• 5 Das ♂ versucht in 31 % der Fälle die Kopulation unmittelbar nach der Flugbalz (n = 96). Nicht Hochhalten der Hinterbeine des ♀ löst beim ♂ einen Kopulationsversuch aus. Die Kopulation dauert mindestens 15 min und vollzieht sich in der “end-to-end position”.
• 6 Das Paarungsverhalten von Pycnopogon fasciculatus läßt sich als Handlungskette darstellen (Tab. 2).
• 7 Das Paarungsverhalten der räuberischen, in einigen Fällen auch kannibalischen Asilidenfamilie ist sehr unterschiedlich. Außer Flugbalz ist Balz auf fester Unterlage und Überwältigung des ♀ durch das ♂ ohne vorausgehende Balzhandlungen bekannt.

     

Functional Monogyny and Dominance Hierarchy in the Queenless Ponerine Ant Pachycondyla (=Bothroponera) sp. in West Java,Indonesia (Hymenoptera,Formicidae, Ponerinae)

Social organization of the queenless ponerine ant Pachycondyla (Bothroponera) sp. was studied in Bogor, West Java, Indonesia. Colonies had no morphologically distinct queens. Worker number per colony was 11 on average. Most colonies contained multiple mated workers (gamergates), however, only one of them laid eggs, whereas additional gamergates were sterile or ovipositing at a low rate, indicating that this ant species shows functional monogyny. Some virgin workers also developed their oocytes and actually laid eggs. Ritualized dominant — subordinate behavior was frequently observed and a linear dominance hierarchy was established among workers. The topranking individual was a fertile gamergate. Sterile gamergates positioned in lower ranks and they often engaged in foraging. Among virgin workers, callows and young workers monopolized higher ranks.     

Reproductive isolating mechanisms contributing to asymmetric hybridization in Killifishes (Fundulus spp.)

When species hybridize, one F1 hybrid cross type often predominates. Such asymmetry can arise from differences in a variety of reproductive barriers, but the relative roles and concordance of pre-mating, post-mating prezygotic, and post-zygotic barriers in producing these biases in natural animal populations have not been widely investigated. Here, we study a population of predominantly F1 hybrids between two killifish species (Fundulus heteroclitus and F. diaphanus) in which >95% of F1 hybrids have F. diaphanus mothers and F. heteroclitus fathers (D♀ × H♂). To determine why F. heteroclitus × F. diaphanus F1 hybrids (H♀ × D♂) are so rare, we tested for asymmetry in pre-mating reproductive barriers (female preference and male aggression) at a common salinity (10 ppt) and post-mating, pre-zygotic (fertilization success) and post-zygotic (embryonic development time and hatching success) reproductive barriers at a range of ecologically relevant salinities (0, 5, 10, and 15 ppt). We found that F. heteroclitus females preferred conspecific males, whereas F. diaphanus females did not, matching the observed cross bias in the wild. Naturally rare H♀ × D♂ crosses also had lower fertilization success than all other cross types, and a lower hatching success than the prevalent D♀ × H♂ crosses at the salinity found in the hybrid zone centre (10 ppt). Furthermore, the naturally predominant D♀ × H♂ crosses had a higher hatching success than F. diaphanus crosses at 10 ppt, which may further increase their relative abundance. The present study suggests that a combination of incomplete mating, post-mating pre-zygotic and post-zygotic reproductive isolating mechanisms act in concert to produce hybrid asymmetry in this system.     

Group hunting in a ponerine ant,Leptogenys nitida Smith

Field observations on the emigration and foraging behaviours of the southern African ponerine ant, Leptogenys nitida, were undertaken at Mtunzini, Natal, South Africa. These colonies have a single ergatoid queen and 200–1000 workers. The nest sites are found in the leaf litter and these nests are moved frequently over distances ranging from 0.5 to 5 m. Leptogenys nitida is a diurnal predator of arthropods dwelling in the leaf litter. Up to 500 workers participate in each foraging trail, and are not led by definite scouts. Ants form clear trunk trails and fan out at various intervals to search for prey. The prey is searched for and retrieved cooperatively. From laboratory tests it was determined that ants will follow pygidial gland extracts, with the poison gland extract eliciting a limited response. The type of army ant behaviour observed in L. nitida seems to be different to that observed in other ponerine ants.     

Le Plantule Della Flora Trapanese

Summary

Secondary wood growth in both a ♂ and a ♀ plant of Schinus Molle has been studied in Bari, Puglia.

The bulk of the secondary wood has been found to be made up of substitution fibres full of starch grains, which it would be more appropriate to call fusiform wood parenchyma cells (according to the « Glossary of Terms Used in Describing Woods», in Tropical Woods, N° 36, 1933).

There are no annual rings: the thickening is due to the addition of successive «growth layers», 3 or 4 per year.

The cambium does not undergo any evident resting period, neither in summer nor in winter, but only periods of suspended activity: the periods of suspended activity do not occur simultaneously in the ♂ and ♀ plants, but in different months.

Each period of cambial activity passes through the three stages of initial, maximum, and decreasing intensity and lasts for 2–3–4 months depending on the season and the sex.

Tipically the growing period starts with the deposition of new elements; next follows lignification of these same elements; during the third stage starch grains appear in the fusiform and radial parenchyma cells of the newly formed wood.

During 1952, 4 growth layers have been formed in the ♂ and 3 in the ♀ plant. In general each episode is slower and more diluted in time in the ♀ plant as compared to the ♂ one. A diversity in cambial behaviour as regards spring reawakening, between the two sexes of dioiceous plants had been already observed in Rhamnus Alaternus (De Paolis, 1949); in both species the ♂ plant appears to be more sensitive and more speedy in its reactions than the ♀ one.

The author does not know of any research on cambial activity in S. Molle carried out in Peru, the native land of this species. Comparing its cambial activity as revealed by the present researches with that of other evergreen plants spontaneous in Puglia the existence of an essential difference is demonstrated: while in S. Molle cambial activity is subcontinuous, in the Mediterranean evergreen trees and shrubs so far studied the cambium undergoes two resting periods, one in winter, and a more or less well defined one in summer. Moreover cambial activity in S. Molle can be taken as continuous if the plants of both sexes are considered, owing to their asinchronism (the same phenomenon happens in flowering times). This lack of a well defined growth cycle corresponds to a similar lack of a well defined seasonal cycle in the climate of the interandine Peruvian valleys, the original habitat of S. Molle, where the mean temperatures of the coldest months vary from 5,8° C to 11,9° C, while those of the hottest months vary from 10,3° C to 15,9° C, with yearly excursions of 4,5 to 5° C.

On the other hand the type of secondary th ckening by successive apposition of wood layers recalls closely that of a group of Mediterranean plants studied in Puglia, that is Ficus carica (Giannuoli, 1949), Olea europaea (Armenise, 1950), and Phillyrea latifolia (Maranò, 1953). In the case of these species the authors had suggested that they belonged, or were related to genera which had originated or developed in regions lacking a well defined seasonal climatic cycle. This hypothesis is confirmed by the fact that S. Molle, a recently imported tropical plant has maintained in our Mediterranean climate not only its original type of secondary thickening, but also its original growth rythm. S. Molle can be considered an experimental test of an hypothesis based so far on the interpretation of observation data.     

Tandem Recruitment and Foraging in the Ponerine Ant <Emphasis Type="Italic">Pachycondyla harpax</Emphasis> (Fabricius)

Tandem running is a common recruitment strategy in ant species with small colony sizes. During a tandem run, an informed leader guides a usually naïve nestmate to a food source or a nest site. Some species perform tandem runs only during house hunting, suggesting that tandem running does not always improve foraging success in species known to use tandem running as a recruitment strategy, but more natural history information on tandem running under natural conditions is needed to better understand the adaptive significance of tandem recruitment in foraging. Studying wild colonies in Brazil, we for the first time describe tandem running in the ponerine ant Pachycondyla harpax (Fabricius). We asked if foragers perform tandem runs to carbohydrate- (honey) and protein-rich (cheese) food items. Furthermore, we tested whether the speed and success rate of tandem runs depend on the foraging distance. Foragers performed tandem runs to both carbohydrate food sources and protein-rich food items that exceed a certain size. The probability to perform a tandem run and the travelling speed increase with increasing foraging distances, which could help colonies monopolize more distant food sources in a competitive environment. Guiding a recruit to a food source is costly for leaders as ants are ~66% faster when travelling alone. If tandem runs break up (~23% of all tandem runs), followers do not usually discover the food source on their own but return to the nest. Our results show that tandem running to food sources is common in P. harpax, but that foragers modify their behaviour according to the type of food and its distance from the nest. Competition with other ants was intense and we discuss how tandem running in P. harpax might help colonies to build-up a critical number of ants at large food items that can then defend the food source against competitors.     

The effects of host race, gender, and host plant distribution on alighting behavior, mating, and oviposition in Eurosta solidaginis

Eurosta solidaginis Fitch (Diptera: Tephritidae) has formed host races on Solidago altissima L. and Solidago gigantea Ait. (Asteraceae), and reproductive isolation between these host races is brought about in part by host‐associated assortative mating. Any non‐assortative mating creates the potential for gene flow between the populations, and we investigated the conditions that favored non‐assortative mating. We hypothesized that the frequency of non‐assortative mating would be influenced by differences in the behaviors of the host races and sexes and by the presence and pattern of distribution of the two host species. To test these hypotheses, we caged flies on four combinations of 32 potted host plants: all S. altissima, all S. gigantea, and cages with both host species arranged in either two pure species blocks or randomly dispersed. We recorded the number of flies of each host race that alighted on each host species and the frequency of mating within and between the host races. Males of both host races were observed on plants more frequently than females. Flies of the host race from S. gigantea (gig flies) were observed on plants in greater absolute numbers, and they mated more frequently than flies of the host race from S. altissima (alt flies). In all treatments, gig flies of both sexes were found on non‐natal host plants significantly more frequently than alt flies, and gig females showed a weaker preference for their host species than did gig males or alt flies of either gender for their respective natal hosts. Assortative mating predominated in all treatments, and flies from each host race mated more frequently in cages containing their own host plant. The frequency of non‐assortative mating varied among treatments, with the matings between alt ♀ × gig ♂ being more common in the pure S. altissima treatment and the gig ♀ × alt ♂ being more frequent in the pure S. gigantea and random treatments. Matings between gig ♂ × alt ♀ were more common overall than the reciprocal mating, because gig males were more active in pursuing matings and in alighting on the non‐natal host plant than alt flies. Non‐assortative matings were more frequent in the random than in the block treatments, but this difference was not significant. Because of strong selection against oviposition into the alternate host, we hypothesized that host plant distribution would not affect oviposition preference. We tested this hypothesis by examining the oviposition behavior of naïve, mated females in two treatments in which both host species were present: either arranged in blocks or randomly dispersed. Females oviposited only into their natal host, regardless of host plant distribution.