Pseudacteon Species Research Articles

A Species delimitation approach to uncover cryptic species in the South American fire ant decapitating flies (Diptera: Phoridae: Pseudacteon).

South American fire ant decapitating flies in the genus Pseudacteon (Diptera: Phoridae) are potential biocontrol agents of the invasive fire ants Solenopsis invicta and S. richteri in the United States and other regions of the world due to their high host specificity and the direct and indirect damage to their host ants. Despite their importance and the fact that several flies have already been released in the US, little is known about the genetic variability and phylogenetic relationships of Pseudacteon flies parasitizing South American fire ants in the Solenopsis saevissima species-group. A species delimitation analysis was conducted using a distance-based method (ABGD) and two tree-based methods (GMYC and mPTP) using COI sequences of 103 specimens belonging to 20 of the 22 Pseudacteon species known from southern South America. Additionally, phylogenetic relationships between the already described and new candidate species were inferred using mitochondrial (COI) and nuclear (wingless) sequences. The species delimitation analysis suggests that species richness in these flies has been previously underestimated, due to the existence of putative cryptic species within nominal Pseudacteon obtusus, P. pradei, P. tricuspis, P. cultellatus, and P. nudicornis. Geographic distribution and host fire ant species seem to support cryptic lineages, though additional morphological data are needed to corroborate these results. All phylogenetic analyses reveal that South American fire ant decapitating flies are grouped into two main clades, with Pseudacteon convexicauda sister and well differentiated relative to these clades. Neither host nor geographic association appeared to be related to the differentiation of these two main clades within South American fire ant decapitating flies. This work provides information that will allow testing whether the putative cryptic phorid fly species show differences in their effectiveness as biocontrol agents against the highly invasive imported fire ants.

Host preference in parasitic phorid flies: response of Pseudacteon curvatus and P. obtusus to venom alkaloids of native and imported Solenopsis fire ants

Pseudacteon curvatus and P. obtusus are two species of parasitic phorid flies (Diptera: Phoridae), introduced in the southern USA for biological control of Solenopsis fire ants (Hymenoptera: Formicidae). The preference of both parasitoid species (P. curvatus and P. obtusus) for native Solenopsis fire ants (S. geminata complex) versus imported fire ants, S. saevissima complex (S. invicta and S. richteri) was investigated in four-choice olfactometer assays. We hypothesized that differences in the venom alkaloid profiles of fire ant species mediate host preference in phorid flies. In olfactometer bioassays, female P. obtusus showed preference for the venom alkaloid (cis + trans alkaloid mixture) of the imported S. invicta fire ant over the venom alkaloid of the native geminata fire ant. Similarly, P. obtusus showed preference for the venom alkaloid of the S. richteri fire ant over that of the native S. geminata ants. Pseudacteon curvatus also showed preference for the venom alkaloid of the S. invicta and S. richteri fire ants over that of the native geminata ants in separate bioassays. The results suggest that host preference in P. curvatus and P. obtusus is mediated, at least in part, by differences in the venom alkaloid profile of various fire ant species. This finding may explain why Pseudacteon species associated with either the S. geminata or S. saevissima complexes have not switched to the other group despite the overlap of native and imported fire ants in some areas of their distributions.

A new species of the ant-parasitizing genus Pseudacteon Coquillett(Diptera, Phoridae) from China.

A new scuttle fly of the genus Pseudacteon Coquillett, P. trapeziformis, from Mt. Changbai, northeat China is described and illustrated. The new species is distinguished from others of the genus mainly by shorter palp bristles and trapezoid sternite VI. A key to known Pseudacteon species of China is presented.

Rearing and Biology of the Decapitating FlyPseudacteon bifidus(Diptera: Phoridae): A Parasitoid of Tropical Fire Ants

The small decapitating fly, Pseudacteon bifidus Brown and Morrison (Diptera: Phoridae), is a parasitoid of the tropical fire ant, Solenopsis geminata (F.) (Hymenoptera: Formicidae). This fly is of interest as a potential self-sustaining biocontrol agent because tropical fire ants are invasive pests throughout the world's tropics, especially on islands of the Pacific. The objective of this study was to develop methods for mass rearing P. bifidus and to study related aspects of its biology. The flies used in this study were collected near the Nueces River north of Catarina, Texas, USA. We found that P. bifidus parasitizes minor workers with an average head width of 0.71 ± 0.10 mm (0.59–1.15, range). The sex ratio of adult flies was moderately skewed to males (58:42%), with males slightly more likely to emerge from the smallest hosts and females from the largest ones. The average generation time was 30 d at 27.6 °C. Average larval development time was 14 d at 27.6 °C, but the pattern was highly skewed with a mode of 11 d and about 15% of individuals in a long tail of slow developing larvae, which extended out to at least 41 d. Male pupae emerged faster than female pupae (0.8 d, 23.5 °C). Unlike other Pseudacteon species, adult females were not ready to oviposit until 8 to 24 h after eclosure. We were able to rear 9,500 ± 2,800 flies per generation primarily by modifying preexisting rearing procedures (1) to provide adults access to water and sugar water so they could live longer, (2) by extending access to hosts for 1 to 2 extra d, and (3) by avoiding reuse of host colonies with poor rates of parasitism. Labor costs were decreased by rearing in discrete generations and the use of an attack box with automatic temperature, humidity, lighting, and mechanical controls that allowed flies to emerge, mate, and parasitize hosts without the need for constant management. The success of these rearing efforts provided a foundation for subsequent studies of P. bifidus host specificity and host suitability.

Comparing longevity of Pseudacteon species of different sizes: effect of sugar feeding

AbstractSugar feeding is known to enhance lifespan in many parasitoid species. Several species of phorid flies in the genus Pseudacteon (Diptera: Phoridae) have been introduced in the Southern U.S.A. for the biological control of imported fire ants of Solenopsis species (Hymenoptera: Formicidae). Pseudacteon species are short‐lived flies and little is known about their nutritional ecology. Results from previous studies in our laboratory show that one of the introduced species, Pseudacteon tricuspis Borgmeier, is capable of feeding on sugar sources with a significant increase in lifespan. However, in Pseudacteon phorid flies, the degree to which sugar feeding can enhance the lifespan of other species is not clear, nor is it known whether there is a relationship between body size and longevity. In the present study, the effect of sugar feeding on the survival of three phorid fly species of different body sizes, Pseudacteon cultellatus Borgmeier, Pseudacteon curvatus Borgmeier and Pseudacteon obtusus Borgmeier, is investigated. For all three species, flies continuously provided with 40% or 20% sucrose solution live longer than individuals provided water only or 40% sucrose solution only on the first day of adult life. However, the degree to which sugar feeding enhanced longevity varies by species and is highest for P. obtusus. The data also indicate that longevity in Pseudacteon phorid flies is related to body size: the largest species (P. obtusus) lives significantly longer than the smaller species (P. cultellatus and P. curvatus). These results suggest that adults of all three Pseudacteon species are capable of feeding on sucrose solution and that sugar feeding can enhance their longevity. The significance of these results is discussed in relation to the field performance of Pseudacteon phorid flies as biological control agents of imported fire ants.

New species and records of Pseudacteon Coquillett, 1907 (Diptera, Phoridae), parasitoids of the fire ant Solenopsis geminata group (Hymenoptera, Formicidae).

The genus Pseudacteon Coquillett (Diptera, Phoridae) has a worldwide distribution and comprises parasitic myrmecophilous species that decapitate host ants. Seventy one species are known in the genus, 41 of them occur in the Neotropical Region and are 25 from Brazil. In northeastern Brazil, there are only records for two species, Pseudacteon dentiger Borgmeier and Pseudacteon antiguensis Malloch. In this paper, two new species of the genus are described from female specimens, Pseudacteon pesqueroi new spec. and Pseudacteon plowesi new spec., and also, new records of three Pseudacteon species for the Brazilian Northeast are given.

Induced Effects on Red Imported Fire Ant (Hymenoptera: Formicidae) Forager Size Ratios by Pseudacteon spp. (Diptera: Phoridae): Implications on Bait Size Selection.

Red imported fire ants, Solenopsis invicta Buren, are adversely affected by phorid flies in the genus Pseudacteon by instigating defensive behaviors in their hosts, and in turn reducing the efficiency of S. invicta foraging. Multiple Pseudacteon species have been released in Texas, and research has been focused on the establishment and spread of these introduced biological control agents. Field experiments were conducted to determine bait particle size selection of S. invicta when exposed to phorid populations. Four different particle sizes of two candidate baits were offered to foragers (one provided by a pesticide manufacturer, and a laboratory-created bait). Foragers selectively were attracted to, and removed more 1-1.4-mm particles than any other bait size. The industry-provided bait is primarily made of particles in the 1.4-2.0 mm size, larger than what was selected by the ants in this study. While there was a preference for foragers to be attracted to and rest on the industry-provided blank bait, S. invicta removed more of the laboratory-created bait from the test vials. There was an abundance of workers with head widths ranging from 0.5-0.75 mm collected from baits. This was dissimilar from a previous study wherein phorid flies were not active and in which large workers were collected in higher abundance at the site. This implies that phorid fly activity caused a shift for red imported fire ant colonies to have fewer large foragers.

Comparative responses of four Pseudacteon phorid fly species to host fire ant alarm pheromone and analogs

Several species of Pseudacteon phorid flies, including Pseudacteon cultellatus, P. curvatus, P. obtusus, and P. tricuspis, have been introduced in the southern United States for biological control of imported fire ants, Solenopsis spp. Previous studies showed that Pseudacteon flies use the alarm pheromone and venom alkaloids of fire ants to locate host ant workers. A recent study by our group demonstrated attraction of one of the species, P. tricuspis, to certain alkylpyrazine analogs of fire ant alarm pheromone. If effective, alkylpyrazine analogs of fire ant alarm pheromone may provide cost-efficient alternatives as phorid fly attractants. To test the hypothesis that other phorid fly species are attracted to alkylpyrazine analogs of fire ant alarm pheromone, we compared electroantennogram (EAG) and behavioral responses of four Pseudacteon species, P. cultellatus, P. curvatus, P. obtusus, and P. tricuspis, to the commercially available alarm pheromone isomer (2-ethyl-3,6(or 5)-dimethyl pyrazine) and six structurally related alkylpyrazine analogs at physiologically relevant doses (ranging from picogram to 10 µg). The alarm pheromone isomer and two analogs, 2,3-diethyl-5-methyl pyrazine and 2 ethyl-3,5 + 6 methyl pyrazine, elicited the strongest EAG activity in all four species at all tested doses. Results from four-choice olfactometer bioassays testing the above three EAG-active compounds showed that all four species were strongly attracted to the alarm pheromone isomer at doses ranging from 0.001 to 1 µg. The flies also showed significant attraction to the two analogs (2,3-diethyl-5-methyl pyrazine and 2 ethyl-3,5 + 6 methyl pyrazine) compared to the solvent control but not as strong as the attraction elicited by the alarm pheromone isomer. These results confirm that multiple species of phorid flies use fire ant alarm pheromone as a host location cue. However, the flies are not strongly attracted to the tested alkylpyrazine analogs of the alarm pheromone.

Habitat Associations of Red Imported Fire Ants (Solenopsis invicta) and Introduced Populations of Pseudacteon spp. Phorid Flies

Habitat selection is one of many aspects of red imported fire ants, Solenopsis invicta Buren, (RIFA) that has been intensively studied since their initial introduction in the United States. Concurrently, innumerable studies have attempted to identify effective chemical insecticides for RIFA control. More recently, several species of Pseudacteon spp. phorid flies (Diptera: Phoridae) have been intensively evaluated to determine their potential utility in biologica control of RIFA. These flies belong to a suite of natural enemies of RIFA in their native South American range, and have been selected for release in the United States as biological control agents against RIFA. It is hypothesized that phorid flies, through parasitism and suppression of RIFA foraging, will provide measureable RIFA population suppression as phorid ranges expand and fly densities increase. Relatively little scientific attention has been paid to habitat selection and partitioning by these flies in their introduced range(s) in the US. We assessed RIFA and phorid (Pseudacteon curvatus and Pseudacteon tricuspis) densities in four central Texas habitats. The habitat types selected represent a continuum of habitat diversity, micro-habitat availability, and plant heterogeneity. Within the habitat types studied, RIFA mound densitieswere significantly different, but foraging intensity was not significantly different in three of four habitat types. Population densities of P. curvatus were determined to be significantly higher in one habitat than all others. Alternatively, P. tricuspis densities followed a trend within the surveyed habitats that correlated with RIFA densities observed in this study. These data imply that the successful release, establishment, and range expansion of P. curvatus may require a greater degree of critical consideration regarding the ecologyof each system into which they are released, as opposed to P. tricuspis. These implications also suggest that the successful establishment of additional phorid species in the US should take into consideration the habitat matrix and habitat-based expansion corridors available to the flies.

Types of Antennal Sensilla of Three Pseudacteon Species (Diptera: Phoridae) Females That Parasitize Red Imported Fire Ants (Solenopsis invicta) ( Hymenoptera: Formicidae)

While antenna is the main organ for insect to accept the external chemical signals, the antennal sensilla that are diverse in structure and function form the insect receptors in chemical communication. Since a variety of Pseudacteon species are important natural enemies of the red imported fire ant, Solenopsis invicta Buren (. invicta), to elucidate the types of Pseudacteon sensilla will promote the study and understanding of the selection behavior of Pseudacteon in parasitizing . invicta. This study has used scanning electron microscope (SEM) to observe and investigate the female’s antennal sensilla of three Pseudacteon species, the Pseudacteon (P.) litoralis, P. obtusus, and P. tricuspis, and demonstrated that there are four types of sensilla, the trichoid, basiconic, coeloconic, and chaetic sensilla, on their antennal flagellum. Among them, the former three are common in all three species, with trichoid sensillum as mostly abundant, while the chaetic sensillum exists only in the antennae of P. obtusus. The trichoid sensilla exhibit significant interspecies variations and are further classified into two subtypes based on the presence or absence of protrusions, the surface of which contains different shades of groove-like or irregular punctate structures. The basiconic sensilla resemble short spines with densely porous structures on the surface and are in the length of 7.3-9.8 μm and the width of 1.3-1.6 μm, upright or slightly bent. The coeloconic sensilla are irregularly formed in the middle and base of the flagellum, without surface pores; each coeloconic sensillum has eight finger-like folds in unequal lengths, while the end of the fold resembles a blunt cone. The chaetic sensilla enlarge at the base, possess multiple fold-like structures and fine-tipped ends, and are approximately 5 μm in length.

Fire ant alarm pheromone and venom alkaloids act in concert to attract parasitic phorid flies, Pseudacteon spp.

Pseudacteon tricuspis, Pseudacteon obtusus and Pseudacteon curvatus are three species of parasitic phorid flies (Diptera: Phoridae), which have been introduced as classical biological control agents of imported, Solenopsis fire ants (Hymenoptera: Formicidae) in the southern USA. Previous studies demonstrated the behavioral response of P. tricuspis to the venom alkaloids and alarm pheromone of the fire ant, S. invicta. In the present study, we compared the responses of P. tricuspis, P. obtusus and P. curvatus to Solenopsis invicta alarm pheromone, venom alkaloids, or a mixture of both chemicals in four-choice olfactometer bioassays. The main hypothesis tested was that the fire ant alarm pheromone and venom alkaloids act in concert to attract Pseudacteon phorid flies. Both sexes of all three Pseudacteon species were attracted to low doses of the fire ant alarm pheromone or venom alkaloids (i.e. 1 ant worker equivalent) alone. However, the flies were significantly more attracted to a mixture of both chemicals (i.e., 1:1 mixture of alarm pheromone+alkaloids) than to either chemical. The results suggest an additive rather than a synergistic effect of combining both chemicals. Comparing the fly species, P. tricuspis showed relatively greater attraction to cis alkaloids, whereas the alkaloid mixture (cis+trans) was preferred by P. obtusus and P. curvatus. In general, no key sexual differences were recorded, although females of P. tricuspis and P. obtusus showed slightly higher response than conspecific males to lower doses of the alarm pheromone. The ecological significance of these findings is discussed, and a host location model is proposed for parasitic phorid flies involving the use of fire ant alarm pheromone and venom alkaloids as long range and short range attractants, respectively.

Occurrence of Phorid Fly (Diptera: Phoridae) Parasitoids of Imported Fire Ants (Hymenoptera: Formicidae) in Georgia

Ten releases of Pseudacteon decapitating flies (Diptera: Phoridae), a classical biological control agent of imported fire ants (Hymenoptera: Formicidae) in the southern United States, were made at 8 different sites in Georgia (USA) over a period spanning 2000 - 2012. Intensive monitoring and survey activities were initiated in 2008 to delineate the dispersal and occurrence of the Pseudacteon spp. in the state. Results show that all 159 counties in Georgia have at least 1 species of Pseudacteon; P. curvatus Borgmeier is established 129 of those counties, P. tricuspis Borgmeier is established in 70 counties, and P. obtusus Borgmeier is established in the county in which it was released. Both P. curvatus and P. tricuspis occur together in 39 counties, and all 3 of the Pseudacteon species occur in the 1 county in which all 3 species were released. These results help target supplemental or additional releases of P. tricuspis, P. curvatus, and P. obtusus as well as releases of other candidate Pseudacteon species.

Dispersal and competitive impacts of a third fire ant decapitating fly (Pseudacteon obtusus) established in North Central Florida

Self-sustaining classical biological control agents offer hope for permanent wide-area control of imported Solenopsis fire ants in the United States because escape from abundant natural enemies left behind in Argentina is a likely reason for unusually high fire ant densities in the United States. The fire ant decapitating fly Pseudacteon obtusus Borgmeier (Diptera: Phoridae) was released as a biocontrol agent of the red imported fire ant (Solenopsis invicta Buren) in Gainesville, FL because it is a common parasitoid of this ant in Argentina and because it has a higher propensity of attacking fire ants along foraging trails than the two Pseudacteon species previously released. Field surveys of a rapidly expanding P. obtusus population (8–12km/yr) proved that this fly was capable of thriving and successfully competing with the much more abundant Pseudacteon curvatus Borgmeier. However, Pseudacteon tricuspis Borgmeier, the first decapitating fly released, was effectively excluded from most sample sites when faced with competition from both P. curvatus and the similar-sized P. obtusus. Despite clear evidence for competitive exclusion, P. tricuspis abundance at sample sites was positively correlated with the abundance of its two competitors—probably because of moderate to strong covariability in the suitability of sample sites for all three congeners. The addition of P. curvatus, the second parasitoid released, increased total parasitism pressure on fire ant populations by about 10-fold. The addition of P. obtusus, the third species, did not measurably improve total guild parasitism rates on imported fire ants in North Central Florida (as assessed by roadside trap counts), but the performance of this species will likely vary with habitat, region, and climate.

Environmental and Habitat Drivers of Relative Abundance for a Suite ofAzteca-AttackingPseudacteonPhorid Flies

Phoridae (Diptera) have widespread impacts on insect communities by limiting host ant behavior. However, phorid-ant interactions may vary with habitat or environmental conditions. Three Pseudacteon species parasitize Azteca instabilis Fr. Smith, a common ant in coffee agroecosystems, and limit A. instabilis foraging, indirectly benefiting other insects. However, little is known about how phorid abundance, behavior, and effects change with environmental conditions. In shaded coffee systems, coffee (Coffea arabica L.) grows under a range of shade conditions and management changes affect species interactions. For example, Pseudacteon spp. more strongly limit A. instabilis foraging in low-shade coffee habitats. We sampled relative abundance of three phorid species around A. instabilis nests in three coffee habitats varying in shade management during dry and wet seasons. We measured canopy cover, tree richness, tree density, leaf litter depth, and number of nearby trees with A. instabilis to determine whether these habitat factors correlate with phorid abundance. P. laciniosus Brown was the most abundant phorid in both seasons. Phorid relative abundance did not differ by habitat, but did differ by season. P. laciniosus accounted for a higher proportion of phorids in the wet season (91.4%) than in the dry season (78.9%), and P. planidorsalis Brown accounted for a larger percent in the dry season (21.1%) than in the wet season (7.3%). Phorid composition did not differ with habitat type, and none of the measured environmental variables correlated with changes in phorid composition. Thus, phorids in coffee agroecosystems respond to large seasonal differences, but not differences between coffee habitats.

Pseudacteon tricuspis: Its Behavior and Development According to the Social Form of Its Host and the Role of Interference Competition Among Females

We studied how the behavior and performance of Pseudacteon tricuspis Borgmeier varies with the social form of its host Solenopsis invicta Buren, in its native range in Argentina where monogyne colonies are more abundant than polygynes (approximately 75 vs. 25%). Female, P. tricuspis took 44% less time (50 vs. 89 s) to attack monogyne than polygyne ants, but oviposition attempts were similar (23 vs. 18 attacks). The presence of the parasitoid affected the average size of foragers on the trail, with the proportion of minor workers increasing on both social forms. In the laboratory, P. tricuspis selected similar host sizes, although pupal survival was 25% higher on monogynes than on polygynes. Developmental times of both genders were similar (33-35 d), although larger females emerged from bigger hosts. The sex ratio of P. tricuspis was more male biased when exploiting polygyne ants. Intraspecific competition significantly affected parasitoid reproductive success, being significantly higher for a solitary female than when three females were present, although the size of workers selected did not vary. The male:female ratio also changed, being 1:1 without competition but 2:1 with competition. We demonstrated for the first time the consequences of interference competition among P. tricuspis females, a common behavior observed in others parasitoids. We discuss why P. tricuspis sex ratios are always biased toward males in both social forms and suggest that similar studies of interference competition within and between already naturalized Pseudacteon species in the United States could help predict establishment patterns.

Distribution ofPseudacteon curvatusandPseudacteon tricuspis(Diptera: Phoridae) in Arkansas

From 1995 to 2009, four Pseudacteon species were released in the U.S. with 3 species, P. curvatus, P. tricuspis, and P. obtusus released in Arkansas. To determine Pseudacteon establishment and expansion, sticky traps were used to monitor phorid fly species at and near 10 release sites, in counties bordering neighboring states, and along regional transects. Pseudacteon flies were captured in 16 Arkansas counties: Ashley, Chicot, Clark, Desha, Drew, Hempstead, Howard, Little River, Montgomery, Nevada, Perry, Phillips, Pike, Polk, Sevier, and Union. Pseudacteon curvatus was found in areas far from release sites, suggesting dispersal from neighboring states. The range of P. tricuspis evidently also expanded from its initial release sites in southern Arkansas.

A Review of Pseudacteon (Diptera: Phoridae) That Parasitize Ants of the Solenopsis geminata Complex (Hymenoptera: Formicidae)

Abstract Some phorid flies of the genus Pseudacteon Coquillett parasitize workers of Solenopsis geminata (F.) complex fire ants. The group is found to comprise at least 21 species of flies, of which 11 are new to science and described here: P. amuletum, P. andinus, P. annulus, P. catarinae, P. deltoides, P. hippeus, P. kungae, P. laticarinatus, P. palomita, P. quinni, and P. robustus. An identification key to the females is included. This expansion of known Pseudacteon species parasitizing the S. geminata complex makes it an interesting system for comparison with Pseudacteon flies that parasitize fire ants of the S. saevissima (Smith) complex.

Distributional Patterns ofPseudacteonAssociated with theSolenopsis saevissimaComplex in South America

Classical biological control efforts against imported fire ants have largely involved the use of Pseudacteon parasitoids. To facilitate further exploration for species and population biotypes a database of collection records for Pseudacteon species was organized, including those from the literature and other sources. These data were then used to map the geographical ranges of species associated with the imported fire ants in their native range in South America. In addition, we found geographical range metrics for all species in the genus and related these metrics to latitude and host use. Approximately equal numbers of Pseudacteon species were found in temperate and tropical regions, though the majority of taxa found only in temperate areas were found in the Northern Hemisphere. No significant differences in sizes of geographical ranges were found between Pseudacteon associated with the different host complexes of fire ants despite the much larger and systemic collection effort associated with the S. saevissima host group. The geographical range of the flies was loosely associated with both the number of hosts and the geographical range of their hosts. Pseudacteon with the most extensive ranges had either multiple hosts or hosts with broad distributions. Mean species richnesses of Pseudacteon in locality species assemblages associated with S. saevissima complex ants was 2.8 species, but intensively sampled locations were usually much higher. Possible factors are discussed related to variation in the size of geographical range, and areas in southern South America are outlined that are likely to have been under-explored for Pseudacteon associated with imported fire ants.

Cytogenetic analysis of three species of Pseudacteon (Diptera, Phoridae) parasitoids of the fire ants using standard and molecular techniques.

Pseudacteon flies, parasitoids of worker ants, are being intensively studied as potentially effective agents in the biological control of the invasive pest fire ant genus Solenopsis (Hymenoptera: Formicidae). This is the first attempt to describe the karyotype of P. curvatus Borgmeier, P. nocens Borgmeier and P. tricuspis Borgmeier. The three species possess 2n = 6; chromosomes I and II were metacentric in the three species, but chromosome pair III was subtelocentric in P. curvatus and P. tricuspis, and telocentric in P. nocens. All three species possess a C positive band in chromosome II, lack C positive heterochromatin on chromosome I, and are mostly differentiated with respect to chromosome III. P. curvatus and P. tricuspis possess a C positive band, but at different locations, whereas this band is absent in P. nocens. Heterochromatic bands are neither AT nor GC rich as revealed by fluorescent banding. In situ hybridization with an 18S rDNA probe revealed a signal on chromosome II in a similar location to the C positive band in the three species. The apparent lack of morphologically distinct sex chromosomes is consistent with proposals of environmental sex determination in the genus. Small differences detected in chromosome length and morphology suggests that chromosomes have been highly conserved during the evolutionary radiation of Pseudacteon. Possible mechanisms of karyotype evolution in the three species are suggested.

A new species of Pseudacteon Coquillett (Diptera, Phoridae) and a new key to the European species

The distinction between Pseudacteon fennicus Schmitz 1927 and P. lundbecki Schmitz 1924 is clarified, and a neotype designated for the latter species. Following this P. lusitanus Schmitz 1938 is synonymised with P. fennicus. P. charnayensis n. sp. is described from France. A revised key to the European species of Pseudacteon is provided.