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L'organisation de la structure musculaire de l'intestin moyen des femelles de l'espece ornithophile Simulium rugglesi et de l'espece mammalophile Simulium venustum a ete examinee par microscopie confocale. De plus, l'association entre les muscles longitudinaux et circulaires de S. rugglesi alimentes a ete determinee par microscopie electronique a transmission. Les muscles longitudinaux et circulaires des deux especes sont places en arrangement orthogonal pour former une structure en grille. Chez S. rugglesi, le nombre de muscles longitudinaux a ete estime a 181 et le nombre de muscles circulaires, a 90, alors que, chez S. venustum, il y a environ 156 muscles longitudinaux et 69 muscles circulaires. Les muscles longitudinaux ont un espacement inegal et ils sont arranges en paquets de faisceaux paralleles. La bifurcation des muscles longitudinaux se produit au hasard le long de l'intestin moyen chez S. rugglesi, mais chez S. venustum, elle se situe anterieurement et posterieurement a la partie la plus renflee de la region posterieure de l'intestin moyen. Les muscles circulaires affichent moins de variation dans l'espacement de leurs faisceaux et leur embranchement est plus regulier. Les faisceaux de muscles circulaires partagent des fibres avec les faisceaux voisins, ce qui les rattache tout le long de l'intestin moyen. L'observation d'intestins moyens de S. rugglesi au microscope electronique a permis de constater l'existence de connections entre les muscles longitudinaux et les muscles circulaires. La comparaison de ces resultats avec ceux obtenus chez les moustiques revele que l'organisation et la structure de la musculature de l'intestin moyen sont semblables chez les deux groupes.

The polytene chromosomes of Simulium noelleri from 16 sites in Armenia, Canada, England, Germany, Russia, and Sweden were analyzed. A standard map is proposed for the S. noelleri species-group that is 11 fixed inversions removed from the original standard map of the subgenus Simulium. Based on one shared subterminal inversion in the IIIL arm, the S. noelleri species-group is in a trichotomy with the S. bezzi and S. ornatum species-groups. Reproductive isolation of S. noelleri and S. decorum is demonstrated chromosomally at sites where both occur together. All analyzed populations of S. noelleri are considered conservatively to represent a single species, although at least three cytotypes are recognized, based primarily on different sex-chromosome systems. Cytotype A, representing the type species of S. noelleri, occurs in Germany, England, and western Russia. Cytotype B occupies Sweden and western Canada, emphasizing the intimate connection between the northern Nearctic and Palearctic simuliid faunas. Cytotype C is known only from Armenia.

Mitochondrial DNA (mtDNA) was extracted from pooled field-collected samples representing six species of black flies (Cnephia dacotensis, Simulium bivittaum, S. johansenni, S. luggeri, S. piperi, S. vittatum) and compared by restriction fragment length polymorphism (RFLP) analysis. Morphospecies were molecularly distinct, with few shared restriction fragments. Eleven populations of S. vittatum were found that appeared to be homogeneous for a single mitochondrial haplotype. Ten other populations of S. vittatum showed extensive mitochondrial heterogeneity. In part, these samples contained mixtures of two cytologically recognized siblings: IIIL-1 and IS-7. About 70% of the mitochondrial genome of a population pure for sibling IIIL-1 was cloned as five HindIII fragments, which were used as hybridization probes to examine individual black flies. Thirteen mtDNA haplotypes involving permutations of 10 HindIII restriction sites were identified in individual black flies examined from 26 populations. DNA from 168 larvae cut with both EcoR1 and HindIII revealed five additional haplotypes. One HindIII haplotype was present in 84% of 390 larvae examined and predominated in every population examined from New York to California and in both the IIIL-1 and IS-7 siblings. Nebraska populations had individuals with nearly all known haplotypes. The most common haplotype was usually the only form present in warm, silty streams with organic enrichment. Rarer haplotypes were found in cool, spring-fed streams but without clear geographic or phylogenetic components.

Resolution of the genetic heterogeneity of closely related insect species depends on the selection of reliable genetic markers derived from representative specimens. We report the results of a survey of genetic variability in nine species of black flies in the subgenusPsilopelmia Enderlein. Three regions of the mitochondrial genome and an amplicon including the internal transcribed spacer 1 of the nuclear ribosomal RNA gene cluster (ITS1) were amplified using the polymerase chain reaction (PCR), and the amplicons were examined for intraspecific and interspecific polymorphisms. Six of the seven Psilopelmia species that yielded PCR products in the ITS1 PCR reaction were found to generate products that were indistinguishable on the basis of size. Similarly, little interspecific variation was noted in the 16S rRNA amplicon among nine species of Psilopelmia assayed by heteroduplex analysis. In contrast, the remaining regions of the mitochondrial genome exhibited both intra- and inter-specific variation when analyzed by heteroduplex analysis. Information collected from the five amplicons could be employed to develop a classification scheme capable of distinguishing the nine species of Psilopelmia examined.

The genus Ectemnia Enderlein consists of four North American species, two of which are described as new. The two previously described species, E. invenusta and E. taeniatifrons, are primarily northern in distribution and their immature stages occupy rocky rivers. The two new species, E. primaeva and E. reclusa, occur in the southeastern Coastal Plain; their immature stages inhabit blackwater swamp streams and sandy rivers. Keys are provided for larvae, pupae, and adults. The polytene chromosomes of all species are analyzed in detail. Seven fixed inversions, 10 unshared autosomal polymorphisms, two centromere-band expressions, and five sex-chromosome systems are recognized among the four species. At least 10 synapomorphies indicate that the genus is monophyletic. Phylogenetically, Ectemnia is in a clade with Metacnephia and Simulium.

All stages of Siinuliutn buissotzi Roubaud, 1906 are redescribed. Larvae, pupae, and males of S. gallinuni Edwards, 1932 are associated with the female holotype and all are fully described. Simulium adamsoni Edwards, 1932, the males of which have enlarged cerci, gonocoxae fused medially, and an enlarged keel-like ,ventral plate, is similarly redescribed. The female holotype and two paratypes of S. inuinfordi Edwards, 1932 are fully redescribed. Males and females of S. hukaense Sechan, 1983 are associated with the pupae and larvae and described. Since all type material of this species is lost, a neotype is designated. Siinuliuin itaense Sechan, 1983 is briefly mentioned. Three new species are described: S. pichoni Craig, Fossati, and Sechan, S. rivierei Craig, Fossati, and Sechan, and S. sechani Craig and Fossati. Brief comments are made concerning the bionomics and the relationships of the species. Resume : On trouvera ici une nouvelle description de tous les stades de Siniuliuin buissorii Roubaud, 1906. Des larves, nymphes et mâles de S. galliiiuin Edwards, 1932 ont ete associes a l'holotype femelle et on peut en lire ici la description. Il en va de meme pour S. adainsoiii Edwards, 1932 dont les mâles, aux cerques renfles et aux gonocoxas fusionnees en leur milieu, possedent une grande plaque ventrale carenee. L'holotype femelle et deux paratypes de S. rnunfordi Edwards, 1932 font l'objet de descriptions entierement nouvelles. Des miles et des femelles de S. kitkaense Sechan, 1983 ont ete associes aux nymphes et aux larves et ils sont decrits ici; tout le materiel type de cette espece a ete perdu et il a donc fallu designer un neotype. Siniulium uaeiise Sechan, 1983 est mentionne. Trois Sechan et S. sechani Craig et Fossati. La dynamique des populations et les relations interspecifiques font l'objet de brefs commentaires. s

We mapped the exact locations of two predaceous stream insects, a caddis larva (Rhyacophila obliterata) and a stonefly nymph (Isoperla grammatica), in 1-m2 sampling windows in relation to their major prey, larval black flies (mostly Simulium sublacustre), which were quantified within the same areas by extensive systematic sampling. In mid-June, when black flies were abundant and highly clustered, R. obliterata was also aggregated on a scale of 5–10 cm. After the emergence of black flies in the beginning of July, the point pattern of the rhyacophilid was random. Isoperla grammatica was abundant only on the earlier sampling occasion, at which time it was randomly distributed. We also analysed spatial association between the two predators and found a weak indication of small-scale inhibition between them. In the range of current speeds of 40–60 cm/s, microhabitat overlap between the caddisfly and black fly was intensive, leading to high consumption of this semi-sessile prey. The second-order methods used proved powerful in detecting small-scale spatial patterns and could be more widely used by ecologists in place of classical single-statistic indices whenever detailed mapping of individuals is feasible.

Effects of episodic experimental pH depressions on water chemistry and structure of benthic communities were investigated in northwestern Ontario, a location with low H+ deposition. Acidity levels were increased from background values of pH 6.9 to 5.0, 4.5, and 4.0 to simulate snowmelt and rainstorm events observed in south-central Ontario. Conductivity and concentrations of Ca and Mn increased significantly for all three pH disturbances. Fe and dissolved organic C remained unchanged at pH 4.5 and 4.0 but Fe increased at pH 5.0. Total and inorganic monomeric Al increased at pH 4.5 and 4.0 relative to reference areas. At pH 5.0, density decreases were observed for Baetis, Ectemnia, Simulium, and Prosimulium, at pH 4.5 for Baetis, Lepidostoma, Micropsectra, Simulium, and Prosimulium, and at pH 4.0 for Baetis, Tanytarsus, and Cricotopus. Densities of collector/filterers and clingers decreased at all pH levels, but diversity decreased in the experimental reach at pH 4.0 only. Drift density of the above genera was correlated with H+ concentration. Large reductions in macroinvertebrate densities occurred at higher pH (5.0) and during shorter exposure times compared with previous experiments in areas of higher H+ deposition in south-central Ontario and New Hampshire.

Infections by insect-pathogenic fungi of the order Entomophthorales in two adult black fly populations were studied in the Réserve Faunique du Saint-Maurice (Quebec). Entomophaga near limoniae infected Simulium verecundum/rostratum in June whereas Erynia curvispora predominantly parasitized Simulium decorum from the 2nd week of July to September and Erynia conica attacked the Simulium venustum complex, Simulium verecundum/rostratum, and the Simulium vittatum complex from May to September. The fungi did not infect the adult stage of Prosimulium species. Possible evidence of host specificity for Erynia conica is discussed.Key words: Entomophthorales, black fly, host specificity, Entomophaga, Erynia, Simulium.

This is the fifth and final paper in a series treating the Simuliidae of Sri Lanka, and it contains a checklist of the species in the country. In this paper are described three new species of Simulium (Simulium): all stages of Simulium subpalmatum and Simulium cruszi (except the larva) and the male and pupa of Simulium paranubis. The female, male, and pupa of Simulium (Simulium) consimile and S. (S.) striatum are redescribed and the larva of S. (S.) consimile is described for the first time. Taxonomic comparisons are made with related species and keys are provided to separate the eight Sri Lankan species in the subgenus.

All life stages and the polytene chromosomes of Simulium (Hellichiella)curriei, a new species from western North America, are described and illustrated. This species is the only Nearctic member of the subgenus with an eight-filamented pupal gill. It is the sister-species of S. rivuli.

Scutal patterns of female Simulium quadrifidum, S. oyapockense s.I., and S. bipunctatum were studied with a light microscope, and these observations were compared with scutal morphology as seen with the scanning electron microscope (SEM). This study has shown that the pruinose scutal patterns seen in these species are due to the distribution of two distinct forms of microtrichia visible only with the SEM, with underlying pigmentation determining the background colouration. The importance of using a standardised lighting method on well-preserved specimens is emphasised.

Larval development of Simulium arcticum Malloch (IIL-3), S. defoliarti Stone and Peterson (IIS-14.15), and S. tuberosum Lundström (FG) was documented for four rivers in southwestern Alberta from 1982 to 1984. Simulium arcticum (IIL-3) and S. tuberosum (FG) had seven instars; these two species were multivoltine with two or three generations per year, while S. defoliarti (IIS-14.15) was univoltine. Water temperature, river discharge, and turbidity were the only environmental factors of five parameters measured that were significantly correlated with the seasonal development of larvae of S. arcticum (IIL-3), S. defoliarti (IIS-14.15), S. vittatum Zetterstedt complex, S. tuberosum (FG), and Prosimulium onychodactylum Dyar and Shannon complex. Also, we speculate on the effect that food quality and quantity have on larval growth and development of blackfly species that breed in mountain rivers.

This paper is the fourth in a series treating the Simuliidae of Sri Lanka and describes the larval, pupal, and adult stages of a new species, Simulium (Byssodon)languidum. Taxonomic comparisons are made with other members of the subgenus.

The paper proposes standard polytene chromosome maps for species groups in the subgenus Simulium. In the S. venustum/verecundum complex we distinguish by their chromosomes a minimum of seven sibling species designated by their IIS sequences, as follows: EFG/C (S. truncatum?) holarctic; EFG, Germany; CC (S. venustum?) North America (with probably CC 1 a separate sibling); A/C venustum, New Hampshire (with AC(gB) venustum Newfoundland probably distinct); CC 2 Ontario, Newfoundland; AA verecundum Newfoundland (with possibly A/C verecundum, Ontario distinct); and ACD (S. verecundum? = sublacustre) holarctic. Three of these taxa (EFG/C, ACD, and CC) were probably recognized by taxonomists, though the circumpolar distribution of the first two was not reliably recorded. The remaining species were not previously resolved. The chromosomal characteristics of all species are described and notes are given on available details of their biology and distribution. Some broad aspects of the relationship between cytological and taxonomic methodology in the group are discussed.

The chromosomal constitutions of three species in the jenningsi-group in the Adirondacks and the Finger Lakes Region in New York State are detailed. No sibling species were found in Simulium jenningsi, S. fibrinflatum, or S. luggeri in these areas. The species were found to differ by 6 fixed inversions and by 19 floating inversions. 4 of which are related to sex determination in S. jenningsi. The major chromosomal differences between the venustutm-group and the jenningsi-group are detailed. The ancestral condition for each arm was determined using ex-group and L′ analyses. A phylogenetic sequence for these three species from the ancestral condition is proposed.

Twenty-one blackfly species from 22 rivers and creeks and 3 additional species from small temporary streams were collected in southern Alberta from 1981 to 1983. Eight Prosimulium, 14 Simulium, 1 Twinnia, and 1 Stegopterna species were identified. The collection of Prosimulium dicum Dyar and Shannon is the first record for Alberta. Simulium verecundum Stone and Jamnback, Simulium vernum Macquart, and Stegopterna mutata Malloch are first records for southern Alberta.

This is the third paper in a series treating the Simuliidae of Sri Lanka; in it are described all stages of a new species of Simulium Latreille belonging to the subgenus Morops Enderlein, Simulium (Morops) trirugosum sp. nov. This is the first species of Morops found in Sri Lanka. Taxonomic comparisons are made with other Oriental species of the subgenus.

Nineteen species of immature simuliids (Diptera: Simuliidae) were collected from streams on Hardwicke Island in the Johnstone Straits of British Columbia. These species were Simulium aureum Fries, S. canadense Hearle, S. decorum Walker, S. hunteri Malloch, S. tuberosum Lundström complex, S. venustum Say – verecundum, Stone and Jamnback complex, Prosimulium dicum Dyar and Shannon, P. esselbaughi Sommerman, P. formosum Shewell, P. travisi Stone, Simulium anatinum Wood, S. bafftnense Twinn group, S. bicorne Dorogostajskij, Rubtzov and Vlasenko complex, S. gouldingi Stone, S. piperi Dyar and Shannon, S. pugetense (Dyar and Shannon), S. vernum Macquart complex, Prosimulium doveri Sommerman, and Stegopterna sp. Enderlein. The last nine species have not been reported previously from British Columbia. Species composition, succession, and seasonal abundance of larvae and pupae were determined from April to August of 1980 and 1981. Simulium venustum–verecundum complex and S. hunteri were the most abundant species in 1980, while S. hunteri and S. vernum complex were most abundant in 1981. Black fly productivity was greatly reduced in 1981.

Abilities of two types of polyethylene samplers to collect black fly larvae were compared with ceramic fireplace tiles, Surber samplers, and Hess samplers. The polyethylene samplers were colonized as well as tiles and were superior to bottom samplers for sampling black flies. The percentages of each species collected from polyethylene samplers were comparable with natural vegetation. Simulium penobscotensis Snoddy and Bauer and Simulium nyssa Stone and Snoddy, which do not readily colonize tiles, were collected by the polyethylene samplers in numbers similar to those collected by vegetation.

Morphometric studies show that there are seven larval instars in Simulium chutteri. A guide for separating instars of S. chutteri is included. Seasonal and sexual size variation and seasonal variation in biomass in larvae and pupae of S. chutteri are statistically analysed and discussed. Water temperature is a major factor in determining larval and pupal size variation and duration of the pupal stage. Growth in S. chutteri follows a geometric pattern and is in agreement with previous works on other species. An overlap of larger and smaller larvae at the same developmental stage resulted from the coexistence of early winter and late summer generations, respectively. Larger larvae and pupae can result in autogenous and more fecund adult females. This is important in the population dynamics of this pest species and can lead to sudden unexpected population explosions. It is suggested that population explosions could be controlled by sharply reducing the population of large individuals of S. chutteri in the spring.

Prosimulium mixtum/fuscum S. &D., Cnephia mutata (Malloch), C. dacotensis D. &S., Simulium aureum Fries, and S. gouldingi Stone are univoltine. S. venustum Say, S. vittatum Zett., S. tuberosum (Lundstrom), S. corbis Twinn, S. latipes (Mg.), and S. decorum Walker are multivoltine. Neomesomermis flumenalis (Welch), Gastromermis viridis Welch, and Isomermis wisconsinensis Welch are univoltine. The eggs of Prosimulium and Cnephia species hatched synchronously with a batch of the eggs of N. flumenalis. The eggs of the first generation of Simulium species hatched synchronously with the eggs of G. viridis and I. wisconsinensis, and the overwintered batch of eggs of N. flumenalis. The larvae of subsequent generations of Simulium species were not infected by mermithids. Mermithid infections occur during the hatching and buildup of blackfly larval population.Caudospora simulii Weiser and C. brevicauda Jamn. occur in Prosimulium and Cnephia species during the cold seasons. Thelohania bracteata (Strickland, 1913), Pleistophora simulii (Lutz and Splendor, 1904), P. multispora (Strickland, 1913), and Coelomycidium simulii Debaisieux occur in Simulium species during the warm seasons. G. viridis and I. wisconsinensis overwinters in the egg stage, and N. flumenalis overwinters partly in the egg stage and partly as parasites in P. mixtum/fuscum. S. venustum and P. mixtum/fuscum are the most prevalent and abundant of the simuliids. N. flumenalis is the most prevalent and abundant of the mermithids and P. multispora is the most abundant of the microsporidans. Aeshna interrupta Walker and Rhyacophila fuscula (Walker) prey on S. venustum larvae.

The vulnerability of three different prey (Baetis spp., Ephemerella spp., and Simulium spp.) to predation by Nigronia serricornis (Megaloptera) and Paragnetina media and Agnetina capitata (both Plecoptera) was studied in recirculating Plexiglas chambers. When each prey was tested individually, Simulium was least vulnerable to predation by A. capitata and equally vulnerable with Ephemerella to P. media and N. serricornis. Baetis was the most vulnerable to predacious stoneflies while Baetis and Ephemerella were equally vulnerable to N. serricornis. When prey were tested in mixed assemblages, similar trends in prey vulnerability were seen, except that Ephemerella was more vulnerable than Simulium to P. media and N. serricornis. Our data suggest that Ephemerella is less vulnerable than Baetis to predation by stoneflies; however, they were equally vulnerable to N. serricornis, possibly because of differences in feeding behavior and morphology between stoneflies and N. serricornis. Significantly more Simulium larvae moved to the sides and ceiling plates of the experimental chambers versus the control chambers; this differential movement suggests that the presence of predators may have stimulated escape to an apparently predator-free refuge.

In most cases the larvae of Wilhelmia equina and W. lineata cannot be distinguished by using classical morphological features. The morphological characteristics of the salivary gland polytene chromosomes allow one to differentiate clearly between the two species. Characteristic for W. equina are the extended region between the centromere, Ctr (transformed centromere), and the nucleolus organizer, NO, in IS, the definitive position of RB (ring of Balbiani) and bulge in IIS, and the fan-shaped IIIL telomere. The chromosomes of W. lineata are marked by complex chromosomal polymorphisms, the altered position of RB and bulge on IIS and by a strong ectopic pairing of centromeres. The comparison of banding patterns provides several intraspecific polymorphic inversions and interspecific fixed rearrangements for species diagnosis. Partial chromosome maps were established. The comparison of the chromosomal banding pattern of Wilhelmia with that of the Simulium standard reveals a whole-arm interchange between chromosomes I and II in Wilhelmia identical with that in Metomphalus, Prosimulium vernale, a form of P. mixtum, and Metacnephia.Key words: cytotaxonomy, Simuliidae, Wilhelmia equina, Wilhelmia lineata, larvae.

A morphological and cytological study on blackflies of the Queen Charlotte Islands, British Columbia, revealed a total of 16 species in three genera: Prosimulium (6 species), Simulium (9 species), and Stegopterna (1 species). Included is a description of Simulium (Hellichiella) nebulosum n. sp. Notes on distribution and bionomics are provided for each species. AH species collected on the Queen Charlotte Islands are also found on the North American mainland. We conclude that the origin of the present-day simuliid fauna is best explained by post-Wisconsinan dispersal from mainland populations. This conclusion does not support the hypothesis of a continuously mild, Wisconsinan refugium on the Queen Charlotte Islands.

The paper describes the polytene chromosomes of Simulium (Ectemnaspis) ignescens from Colombia and compares them with those of a sympatric undescribed species designated Simulium "C." The two species share the entire banding sequence of chromosome arms IS, IL, and IIIS. Their IIIL arms differ by a single fixed inversion, but their second chromosomes exhibit multistep arrangement differences that were not completely resolved. The nucleolar organizer of S. ignescens is in the base of IIIS, that of Simulium "C" in the base of IS. Polymorphisms in IIS of S. ignescens exhibit partial sex linkage; no sex differential segments were demonstrated in Simulium "C." Common autosomal polymorphisms of both species involve preferentially chromosome III. Preliminary studies indicate that the chromosome maps of S. ignescens can serve as standards for other neotropical Simulium Latreille species including the important vectors of Onchocerca volvulus and Mansonella ozzardi.

This paper on the Simuliidae of Sri Lanka provides notes on Simulium (Eusimulium) aureohirtum and describes three new species of Simulium (Gomphostilbia): Simulium krombeini, Simulium ela, and Simulium dola. The females of S. (G.) pattoni and Simulium (Gomphostilbia) ceylonicum comb.n. are redescribed and the males, pupae, and larvae are described for the first time. Keys are provided for separating the subgenera of Simulium and the species of Gomphostilbia in Sri Lanka. Some taxonomic comparisons are made with related Oriental species and a few biological notes are given.

The population dynamics and drifting behavior of Simulium virgatum, S. trivittatum, and Simulium species A were investigated at nine stations in a south-central Oklahoma limestone stream from June 1972 through August 1973. Simulium species A abundance paralleled vernal periphyton increases and exhibited characteristic midsummer and midwinter decreases, while S. virgatum and S. trivittatum were most prevalent during late summer and early fall. Simulium species A and S. virgatum oviposited during the evening crepuscular period. In Simulium species A synchronous egg maturation and eclosion apparently imparted a degree of drift rhythmicity in the early instars which progressively degenerated during larval development. Simulium virgatum drift rates were highest during midafternoon and were correlated with diel changes in water temperature. Simulium trivittatum drift rates were highest during the evening and morning and lowest during the afternoon.

To test whether Simulium furculatum (Shewell) belongs to the Simulium vernum (Macquart) species-group, a comparison was made of the polytene chromosomes of S. furculatum and the S. vernum "Knebworth" standard. Only two chromosome arms (of six) could be completely analysed. It is argued that S. furculatum does not belong to the S. vernum species-group. A complex sex-chromosome system (X1, X2, Y1) is found in both eastern and western Canada. Phylogenetically, the single Y chromosome is intermediate between the two X chromosomes. Intraspecific inversion polymorphisms, which serve to differentiate eastern from western populations, are also identified. Only one sibling is indicated.Key words: black flies, Simulium furculatum, Simulium vernum, cytotaxonomy, polytene chromosomes, sex chromosomes.

The polytene chromosomes of the black fly species Simulium (Nevermannia) costatum are joined at the centromeres in a strongly heterochromatic chromocentre. Examination of the larval salivary gland chromosomes revealed two populations with a unique polymorphism for attachment to the chromocentre involving all centromeres. All three homologous pairs of chromosomes are polymorphic for centromeres that do not join to the chromocentre. Samples from one of these populations were large enough for thorough study. In this population, the attachment polymorphism is in Hardy-Weinberg equilibrium for two of the centromeres and was in the same frequency for 2 successive years of sampling. The polymorphism could be either primary, retained from an ancestral nonchromocentric state, or secondary, evolving independently or introduced via hybrid introgression. The evolution of chromocentres is discussed in the context of species in the Simulium vernum group.Key words: black fly, polytene chromosome, chromocentre, polymorphism, evolution.

Acute toxicity tests with the predaceous trichopteran larvae of Rhyacophila dorsalis and Hydropsyche pellucidula in a laboratory-simulated stream have shown that both species have a high survival rate after short exposures, 15 min to 1 h, to concentrations of temephos(Abate) and chlorpyrifos methyl that produce a mortality of 90% or more in late-instar Simulium larvae. A similar differential reaction to the synthetic pyrethroid permethrin was demonstrated in the case of Hydropsyche but not Rhyacophila.The implication of these findings is discussed firstly in relation to the environmental impact of Simulium control programmes based on application of larvicides to rivers and streams, and secondly to the wider problem of evaluating the impact of pesticides and allied toxic chemicals on stream ecosystems.It is concluded that although significant differences in tolerance level to some insecticides between prey (Simulium) and predator (trichopteran larvae) can be demonstrated in the laboratory, the margin of safety is not sufficiently wide to assure selective control of Simulium larvae by these chemicals under the less precise conditions of larvicide application in the field.

Clinical toxicosis, used as an indicator of black-fly tolerance, was not as pronounced in Charolais bulls as in Angus or Hereford bulls, whether or not they were treated dermally with 25 mg phosmet/kg body weight. During the first 20 days after exposure to black flies, the combined weight loss of the treated and untreated Charolais bulls was lower than that of their Angus or Hereford counterparts. Up to 33 days PT, the combined gains of the treated bulls of the three breeds were intermittently higher than those of the controls. Key words: Black flies, beef bulls, phosmet, response, toxicosis, weight gains

Hatching from the egg was studied in Simulium (probably venustum). In early stages of development the embryo completely fills the space enclosed by the chorion, but in later stages there are sizable fluid-filled spaces between the prolarva and the shell. Initiation of hatching involves the beginning of swallowing and absorption of the intrachorionic fluid. As a result of this the prolarva swells until it not only completely fills the egg again, but also exerts considerable pressure from within against the shell. This process brings the cephalic egg burster into strong contact with the chorion. Eventually the burster penetrates the shell and thus initiates the rupture which permits the first instar larva to escape from the egg, always head first.

An extensive survey was made of the blackflies of insular Newfoundland. Collections from 212 streams contained three genera and 21 species of simuliids, viz., 3 species of Prosimulium, 2 species of Cnephia, and 16 species of Simulium. Simulium croxtoni, S. excisum, and S. gouldingi are new records for the island. Data on the bionomics and seasonal occurrence of the species are also presented.

The response of Simulium euryadminiculum Davies towards silhouettes of different shapes and colors was observed, using an ether extract of the uropygial gland of the common loon to concentrate the flies in the experimental locality and "Tanglefoot" to trap the flies on the silhouettes. These simuliids were attracted primarily to the head–neck portion of duck decoys, most flies landing on the leeward side. The simuliids were attracted to the terminal (and most prominent) portions of artificial head–neck or cylindrical silhouettes; the distribution was not markedly influenced by the color of the silhouette. The number of flies attracted to three-dimensional targets was markedly influenced by color, black being the most attractive and white relatively unattractive. This selection of black by the fly may reflect its adaptation to the color of its natural host, the loon. Response of the flies to a two-dimensional (plane surface) target was essentially similar except that white was the most attractive color and black was relatively unattractive. Possibly this selection of white is associated with a resting response.

A new method for tagging large numbers of black flies (Simulium spp.) for flight range investigations is described. Radioactive adults that were readily detected with a Geiger counter were reared from larvae that had been kept for 24 hr. in a very dilute solution of radioactive phosphorus (0.2 μc. per ml.) and then returned to nonradioactive water to complete their development. The treatment did not visibly harm larvae or the adults that emerged from them. Larvae were reared in small containers of water circulated and aerated with air jets in the laboratory, and with a simple arrangement of stream-driven paddle wheels in the field. By this method, tagged larvae, pupae, and adults were produced which could be readily detected with a Geiger counter. In field tests, tagged larvae and pupae were found as far as 520 yd. downstream from the point of release. Tagged adults were taken in cages placed over radioactive larvae and pupae in the stream, but only one was recaptured in the open, and this only 100 yd. from the stream. Failure to capture more tagged adults was believed due mainly to inadequate collecting methods. A short treatment period is desirable when tagging large populations of black-fly larvae in the field because other methods, such as rearing in a small container for a long period of time, or treating the stream with a radioactive isotope, are impractical. This method would be applicable in studies on dispersal and predation of immature and adult stages of other insects of streams and ponds.

The immature stages of Simulium villatum Zett., venustum Say, meridionale Riley, tuberosum Lund., decorum Walk., bivittatum Mall., griseum Coq., arcticum Mall., and aureum Fries, in decreasing order of abundance, occurred in 79% of 341 collections from 69 irrigation canals and 18 adjacent rivers and streams in irrigated areas of southern Saskatchewan and Alberta in the years 1950 to 1958. Canals less than 4 years old were infested despite a scarcity of aquatic vegetation which formed the main attachment sites. Factors reducing larval abundance included interruption of flow, lack of food and attachment sites, parasites, invertebrate and vertebrate predators, and possibly insecticides from irrigated crops. In some canals up to 100% of the larvae of S. vittatum were infected with nematodes, Hydromermis sp., which prevented pupation. Other larvae were infected with microsporidia. Most canals were dry in the winter. Larvae of only S. vittatum overwintered in two canals which flowed all winter; no viable overwintered eggs of any species were found in any canals. It is presumed that canals are reinfested each spring by gravid females immigrating from permanent breeding sites, mainly in the larger streams and rivers. Although black flies, especially S. vittatum, were occasionally abundant and all are blood feeders, they were generally less of a nuisance than mosquitoes. Leucocytozoon occasionally occurred in turkeys.

Neomesomermis flumenalis (Welch) infected 11.2% of 3144 blackfly larvae from five localities. Thelohania bracteata (Strickland) and Pleistophora simulii (Lutz &Splendor) and P. multispora (Strickland) parasitized 4.9% of the larval population. The mermithid infected the larvae of Prosimulium mixtum Sime &Davies and Simulium corbis Twinn, S. decorum Walker, and S. venustum Say. The larvae of the last three species were also infected with at least one of the species of Microsporida. Joint infections of T. bracteata and P. simulii occurred in S. venustum. Only 5 of 18 localities had infected blackfly populations. One nematode per host occurred most often, and emergence was from the host abdomen.