Simulium (Psaroniocompsa) minusculum Lutz

Hernández, Luis Miguel, Shelley, Anthony John, Herzog, Marilza Maia - & Dias, Antonio Paulino Andrade Luna, 2011, Morphological variation in S. minusculum Lutz and S. oyapockense Floch and Abonnenc and a redescription of the larva of S. minusculum (Diptera, Simuliidae) in the Neotropical Region, Zootaxa 2863, pp. 35-48 : 36-42

Simulium (Psaroniocompsa) minusculum Lutz

(Figs. 1–32)

Variation was observed in pupal gill configuration and adult scutal pattern from various localities. We compared body size of specimens with these gill variations and related them to scutal pattern. A full description of the larva is given.

FEMALE. Body size. Typical S. minusculum from many localities including the type locality (Lassance) and Dardanelos, Aripuanã River [both figured] in Brazil and Argentina [see Material Examined] [all with median primary branch bifurcating at mid length of gill and more distal to dorsal and ventral primary branch bifurcations, Fig. 23]: Body length 1.5–2.2 mm (mean= 1.8 mm, s.d.=1.58, n=25), wing length 1.1–1.6 mm (mean= 1.4 mm, s.d.=0.10, n=25), wing width 0.5–0.8 mm (mean= 0.6 mm, s.d.=0.08, n=25).

Specimens from Maranhão (Tocantins River), Tocantins (Barreiros River) and Pará States (Iriri River) [all with median primary branch located more basally and more distal to dorsal and ventral primary branch bifurcations, Fig. 24]: Body length 1.1–1.4 mm (mean= 1.2 mm, s.d.=0.12, n=4), wing length 0.8–1.2 mm (mean=1.0 mm, s.d.=0.24, n=3), wing width 0.6–0.7 mm (mean= 0.6 mm, s.d.=0.06, n=3).

Specimens from Mato Grosso, Chapada dos Guimarães, Brazil [all with median primary branch dividing more apically at about same level as bifurcations on the dorsal and ventral primary bifurcations, and gill filaments much longer than in typical S. minusculum , Fig. 25]: Body length 1.6–1.8 mm (mean= 1.6 mm, s.d.=0.11, n=3); wing length 1.4–1.8 mm (mean= 1.5 mm, s.d.=0.16, n=5); wing width 0.5–0.8 mm (mean= 0.6 mm, s.d.=0.12, n=4).

Thoracic pattern. The thoracic pattern in the typical form of S. minusculum and the specimens found in Chapada das Guimarães is different. In the typical form observed in the lectotype (Figs. 1, 2), link–reared specimens from Mato Grosso, Dardanelos (Figs. 3, 4), Argentina (Rio de las Piedras, Jujuy Province) (Figs. 7, 8), and Maranhão (Tocantins River) (Figs. 9, 10), Tocantins (Barreiros River) and Pará States (Iriri River), Brazil, and man–biting specimens from Dardanelos, Mato Grosso State (Figs. 3, 4) and Peixe, Tocantins State (Figs. 5, 6), Brazil, the scutal pattern changes in relation to illumination. With anterior lighting the black areas of the scutum on silver a pruinose background are as follows: wide median black vitta extending from anterior border for almost four fifths of scutum, rounded and twice as wide posteriorly as on anterior scutal border; 1+1 disc–shaped vittae between median vitta and lateral margin of scutum, beginning in second quarter of scutum and extending to same posterior limit as median vitta; 1+1 intervittal triangular marks on anterior scutal border in pruinose area between median and disc–shaped vittae – tails extending for half length of scutum may be seen by tilting specimen slightly. With posterior lighting median black vitta remains the same while the disc–shaped vittae extend to anterior margin of scutum to form bands of the same length and run almost parallel to median vitta; intervittal triangular marks become silver pruinose.

In link–reared females collected from Chapada dos Guimarães, Mato Grosso State, Brazil, the thoracic pattern is somewhat similar to the above patterns. However, with light anterior the 1+1 silver pruinose bands diverge at the mid–point of the scutum towards the lateral margins and they are of the same width along their lengths (Fig. 11). With light posterior the 1+1 silver pruinose bands have a black area in the anterior half leaving a 1+1 rounded disc on the anterior third of the scutum and 1+1 tails near the posterior margins (Fig. 12).

MALE. Body size. Typical S. minusculum from many localities including link–reared specimens with typical gill configuration, and specimens from Minas Gerais (Rio das Velhas) and Goiás (Mucambão River) States of Brazil (see Material Examined) [all with median primary branch bifurcating at mid length of gill and more distal to dorsal and ventral primary branch bifurcations, Fig. 23]: Body length 1.4–1.9 mm (mean= 1.7 mm, s.d. =0.16, n = 6), wing length 1.4 mm, wing length 1.4 mm range (mean= 1.4mm, s.d.=0.02, n=4), wing width 0.5–0.9 mm (mean= 0.7 mm, s.d.=0.19, n=4).

Specimens from Maranhão State (Tocantins River), Tocantins (Barreiros River) and Pará States (Iriri River) [median primary branch bifurcating basally – Fig. 24]: Body length 1.4–1.5mm (mean= 1.4mm, s.d.=0.09, n=3), wing length 0.8–1.3mm (mean= 1.1mm, s.d.=0.28, n=3), wing width 0.5–0.9 mm (mean= 0.7 mm, s.d.=0.19, n=3).

Specimens from Mato Grosso, Chapada dos Guimarães, Brazil [link–reared specimens with median gill dividing more apically – Fig. 25]: Body length 1.4–1.8 mm (mean= 1.5 mm, s.d.=0.20, n=3), wing length 1.3–1.6 mm (n=2) wing width 0.7–0.8 mm (n=2).

PLATE 1. Figs. 1–12. Female thoracic patterns of S. minusculum . The first figure shows anterior light direction and the second posterior light direction.1, 2: Lectotype, Minas Gerais State, Lassance, Brazil; 3,4: Mato Grosso State, Dardanelos, Brazil; 5,6: Tocantins state (Peixe); 7–8: Jujuy Province, Rio de las Piedras, Argentina; 9–10: Maranhão State, Tocantins River, Brazil; 11– 12: Mato Grosso State, Chapada dos Guimarães, Brazil (new morphoform).

Thoracic pattern. In typical S. minusculum the scutum is velvet black with a pattern varying with direction of light source. With anterior light source 1+1 sub–median thin silver lines arising from anterior scutal border and extending for two thirds scutal length, occasionally silver lines reduced to small bands on anterior scutal border (Figs. 13, 15, 17, 19). With posterior light source thorax black, though sometimes silver pruinosities present and hard to distinguish (Figs. 14, 16, 18, 20).

In specimens from Chapada dos Guimarães, the thorax is black with 1+1 thin silver lines centrally that extend from the anterior third of the scutum to nearly the posterior margin with anterior and posterior light sources (Figs. 21, 22).

PUPA. The cocoon shape, and morphology of the pupal frontoclypeus, thorax and abdominal onchototaxy in specimens from Argentina and the material collected in Chapada dos Guimarães, Mato Grosso, Brazil falls within the variation of typical S. minusculum . However, variation in the bifurcation of the median primary branch was found.

PLATE 2. Figs. 13–22. Male thoracic patterns of S. minusculum . The first figure shows anterior light direction and the second posterior light direction. 13–14: Minas Gerais, R. dos Velhas, Brazil. 15–16: Goiás State, Mucambão River, Brazil; 17–18: Maranhão State, Tocantins River, Brazil; 19–20, Pará State, Iriri River, Brazil; 21–22, Mato Grosso State, Chapada dos Guimarães, Brazil (new morphoform).

Body size. Typical S. minusculum (following Shelley et al. 2010) (Fig. 23): Cocoon length dorsally 1.4–1.9 mm (mean= 1.7 mm, s.d.=0.13, n=12), ventrally 1.6–2.0 mm (mean= 1.8 mm, s.d.=0.16, n=12), pupa length 1.3–1.7 mm (mean= 1.6 mm, s.d.=0.14, n=6), gill length 1.1–1.3 mm (mean= 1.2 mm, s.d.=0.05, n=4).

Specimens from Maranhão State (Tocantins River) and Pará State (Iriri River) [median primary branch bifurcating basally] (Fig. 24): Cocoon length dorsally 1.4–2.2 mm (mean= 1.9 mm, s.d.=0.34, n=4), ventrally 1.8–2.7 mm (mean= 2.2 mm, s.d.=0.41, n=12), pupa length 2.2–2.7 mm (mean= 2.4 mm, s.d.=0.18, n=4), gill length 0.8–1.3 mm (mean= 0.9 mm, s.d.=0.21, n= 5).

Specimens from Chapada dos Guimarães, Mato Grosso State, Brazil [median primary branch bifurcating more apically] (Fig. 25): cocoon length dorsally 2.1–2.6 mm (mean= 2.3 mm, s.d.=0.1, n=12), ventrally 2.3–3.1 mm (mean= 2.6 mm, s.d.=0.26, n=12), pupa length 2.3–3.5 mm (mean= 2.6 mm, s.d.=0.31, n=12), gill length 1.5–2.5 mm (mean= 1.8 mm, s.d.=0.27, n=12).

In typical S. minusculum (Minas Gerais State, Lassance, Rio das Velhas), and from many other localities across Brazil (see Material Examined) and Argentina, the pupal gill main trunk gives rise to three primary branches (Fig. 23): dorsal with bifurcation in basal tenth of gill, median (sometimes arising from base of dorsal primary branch) with bifurcation in basal third of gill, and ventral which bifurcates in basal fifth of gill. This is the most common pattern found in S. minusculum . However two variations have been found in specimens collected from Brazil. In specimens collected in the states of Maranhão (Tocantins River), Tocantins (Barreiros River) and Pará (Iriri River), Brazil (Fig. 24) the bifurcation of the median primary branch is more basal and at the same level as that of the ventral primary branch. An extreme variation from the typical pattern was found in specimens from Chapada dos Guimarães, Mato Grosso State, Brazil (Fig. 25) in which the median primary branch bifurcates at the mid point of the pupal length and nearly at the same level of the dorsal and ventral primary branches.

PLATE 3. Figs. 23–25. Variation in the pupal gill configuration in S. minusculum . 23: Typical configuration, Minas Gerais State, Lassance, Rio dos Velhas, Brazil; 24: Gill with median primary branch more basal, specimens from Maranhão State, Tocantins River, Brazil; 25: Gill with median primary branch more apical and bifurcating at mid length of gill, specimens from Mato Grosso, Chapada dos Guimarães, Brazil. Arrows indicate the different bifurcations in the gill filaments.

PLATE 4. Figs. 26–32. Larval morphology of S. minusculum based on specimens collected in Chapada dos Guimarães, Mato Grosso State, Brazil. 26: Body (lateral view); 27: Head (dorsal view); 28: Head (ventral view); 29: Hypostomium (ventral view); 30: Hypostomial teeth; 31: Mandible; 32: Mandibular teeth.

LARVA (Final instar). A short description of the larva of S. minusculum was provided by Coscarón (1991) and Coscarón and Coscarón–Arias (2007), who only illustrated the larval antennal segment. The general morphology of larvae from Chapada dos Guimarães, Mato Grosso in Brazil falls within the morphological variation of larvae in typical S. minusculum collected in Minaçu, Goiás State, Brazil, except the morphology of the median primary branch in the dissected gill histoblast. Hence, we provide a detailed description of this life stage and illustrate the key diagnostic characters, which are the morphology of the postgenal cleft, and number of hypostomial and mandibular teeth.

Body size. Typical S. minusculum (Minaçu, Goiás State, Brazil, n=1): Body length 2.7 mm, length of head capsule 0.4 mm, width of head capsule 0.3 mm.

Specimens from Chapada dos Guimarães, Mato Grosso State, Brazil: Body length 3.2–4 mm (mean= 3.5 mm, s.d.=0.29, n=6); length of head capsule 0.4–0.5 mm (mean= 0.4 mm, s.d.=0.05, n=6); width of head capsule 0.4 mm (n=6). Body colour dark grey dorso–laterally, whitish ventrally (specimens preserved in Carnoy’s solution and/or alcohol). General body form as in Fig. 26.

Head — mainly pale brown, anterior region of cephalic apotome yellowish. Numerous small setae present on all surfaces and head capsule slightly wrinkled. Head pattern lightly positive (Fig. 27). Cervical sclerites small, elliptical, fused to membrane. Postgenal cleft deep and broad mesally wide on median region; postgenal bridge 3.5 times shorter than length of hypostomium (Fig. 29). Hypostomium strongly pigmented on anterior margin, with approximately nine apical teeth evenly distributed along anterior margin; median tooth weakly reduced and nearly of same height as sublateral teeth; 3+3 sublateral teeth, all of same length; 1+1 produced lateral teeth much longer than remaining teeth; 6+6 small lateral serrations; 1+1 lines of 5 or 6 hypostomial setae parallel to lateral margin; 1+1 long, simple setae in posterior half of hypostomium (Fig. 30). Sub–esophageal ganglion lightly pigmented. Antennal segments longer than labral fan stalk, segments I–III weakly pigmented brown, length of segments excluding the sensillum 0.08–0.1:0.04–0.07: 0.06–0.08 mm (n=7). Mandible with three apical teeth, the first one longer than second and third teeth; mandibular comb with approximately 13 teeth, the first three more prominent than others; two mandibular serrations, the anterior more prominent and longer than posterior (Figs. 31, 32). Lateral mandibular process absent. Maxillary palps heavily pigmented; one and a half times as long as wide at base. Labral fan with 23–43 rays with fine, single line of spines in a row (n=4).

Thorax — pale grey dorsally and ventrally. Cuticle without setae. Proleg with plate heavily sclerotised with single band of approximately 19–25 processes of 9–15 hooks (n=4). Pupal respiratory gill histoblast greyish; dissected gill histoblast with six filaments, with dorsal, median and ventral primary branches all branching at mid length; filaments rounded apically.

Abdomen — dark grey with segments having greenish tinges. Ventral nerve cord pale white. Ventral papillae absent. Cuticle lacking setae except area around anal sclerite and rectal gills [ Coscarón (1991) reported abundant simple and bifid setae on the larval cuticle]. Anal sclerite well sclerotised with anterior arms extending to one third of posterior circlet anteriorly, with weakly sclerotised areas between arms. Posterior circlet with 52–64 rows of 12– 19 simple hooks (n=5). Rectal gills not everted in all specimens examined.

Discussion. The convoluted history of the taxonomy of S. minusculum was first reviewed by Shelley et al. (1982, 1984). The adults and pupa were later redescribed by Shelley et al. (2000) whilst studying the Simuliidae of Minaçu, Goiás State in Brazil. More recently, Shelley et al. (2010) have also covered the taxonomy of this species and selected a female lectotype, which is housed at the IOC (Material Examined). Further descriptions of S. minusculum are given in Coscarón (1991) and Coscarón & Coscarón–Arias (2007).

Variation in the adults’ thoracic pattern and in the pupal gill configuration has been recorded in S. minusculum from link–reared specimens from Chapada dos Guimarães, Brazil. These differences from typical S. minusculum are sufficient to designate the latter as the “Chapada dos Guimarães form of S. minusculum ”. Preliminary studies on the COI DNA Bar–coding region on three specimens (two specimens typical of S. minusculum – Goiás and Mato Grosso states, and one specimen from Chapada dos Guimarães) [L.M.Hernández, X.Zhou, P.N.D.Hebert– BIO, Canada, unpublished data] formed a single cluster, in which the specimen from Chapada dos Guimarães is separated by a>2% genetic divergence from the other two typical S. minusculum in the Neighbour Joining analysis tree [as in BOLD working platforms: www.boldsystems.org]. However, the paucity of data at this stage precludes any further comment on the taxonomic status of the Chapada dos Guimarães form.

The larva of the Chapada dos Guimarães form of S. minusculum can be distinguished from the other morphologically similar species S. oyapockense and S. roraimense by the centrally expanded and oval postgenal cleft (Figs. 28, 29), a very short postgenal bridge (Fig. 29) and the median and sublateral teeth (Fig. 30) of the mandible not extending to the apex of the lateral teeth (see also Shelley et al. 1997, Figs. 241, 252). However, the larva of the Chapada das Guimarães form of S. minusculum can be identified from the latter two species by the absence of ventral papillae and the filaments of the dissected gill histoblast being long and bifurcating at mid length of gill (as in Fig. 25). In S. oyapockense and S. roraimense the ventral papillae are present and filaments in the dissected gill histoblast is different as detailed in Shelley et al. (1997).

Simulium minusculum is a common anthropophilic species in Brazil found from south of Amazon River into parts of Argentina and possibly Paraguay (Shelley et al., 2010; Material Examined). All specimens from Chapada dos Guimarães were collected in medium to large rivers (varying from 1 to 10 m wide and 50 cm to 2 m deep) of clear, fast flowing waters with rocky and sandy riverbeds, temperatures of 21ºC to 24.5ºC and pH 4–5.