FNZ 68 - Simuliidae (Insecta: Diptera) - General structure and terms
Craig DA, Craig REG, Crosby TK 2012. Simuliidae (Insecta: Diptera). Fauna of New Zealand 68, 336 pages.
(
ISSN 0111-5383 (print),
ISSN 1179-7193 (online)
;
no.
68.
ISBN 978-0-478-34734-0 (print),
ISBN 978-0-478-34735-7 (online)
).
Published 29 June 2012
ZooBank: http://zoobank.org/References/9C478D54-FEB2-45E8-B61C-A3A06D4EB45D
General structure and terms
Terms for morphological structures of Simuliidae are not universal in application, so we use here in large part those terms proposed by Adler et al. (2004), and less so Crosskey (1990) and Craig (1977). Those works should, however, be consulted for more detail on morphological structures. Dumbleton (1962) described structures for immature simuliids (larvae, pupae, and the cocoons), however, many of the terms he used have been superseded. In the following section we deal mainly with those structures that are key characters used to separate the sexes and to identify New Zealand species, and which are used in the descriptions and keys of this revision. In the main, key characters are pupal, cocoon, and larval structures. Internal structures of the larvae are of phylogenetic value, and gut morphology was used by Kim & Adler (2009) to permit generic diagnoses: we did not use this latter character.
Adults of New Zealand Austrosimulium, with few exceptions, are almost impossible to identify to species, even under moderate magnification, without mounting some parts for detailed examination. A hand lens of 10–15× magnification will, however, allow the sex of adults to be determined and placement of a specimen to a species-group, and possibly in one instance to species. Some pupae may also be identified to a species-group at moderate magnification, as can some final instar larvae. As males are extremely rarely encountered during collecting, they need to be reared from pupae; however, an important point to consider when examining reared male specimens killed very soon after emergence is that their structures and coloration may not be fully developed. Definitive identification usually requires specialised dissection and mounting of body parts, and then examination under high magnification; the techniques involved are provided in the Methods section (p. 49).
Adult
Body. Simuliid adults are relatively easy to distinguish from other small Diptera. The thorax is markedly humped (Frontispiece; Fig. 1) and in the U.S.A. simuliids are often called buffalo gnats for this reason. New Zealand simuliids are all evenly dark brown to black; the males are an elegant dull black with a vestiture of golden scales. The legs tend to be slightly lighter in colour. Females tend to be larger than males.
Head. The heads of male and female adults show a major difference in the structure of the compound eye. In females (Fig. 1, 15) the eyes are composed of arrays of equal-sized ommatidia (individual corneal facets), with the 2 eyes separated by the frons or forehead. This arrangement of the eyes in the female is known as dichoptic. The eyes of the male (Fig. 16) have 2 sizes of ommatidia, larger ones on the upper eye and smaller below. The larger ommatidia meet across the front of the head, a condition of the head referred to as holoptic. In life the male eyes tend to be more reddish than those of the female. These eye differences readily separate the two sexes. We use the frons width : head width ratio as a descriptor for head shape in females, in addition to providing measurements of head width and head depth.
Protruding from the front of the head are 2 antennae (Fig. 15, 16). The number of divisions of the antenna is an important diagnostic feature within Simuliidae. However, the use of terms in describing the divisions is problematic. Strictly the antenna consists of a basal scape (1st antennal segment), a pedicel (2nd antennal segment), and a flagellum (3rd antennal segment): this 3rd antennal segment is divided into a number of flagellomeres or articles (Fig. 18). Therein lies the problem — flagellomeres are also often referred to as segments (e.g., Dumbleton 1973; Crosby 1974b) and consequently the antenna of Austrosimulium has been described as “10 segmented”. Here we take the pedicel and scape as given and refer only to the number of flagellomeres; for Austrosimulium this is normally 8. The antenna is usually of even coloration, with occasional patterning on the antenna being of taxonomic value, particularly the 1st flagellomere (base of the 3rd segment). There are subtle differences in shape of male and female antennae; those of the male are more tapered (Crosby 1974b, Craig & Crosby 2008).
In the centre of the head and below the eyes is the clypeus, a domed region that internally supports muscles of the cibarial pump; hence the clypeus is larger in females than in males. The clypeus is variably setose. Below the clypeus are the mouthparts or proboscis. We use the relative length of the proboscis to the head depth as a measure of mouthpart size, as that provides an indication as to whether the female might bite or not. Articulated to the clypeus is a triangular labrum. The labrum has a groove on its inner surface forming the upper surface of the food channel. Apically, the labrum has a series of robust teeth that are absent in the male. Lying under the labrum and forming part of the bottom surface of the food channel is the hypopharynx. Articulated basally to that is the cibarium, a major pump for fluid ingestion. The internal end of the cibarium in Austrosimulium is smooth (e.g., Fig. 24–32), but in other simuliid genera there may be ornamentation. There are 2 sclerotised arms or cornuae laterally; these are variably sculpted apically and ornamented; of minor value here in species identification.
Hidden under the labrum are 2 flat mandibles (Fig. 17) with an array of minute teeth only on the inner (medial) side of the apex. This character state is of some phylogenetic importance since most other simuliids have teeth on both sides. Posterolateral to the mandibles are 2 laciniae, articulated 1 each on the base of a maxillary palp. Each lacinia has a series of sharp retrorse (recurved) teeth (Fig. 17) on each side. The number of teeth may be of some significance in regard to blood feeding. Mandibles and laciniae are untoothed in males. Protruding from the proboscis base, but often folded backwards, are 2 maxillary palps. These consist of 5 segments with the 2 basal ones being small. The 3rd segment is expanded to accommodate a sensory vesicle (Lutz’s organ). This contains odour- and carbon dioxide-sensing organs that, in blood-feeding females, are used to locate the host. The length of the 5th segment may be of significance in evolutionary relationships, but is not used by us.
Behind and tending to surround the other mouthparts is the substantial labium. Apically the labium is expanded into 2 fleshy labella. These are spread out over the skin to collect the blood when the female is biting; normally they are in a retracted position.
Thorax. The thorax of Simuliidae in almost all instances is distinctly convex, more so in the male. Of its 3 segments, the prothorax is much reduced, but remnants are the postpronotal lobes on the anterolateral margins of the larger scutum (2nd thoracic segment). These 2 postpronotal lobes are sometimes lighter in colour than the scutum and of minor diagnostic use. The scutum is by far the largest part of the thorax and its colour, and various patterns, are of important taxonomic use elsewhere in Simuliidae; however, the scutum is of little taxonomic value for Austrosimulium. The scutum is densely covered with a vestiture of microtrichia plus hairs that are often scale-like and of different colours; these are mainly golden in Austrosimulium. In older specimens the vestiture and hair may become worn and the scutum appears shiny. Posteriorly the scutum has a concave region referred to as the scutellar depression with vestiture of sparse, stiff, black hairs. Behind this is the scutellum, a raised subtriangular area with similar hairs to the scutellar depression arranged in an irregular row. The scutellum is often slightly lighter in colour than the scutum. Between the scutellum and the 1st segment of the abdomen is the bulbous postnotum (3rd thoracic segment), bare and usually concolorous with the scutum. In dried specimens the depression, scutellum, and postnotum appear pollinose (covered in dust) when lit from the posterior. Under such illumination lighter bands or vittae may appear lengthways on the scutellum. Laterally the thorax consists of the pleuron, derived almost entirely from the mesothorax; it is of little taxonomic importance in Austrosimulium, but elsewhere in Simuliidae it is useful. The pleuron is usually lighter in colour than the scutum, and under some lighting it appears pollinose. One major feature of the pleuron is a membranous region termed the anepisternal membrane, that in some simuliids is haired and a useful taxonomic feature, but is bare in Austrosimulium. Ventrally between the coxae of the hind legs is the invaginated sternum, or furcasternum — of reputed taxonomic value, but not so here.
Wings. The wings of Austrosimulium are hyaline and, as in all simuliids, clothed in microtrichia. The wing venation referred to below follows that of Adler et al. (2004). In some Austrosimulium species the wings are slightly dusky, a condition of diagnostic value. The leading veins of simuliid wings are strongly expressed — the costa (C), subcosta (Sc), and radius (R) (Fig. 1–3). The costa has well-developed spines as well as hairs. The subcosta has ventral hairs, either fully or partly, along its length. The basal section of R is haired on the dorsal surface and lacks spines. The anterior branch of the radius, known as R1, is haired, but lacks spines. The posterior branch of R1 is the radial sector (Rs) and is unbranched. In Austrosimulium there are spiniform setae on the costa and the apex of the subcosta. More posterior veins are the branched media (M1 and M2), 2 anterior cubital veins (CuA1 and CuA2), posterior cubitus (CuP), and single anal vein (A1). A false vein (sometimes labelled medial–cubital fold or M3+4 or submedian fork) lies between M2 and CuA. There is a short humeral cross vein (hm) between the base of the costa and subcosta. A small basal medial cell (bm) is present at the base of the media and CuA1 veins. The remnant of the hind wing in Diptera is the haltere, a knob-like structure on a stalk. Normally in New Zealand Austrosimulium the halteres are tan in colour, but occasionally they are white and of diagnostic value.
Legs. The legs of Austrosimulium probably carry more taxonomic information than we have used here, and the implications of various patches of hairs and their expression have not been examined. Legs are generally of lighter colour than the rest of the body, but are still dark (yellowish gray and black). The cuticle at articulations tends to be darker than intervening regions. The basal coxae are short and conical, trochanters small, and the femora spindle-shaped. The tibiae are more elongated and tend to be slightly expanded apically, particularly on the hind legs. Of importance is the basitarsus, which is furnished with a comb along its ventral edge and apically has the calcipala, a variously developed flattened flange extended over the articulation to the next tarsal segment, and present in both sexes (Fig. 19). The remainder of the tarsus consists of 4 divisions, or tarsomeres, the taxonomically important tarsomere being the basal one. In some simuliids there can be an incised area termed the pedisulcus, partly covered by the calcipala. However, it is not particularly well developed in Austrosimulium (Fig. 19); it is a weakening of the leg cuticle that enables the leg to fold up under the wing pad of the pupa during development. In some genera that are thought to be related to Austrosimulium, the pedisulcus is poorly developed and is little more than a series of wrinkles. It is absent in even more basal simuliids such as Helodon Enderlein.
On the hind basitarsus there may be a ventral row of stout setae (Fig. 19, 42–49). These setae are a plesiomorphic character for Simuliidae, occurring even in the basal genus Parasimulium, but not distinctly. In Austrosimulium their absence in both sexes helps define one of the species-groups. The apical claws on simuliid legs are of considerable importance, taxonomically and also provide biological information. At their simplest the two claws resemble elegantly curved talons (Fig. 50–54). This state is usually seen in mammophilic species and is perhaps an adaptation for dealing with hair. The shape of such simple claws is variable, and in some Austrosimulium species the base of the claw is substantial and has been referred to as a heel (Fig. 56). Ornithophilic species have a bifid claw with a variously developed basal lobe (Fig. 62). Terms for the lobe differ and here we use basal tooth as opposed to thumb-like lobe (Adler et al. 2004). In Austrosimulium when a claw tooth is present the tooth has a basal notch. There is considerable variation in claw structure and its various states do not unambiguously indicate feeding behaviour of the female. For instance, females with bifid claws are not restricted to just bird feeding. Adler et al. (2004: 41–42) discussed this aspect of simuliids in some detail. Simuliid males all have a series of grappling hooks (claw pad) dorsal to the base of the claw (Fig. 68, 69) that are used to engage the female vestiture during copulation (Craig & Craig 1986). Not mentioned in the literature, as far as we know, is that males possess a lobe-like structure superficially similar to the basal tooth of female claws. It is, however, more medial, so is unlikely to be a homologue (Fig. 68). There are small hairs and pegs at the junction of the claw and the lobe. This lobe-like structure appears to be plesiomorphic in simuliids since it occurs in male Parasimulium (Adler et al. 2004: Fig. 4.58).
Abdomen. The abdomen has 11 segments. The 1st segment is distinctly modified and is usually referred to as the basal scale. It supports a row of long fine hairs, the basal fringe. In males these hairs may extend back to the middle of the abdomen. Their colour can be useful taxonomically. On the dorsum (tergum) of each segment, a sclerotised portion is referred to as a tergite, and the relative size of these to each other is of considerable taxonomic value as well as indicative of feeding behaviour. Blood feeders that require the abdomen to expand during engorgement have smaller tergites (Fig. 70, 82); non-blood feeders have larger ones (Fig. 76). In males the tergites occupy the full width of the tergum (Fig. 88), as befits non-blood feeders. Vestiture is variable, mainly sparse short black hairs on the tergites, more pronounced on posterior tergites. Ventrally the sternites are poorly developed and of no taxonomic value. Membranous areas, such as the lateral pleuron, are pleated to allow expansion.
Female terminalia. The 8th sternite (VIII) is modified on its posterior edge as a pair of hypogynial valves that function as ovipositor lobes (Fig. 20, 22). The shape and arrangement of these is taxonomically important in simuliids, but less so in Austrosimulium where they are all rather similar (cf Fig. 90–107), but are, however, of minor use to discriminate species. The 9th tergite (IX) is well developed and connects laterally to a markedly modified 9th sternite, the genital fork (Fig. 20). This inverted Y-shaped structure is reflexed internally with its long arm directed anteriorly. The length and sclerotisation of the long arm and shape of the lateral arms and plates, and an anteriorly directed apodeme, are of considerable taxonomic importance both for species-groups and at higher levels (Fig. 108–125). The 10th segment (X) has a small tergite and its sternum consists of the anal lobes (paraprocts), each broadly connected posteriorly to a cercus (Fig. 20, 23, 90). Of importance taxonomically elsewhere in simuliids, these two structures are of minor taxonomic use in Austrosimulium. At the junction of the genital fork arms is the opening (e.g., Fig. 94, 95, 97) to the spermathecal duct that leads to the spermatheca. This globular structure is used to store sperm from the male. Of considerable importance taxonomically at higher levels in simuliids (Evans & Adler 2000), the spermatheca in Austro-simulium is rather uniform in character, albeit the clear area at the junction of the duct and spermatheca is variable in size. Although lacking external patterning, sometimes fine, sparse microtrichia can be observed internally.
Male terminalia. The terminalia of the male consist of the genitalia plus the small tergite of segment 10 and small cerci that arise from the reduced 11th segment (Fig. 21). The genitalia are of major importance taxonomically for Simuliidae and in large part Austrosimulium (Fig. 126–134). Tergite 9 (IX) is well expressed and connected laterally to a band-like sternite 9 (IX). The gonopods (claspers) are the most obvious part of the genitalia at first glance. These are 2-segmented appendages, with a basal gonocoxite and an apical gonostylus. The gonocoxites are subconical in structure and show some characters of taxonomic value. The gonostyli are variable in shape and have distinct numbers of substantial terminal spines (spinules) — both states useful in delimiting species-groups. The aedeagus is a complex medial structure. Most obvious is its ventral plate, a shield-like structure with 2 anteriorly directed basal arms, or apodemes. The posterior shape of the plate and the development of a median ridge are taxonomically useful. Arising from the dorsal surface of the ventral plate is a strap-like median sclerite; poorly expressed in Austrosimulium, its development and shape are important at the generic level elsewhere in other Simuliidae. The aedeagal membrane is a thin, transparent cuticle, which in Austrosimulium has extremely small microtrichia and superficially may appear bare.
In many simuliids there are sclerites that support the aedeagal membrane laterally. These are the parameres and they extend from an apodeme on the dorsomedial base of the gonocoxite to the apex of the basal arms of the ventral plate. In some simuliids the paramere is a quadratic or subtriangular plate. A further posterior extension may possess 1 to many parameral spines of various configuration. In New Zealand Austrosimulium the paramere is poorly expressed, merely consisting of a short rod on the gonocoxite and a difficult-to-see twisted rod of cuticle basolateral to the aedeagal membrane. There are no parameral spines (Fig. 135–142), although they occur in some Australian Austrosimulium.
Pupa
The pupa is basically the same shape as the adult developing within; with the appendages closely applied to the body (Fig. 4). Of importance in Austrosimulium is the cephalic plate and antennal sheaths (Fig. 197–201, 202–233). The cephalic plate, along with the anterior of the thorax, may be concave in some species. In males the antennal sheaths only reach halfway over the underlying eye, but fully in females, so pupae and their exuviae can be sexed relatively easily. The disposition of various setae on the cephalic cuticle is important. Facial setae are always present; frontal, epicranial, genal, and ocular setae may be present or absent (Fig. 196, 197–201). Thoracic setae are of good taxonomic value, but in New Zealand Austrosimulium, since they are more often than not damaged, or missing, they are of superficial use only. Dumbleton (1973: 481, 500) referred to 3 of the more obvious ones as dorsocentral setae; elsewhere such setae are known as trichomes. Of considerable importance is the sculpting of the thoracic and cephalic integument. When microtubercles are present they may be grouped into patterns (Fig. 234–236, 241). When tubercles are absent the cuticle can be corrugated or mammillated (Fig. 238, 239).
Respiratory gills are of major value for species identification, both in pupae and final instar larvae. These are bilateral cuticular projections, located anterolaterally on the pupal thorax, and are generally directed forwards and occasionally downwards. The gills can usefully be considered as 3 components (Fig. 255–267); a base that attaches the gill to the pupa and is usually more pale coloured; an apical black horn (when present); and filaments. The gills are covered by a very thin outer layer of cuticle and this is supported by cuticular posts, or trabeculae, of various sizes — on the base they show as black spots. Also on the base, and lateral, is a basal fenestra (Fig. 268, 271, 275) — a weak spot that bursts at the larva–pupal ecdysis (moult), thought to equalise water pressure inside and out of the gill. The shape of surface sculpting (often of small sharp cones, termed scobinate) of the black horn can be of considerable diagnostic value (Fig. 268, 278). The number of gill filaments is variable in Austrosimulium, as is their manner of taper. Furthermore, the surface of the gill along its length can be annulated or reticulated (Fig. 286–302); these are characters states of good taxonomic use.
The chaetotaxy (disposition of hairs and spines) of the abdomen is of considerable taxonomic importance in Simuliidae, but of little importance within Austrosimulium where all species are close to the same (Fig. 194). Of note, however, is that the genus lacks spine combs that are rows of sharp projections across the posterior abdominal terga. However, some pupae possess grapnel hooks. These are modified, curly, sharply-pointed hairs (Fig. 195) on the last segment (IX) and are of value in defining species-groups.
Cocoon
The cocoon is spun by the pharate pupa, that is, while the pupa is still inside the final larval instar cuticle. The larva–pupa ecdysis takes place within this cocoon. The cocoon fabric is salivary-gland silk, and while the strand-like nature of the silk can usually be observed in the cocoon (Fig. 185), in some Austrosimulium species the fabric appears to be foamy (giving an alveolate appearance) (Fig. 184); in others it appears to have a waxy consistency where the original strands of silk are not obvious. Cocoons in Simuliidae range from a mere strand or two of silk holding the pupa to the substrate, to complex woven structures covering the complete pupa except for the gills. Some may have fenestrae, probably to allow water flow to ventilate the gills. The basic cocoon shapes in Austrosimulium are relatively conservative and range from slipper- to shoe-shaped. The former has little in the way of a ventral connection around the front edge of the cocoon opening, whereas the latter can have a considerable edge. Normally, the cocoon in the shoe shape is high and fits moderately closely around the pupa. However, in some species (Fig. 143, 160) the cocoon is patellate (round and flattened like a knee cap; Crosskey 1990). There is a considerable range of projections from the anterodorsal edge of the cocoon including ridges along the dorsal surface (Fig. 154–159); so distinct are these that they are species-defining characters for some Austrosimulium. Indeed, Dumbleton (1973: 522) only listed the cocoon as holotype for a subspecies he erected, although he also had the pupa that formed this cocoon and the pharate female (alveolatum p. 117). Cocoon-spinning behaviour of Simuliidae is of considerable phylogenetic importance (Stuart & Hunter 1995, 1998). Austrosimulium australense was investigated by Stuart (2002) who showed that it was the sister to Simulium in terms of cocoon-spinning, differing in one behavioural characteristic. Stuart’s work is of significance since she showed that a similar end-product shape of cocoon spinning could be achieved by different spinning behaviours. Therefore, using shape for phylogenetic purposes, but lacking knowledge of the spinning behaviour, might result in use of homoplasious characters. Since spinning behaviour has been reported only for A. australense, we did not use cocoon shape in the phylogenetic analysis.
Larva
The general habitus of a simuliid larva is that of a well developed cuticular head capsule and a dumbbell- or amphora-shaped body (e.g., Fig. 6, 303–319). With few exceptions the head bears 2 anterodorsal labral fans (cephalic or head fans). These are often considered as mouthparts, and although intimately connected with food gathering, they are actually markedly modified parts of the lateral labrum and are homologous with the lateral palatal brushes of Culicidae and Dixidae larvae (Craig 1974). The fan proper is held away from the head by a substantial fan stalk, or stem (Fig. 9, 10, 362). Fans consist mainly of an array of variable-length, elegantly curved rays, beset with a row of microtrichia along the ventral edge (Fig. 7, 7a). There are other minor rows of rays, variously developed. Since larvae tend to turn the body so the fans intercept the water flow, the microtrichia are on the leading edge of the ray and in large part responsible for capture of particles in the water.
Immediately posteromedial of the fan stalks are the anterolaterally-directed antennae. In later instar larvae these consist of a basal, medial, and apical article and an apical sensillum (Fig 10, 367–383). The proportional length of the apical article provided in descriptions does not include the apical sensillum. In 1st instar larvae there is only the apical article and sensillum; the other articles are added in subsequent instars (Dumbleton 1964b; Crosby 1974a, b; Adler et al. 2004). The long, narrow, apical article is unusual for simuliids: although of only minor use taxonomically within Austrosimulium, it is an important phylogenetic link to the South American Paraustrosimulium.
The mouthpart structures are the medial labrum, paired mandibles and maxillae, and a fused labium and hypopharynx. All these structures are underlain by a hypostoma (Fig. 8, 9, 13). The labrum is the anterior continuation of the cephalic apotome and there is no delimitation between the two. Ventromedially the labrum merges with the labropalatum, or epipharynx. The mandibles (Fig. 14) are apically complex (Craig 1977), and in Austrosimulium show useful characters at the species-group level. Apically and protruding is the apical tooth, aborally are 3 variously developed outer teeth, adorally and more basal are 3 preapical teeth, then 9–11 sharp spinous teeth. In other simuliid larvae that have been examined there is normally a single substantial serration with a basal sensillum and finally a blade region (Fig. 421). In Austrosimulium, and probably diagnostic for the genus, the sensillum and serration are complex and which structure is which is not obvious. The maxillae are mitten-shaped with the palp representing the thumb. Maxillae show no useful variation and are not dealt with here, and neither is the complex labiohypopharynx.
The hypostoma is the anteroventral portion of the head capsule and underlies the labiohypopharynx. A trapezoidal toothed structure, it is delimited posteriorly from the postgenal bridge by the hypostomal groove (Fig. 13). Laterally, the hypostoma has an array of sublateral setae on each side, some 4 or 5 in Austrosimulium, often more in other genera. They are of little taxonomic interest here. However, family-wide, the 11–13 teeth on the dorsal side of the hypostoma are of considerable importance and of some importance for Austrosimulium. There is a single median tooth, 3 sublateral teeth on either side of it and usually of lower prominence, and then a single more prominent lateral tooth on each side. Next are variously developed paralateral teeth — 1 is normally obvious; a 2nd may or may not be so. Not part of the teeth series, and actually on the ventral wall of the hypostoma, are sporadically-developed, rounded lateral serrations (Fig. 404). The teeth are more often than not (cf Fig. 408 and 418) obscured by the ventral wall of the hypostoma, and the degree of this obscuring is of some diagnostic significance for Austrosimulium.
The region posterior to the hypostomal groove (Fig. 388) is termed the genal bridge and is generally featureless except for some colour patterning of moderate diagnostic value. Posterior to that is a region of the head capsule which is weakly sclerotised and non-pigmented — the postgenal cleft. It is defined laterally by the posterior tentorial pits — 2 heavily sclerotised and pigmented areas continuous laterally with the postocciput; and anteriorly, usually, by the ill-defined edge of the postgenal bridge. The shape and depth of the cleft are of major taxonomic importance in Simuliidae and can be used with value in Austrosimulium. However, the cleft is not significantly developed, if at all, in some species. Of some diagnostic use is that in the australense species-group the suboesophageal ganglion, which is pigmented, shows through the cleft (cf Fig. 401, 402).
Perhaps of most taxonomic use elsewhere, and for Austrosimulium larvae, is the frontolabral apotome (cephalic apotome), the central area of the head delimited laterally and posteriorly by ecdysial lines (Fig. 10, 321, 334). Muscle attachments for the mouthparts on the apotome are often pigmented (positive) (Fig. 320), or unpigmented (negative and neutral) (Fig. 322), and their patterns and arrangement are of considerable diagnostic value. The spots are usually described as an anteromedial-, posteromedial-, anterolateral-, and posterolateral group. Sometimes this latter group is fused with pigment along the posterior of the apotome and continuous with the well-sclerotised, rim-like postocciput laterally and ventrally around the back of the head (Fig. 320–325). There may be other pigmentation that is of use, such as an irregular gray mottling anterior to the muscle spots (Fig. 332). The course taken by the ecdysial lines and consequent shape of the apotome are often of use. Posterior to the apotome are 2 cervical sclerites. In some simuliids these are part of the postocciput, but in most simuliines they are separate. Their shape is of minor diagnostic use in Austrosimulium. There is occasional sexual dimorphism in head pigmentation, the female usually being the darker (cf Fig. 320 and 321).
Laterally on the head capsule are the 3 stemmata (ocelli or eye spots), although they appear as 2; a large and a smaller one (Fig. 321). Above them can be a curved pigmented eyebrow stripe. These characters are not of diagnostic value in Austrosimulium.
Immediately behind the head on the ventral surface of the thorax is the single thoracic proleg (Fig. 6, 306). Armed with a circlet of hooks apically, this structure is important to the larvae in dealing with salivary silk and locomotion. It is not of taxonomic value in Austrosimulium.
In later instar larvae, and in particular, mature final instar larvae, the developing histoblasts of the pupal gills, adult wings, and legs begin to show through the thoracic cuticle of the larva (Fig. 6, 305). The development of the wing and haltere histoblasts and venation was examined in detail for Austrosimulium by Crosby (1974c). Here, for Austrosimulium, the state of the pupal gill histoblast, when fully mature just prior to pupal ecdysis, is of equal importance to the pupal characters for species identification (Fig. 339–350, 351–366).
The colour of the abdomen is of minor use in diagnoses, and, as for the head, there might be sexual dimorphism in colour. Posteroventrally on the abdomen and immediately anterior to the anal proleg are 2 ventral tubercles (posteroventral tubercles); cone-shaped protuberances of the abdominal wall (Fig. 6, 12, 303). They are of minor use diagnostically.
Posteriorly is the anal proleg (Fig. 11, 12). This consists of an anal sclerite, semicircular sclerite (usually attached to an accessory sclerite), and a posterior circlet of hooks (crotchets), structures all of considerable taxonomic value (Fig. 437–453). The anal sclerite is usually X-shaped with variously developed anterior and posterior arms, and a median plate. In Austrosimulium, lateral to the median plate are rod-like interarm struts, also referred to as “backward struts” by Dumbleton (1973). Arising from the anterior arms the struts do not join the posterior arms, but in mounts they may appear to merge with them. The struts may taper and are of variable thickness, hence are useful for species diagnosis. The apices of the anterior arms are variously flared and emarginated as befitting a major attachment of abdominal muscles. The posterior arms taper posterolaterally, are of variable length, and are a minor diagnostic character.
Most Austrosimulium larvae possess a semicircular sclerite, a band of pigmented cuticle that surrounds the posterior circlet of hooks. All New Zealand species possess this. A superficially similar sclerite occurs in Gigantodax Enderlein, and sporadically in other genera. In them, more often than not, the posterior arm of the anal sclerite is continuous with this semicircular sclerite, but not so in Austrosimulium. The dorsolateral terminal expansions (=accessory sclerite) of the semicircular sclerite are variously flared and emarginated, and in some instance tapered, not flared. These states are of considerable use taxonomically.
The semicircular sclerite has been used as a character in phylogenetic analyses involving Austrosimulium (present in New Zealand species, absent in some in Australian), Paraustrosimulium (absent), Cnesiamima Wygodzinsky & Coscarón (absent), Lutzsimulium d’Andretta & d’Andretta (absent), and Gigantodax (present) (Dumbleton 1963b; Wygodzinsky & Coscarón 1962). There are, however, distinct possibilities that these various expressions of the sclerite are not homologous, but rather are homoplasious (arisen more than once). In all simuliid larvae examined (DAC, pers. obs. and unpublished) there is an unpigmented ring of cuticle underlying and supporting the circlet of hooks. It is this ring that is pigmented when a semicircular sclerite is deemed present, and can be seen in A. bicorne and A. unicorne where the ring is only partially pigmented (Fig. 451, 453) and the pigmented “semicircular sclerite”, as such, is narrowed. Additionally, many simuliid larvae have small accessory sclerites ventrolateral to the posterior arms of the anal sclerite. These sclerites are where abdominal muscle insert and there are associated campaniform sensory organs. Crosby (1974b) showed in A. tillyardianum that the 2nd larval instar, while possessing an anal sclerite, lacked both accessory and semicircular sclerites, but the 3rd instar had accessory sclerites. The semicircular sclerite occurred in subsequent instars and incorporated the original accessory sclerites, terminally. The position of the earlier accessory sclerites shows that the terminal expansions of the semicircular sclerite are merely modified accessory sclerites and dissection shows that muscles do attach at that location (Puri 1925; DAC, pers. obs.). Adler et al. (2004) noted for their character #148, in Gigantodax — “Anal sclerite with posterior arms extended ventrally around abdomen, forming a complete ring around base of posterior circlet of hooks” — that it should not be confused with the semicircular sclerite of Austrosimulium. Takaoka & Craig (1999) suggested that possession of such sclerites was an adaptation to higher velocity water. Since the semicircular sclerite appears to be homoplasious, as a character it should be used with caution in higher level phylogenetic analysis.
The posterior circlet consists of numerous rows of hooks (Fig. 11, 12, 448). Each hook has an elegant shape — en masse designed to attach to pads of salivary silk that the larva deposits on the substrate. These are intimately involved in locomotion and filter feeding (Barr 1984). There is a strong correlation between the number of hooks and the velocity of water inhabited by larvae (Palmer & Craig 2000), with higher hook numbers found in species in higher velocity. The same applies to Austrosimulium larvae, so hook number appears of some significance in habitat choice by larvae; particularly so for A. longicorne (p. 111).
Protruding from the anus, located between the anterior arms of the anal sclerite, are thin-walled, unpigmented rectal papillae, used for chloride ion uptake for osmotic regulation (Fig. 6, 11, 12, 308). Usually, in specimens preserved in lower percentage ethanol, these papillae are retracted. Often multibranched elsewhere, in Austrosimulium they are merely 3 simple lobes and of no taxonomic value. In some simuliids considered related to Austrosimulium, such as Paraustrosimulium, there are rectal scales on the anal sclerite and around the base of the papillae. Such scales are absent in New Zealand Austrosimulium species, but in some species (Fig. 439) there are a series of small sensillae on raised bases medially on the anal sclerite. They are probably campaniform sensilla.