Ilyobius chilensis (McLachlan)

Berrondo, Matias, 2017, Description of the larva of Ilyobius chilensis (McLachlan) (Megaloptera: Sialidae) and notes on the adult morphology, Zootaxa 4318 (1), pp. 177-186 : 178-183

Description of the mature larva of Ilyobius chilensis (McLachlan)

Diagnosis. Head capsule subrectangular, slightly wider than long, flat, bearing six stemmata on each side arranged in semicircle; circular ridge long, almost reaching coronal line; gula absent. Labrum sharply pointed. Antenna with second antennomere longer than third; third antennomere subequal to fourth. Mandibles with two preapical teeth (retinacula), basal tooth more than half the size of distal one, with two preapical teeth. Maxilla with second palpomere slightly longer than third and fourth palpomeres combined.

General appearance ( Figs. 1, 2 ): Length: 18–21 mm (not including caudal filament); maximum width: 3.0– 3.3 mm. Larva campodeiform, head and thorax strongly sclerotized, abdomen poorly sclerotized, tapering towards posterior end, ending in a long caudal filament. Color: head capsule light brown to pale yellow, with distinct pattern of dark brown marks and two black ocular areas; thoracic tergites light brown with patterned areas extensive; ventral surface of thorax and all abdomen dark purplish brown, abdominal gills lighter ventrally; legs light brown. Body elongate, wider at prothorax, tapering towards abdominal apex.

Head capsule ( Figs. 2–4 ): Length: 2.6–2.7 mm (not including labrum); width: 3.0– 3.3 mm. Subprognathous slightly retracted into prothorax. Light brown to pale yellow, smooth and shiny; clear areas smooth, darker areas areolate-rugulose; several sparse slender setae distributed on head capsule. Subrectangular, slightly broader basally, flattened. Six stemmata arranged in a semicircle. Clypeus well developed, narrow and broad, separated from frontal region by the frontoclypeal suture. Frontal lines coming together before mid-head, fusing with long coronal line, demarcating a small subtriangular frons and two large parietal regions; an incomplete neck delimited by a ridge extending dorsally from the sides of the coronal line into the ventral side of head capsule, reaching the base of mandibular acetabulum. Gular sclerite absent. Foramen occipitale slightly narrowed, transverse and subtrapezoidal.

Labrum ( Figs. 6, 7 ): Connected with clypeal margin by a narrow membrane; large, subtriangular, pointed distally, concave on lateral margins; basally with two sharp apophyses that serve for muscle attachment; dorsally with several long and slender setae (trichobothria) and two pairs of shorter apical setae, inner pair minute, outer pair short and stout; all outer margins bearing minute, peg-like setae.

Antenna ( Figs. 8 , 12): Four-segmented, slender, about half as long as length of head capsule. Inserted on anterolateral corners of head capsule. First antennomere about twice as long as wide, wider than remaining antennomeres. Antennomere 2 the longest, about 5Χ long as maximum width, more slender basally, gradually widening distally; with one small apical conical sensillum, accompanied by several small peg-shaped sensilla and pores. Antennomere 3 slender, slightly longer than antennomere 1 and slightly shorter than 4, lacking sensilla. Antennomere 4 very slender, rounded apically, with 6 short setiform sensilla distally.

Mandible ( Figs. 9, 10 ): Symmetrical, slender and long, reaching as far as antennae; sharply pointed, with two preapical teeth (retinacula) at about midlength, distal one stouter and slightly serrated; a minute toothlet may be visible at about basal third (difficult to see in specimens with worn out mandibles); outer margin convex, bearing one long and slender basal seta and one short slender seta at about midlength; inner margin concave.

Maxilla ( Figs. 11 , 13): Free, not connected with labium; formed by cardo, stipes, palpifer, a 4-segmented palpus, galea and lacinia. Cardo large, undivided, bearing one long, central seta ventrally. Stipes subriangular, smaller than cardo, outer margin with five long setae; two additional setae ventrally, one near central area, the other subbasally on inner margin. Lacinia attached to dorsal inner margin of stipes, large, hook-shaped, basally with six rather large setae on inner margin, two very stout setae at midlength on inner margin, four or five more short and stout setae on distal half of inner margin. Palpifer attached to ventral distal margin of stipes, large, subrectangular, longer than wide, bearing three slender setae on ventral side. Galea one-segmented, cone-shaped, with one slender seta on inner margin at midlength and several small apical sensoria. Palpus four-segmented, palpomere 1 the shortest, wider than long, bare; palpomere 2 the longest, with one long apical seta on outer margin; palpomere 3 shorter, with two setae, one short on inner margin, the other long, apical, on ventral side; palpomere 4 as long as palpomere 3, narrower, cone-shaped, bare.

Labium ( Figs. 11 , 14): Formed by submentum, mentum and prementum. Submentum large, subrectangular, wider at midlength, twice as long as wide, ventrally with a pair of lateral stout and long setae on basal third, two smaller and slender setae closer to midline at distal third. Mentum subquadrate, slightly narrower than prementum, incompletely sclerotized, distal half membranous; three lateral slender setae at midlength on each outer margin. Prementum wider than long, with well-developed membranous ligula, bearing several pairs of long and slender setae on dorsal, lateral and ventral sides. Palpus three-segmented, basal palpomere the shortest, slightly wider than long, with two apical setae on outer margin, one long and stout, the other very short; second palpomere the longest, twice as long as wide, curved inwards, bearing two long setae, one on basal third, the other subapical; third palpomere cone-shaped, bearing several apical sensoria.

Thorax ( Figs. 1, 2, 5 ): All thoracic tergites with sagittal line. Pronotum large, subrectangular and slightly wider than long, anterior margin straight, posterior corners and margin slightly rounded; meso- and metanotum shorter, ca. 2Χ wider than long, anterior margins slightly concave, lateral and posterior margins rounded. Legs well developed, six-segmented, with unequal claws. Coxa the largest segment, those of meso- and metathoracic legs larger; trochanter narrow and elongate; femur large, flattened antero-posteriorly, with longitudinal rows of swimming hairs on dorsal and ventral margins; tibia shorter and narrower than femur, not flattened, with longitudinal rows of swimming hairs on dorsal and ventral margins; tarsus one-segmented, ca. 0.3–0.4Χ the length of tibia, bearing rows of swimming hairs; anterior claw longer in pro- and mesothoracic legs, shorter in metathoracic leg.

Abdomen: Ten-segmented, wider anteriorly, tapering towards posterior end; first segment the shortest. ca. 0.5Χ the length of second segment. Lateral abdominal filaments (gills) present on anterior seven segments, segments eight and nine lacking filaments, segment ten small, bearing a long terminal filament ca. as long as length of abdomen. Lateral filaments five-segmented, tapering, basal segment shortest. Spiracles: Present on mesothorax, ventrally just in front of procoxa, and in first eight abdominal segments, laterally, anterior to lateral filaments on first seven abdominal segments.

Figures

FIGURES 1 – 5. mature larva of Ilyobius chilensis. 1) Partial habitus, dorsal view; 2) head and thorax, lateral view; 3) head capsule, dorsal view; 4) head capsule, ventral view; 5) prothoracic leg, frontal view. Scale bars, Figs. 1 – 2 = 2 mm; Figs. 3 – 5 = 1 mm.

FIGURES 6 – 11. mature larva of Ilyobius chilensis. 6) Labrum, dorsal view; 7) detail of apical portion of labrum, dorsal view; 8) antenna, dorsal view; 9) mandible, dorsal view; 10) mandible, ventral view; 11) maxillolabial complex, ventral view. Scale bars, Figs. 6, 8 – 11 = 0.5 mm.