Armostus ohyamatensis Hoshina and Satô, 2006

Minoshima, Yûsuke N., 2018, Larval Morphology of Armostus ohyamatensis Hoshina and Satô (Coleoptera: Hydrophilidae: Megasternini), The Coleopterists Bulletin 72 (4), pp. 767-778 : 768-777

Armostus ohyamatensis Hoshina and Satô, 2006 ( Figs. 1–8 )

Material Examined. One L1: YNME #16-16; two L3: YNME #16-1, 16-15 (see Table 1).

Additional Material Examined. One L3, one L2: Loc. #2016-11, collected together with YNME #16-15, 16-16 (see Table 1).

Diagnosis of Larva. Body moderately stout ( Fig. 1A ). Frontal lines lyriform (clearly visible in L1 and L2) ( Fig. 2A ). Surface of head capsule bearing fine microtrichiae ( Fig. 1B ) and granulate posterolaterally (especially in L3) ( Fig. 1C ). Egg busters present ( Fig. 2A ). Antenna short; antennomere 2 with 1 additional pore; antennal sensorium SE1 stout, as long as antennomere 3 ( Figs. 3A , 6A ). Mandible asymmetrical, left mandible without inner tooth, right mandible with single inner tooth ( Figs. 3B, C , 6B, C ). Stipes asymmetrical, bearing cuticular hair-like projections, MX 8–11 intermixed with 3 additional setae, forming gMX2 ( Figs. 3D, E , 7A, C ). Maxillary palpomere 1 cylindrically sclerotized, whereas palpomere 2 incompletely sclerotized dorsally (often sclerite narrowly attached dorsally) ( Figs. 3E , 7A, C ). Submentum with a pair of deep lateral notches ( Fig. 2C ). Mentum and prementum short, ligula reduced, hypopharyngeal lobe present ( Figs.3G, H , 7A ). Dorsal plate of spiracular atrium very large, posterior edge sinuate, with a triangular posterior projection (different intraspecifically, more weakly sinuate in early instars) ( Figs. 1E, F , 4F ). Spiracular atrium without finger-like lateral projections.

Description of First Instar. General Morphology. General morphology of L1 similar to that of L2 and L3. Frontal lines clearly visible, lyriform ( Fig. 2A ). Surface of head capsule sparsely covered with very fine microtrichiae, not to very weakly granulate posterolaterally. Hair-like cuticular projections on stipes sparser than those on L3 ( Fig. 3D, E ). Posterior edge of dorsal plate of spiracular atrium weakly sinuate ( Fig. 1F ).

Primary Chaetotaxy of Head. Frontale ( Fig. 2A, D ): Rather long seta FR1 on posterior fourth, close to frontal line; pore-like sensillum FR2 and short seta FR3 on midlength of head capsule,

posteromesal to FR5. Long seta FR5 and rather short seta FR6 behind antennal socket, FR6 lateral to FR5. Pore-like sensillum FR4 posteromesal to antennal socket. Position of sensilla on anterior part of frontale asymmetrical. Right side: Aggregated sensilla FR8–10 and FR14 anteromesal to antennal socket; short seta FR10 behind moderately long setae FR8 and FR9, FR8 mesal to FR9. Pore-like sensillum FR14 close to inner margin of antennal socket. Short seta FR 12 anterior to FR8, pore-like sensillum FR13 posteromesal to FR12, FR11 may be absent (see Remarks). Pore-like sensillum FR15 posterolateral to egg burster. Left side: Aggregated sensilla FR8–10 and FR12–14 anteromesal to antennal socket; FR14 very close to inner margin of antennal socket; FR9 between FR14 and FR8; FR10 behind FR8, FR12 and FR13 anterior to FR9 and FR8 respectively. FR15 posterolateral to egg burster. gFR1 with 5 setae on anterior margin and 2 minute setae below egg bursters.

Parietale ( Fig. 2A–C ): Dorsal surface with group of 5 sensilla (PA1–5) posteriorly [PA2 missing on right side of molecular voucher, for left side see Fig. 2B ], PA3 pore-like, remaining ones minute setae; PA1, PA2, PA4, and PA5 forming irregular longitudinal row, PA3 mesal to PA1 and PA2. PA6 pore-like, close to posterior end of frontal line. PA7 moderately long seta situated dorsally on posterior third; long seta PA8 and pore-like sensillum PA10 dorsally on anterior third, PA8 mesal to PA10. PA9 and PA19–22 aggregated on anterior corner of head capsule, forming irregular transverse row, from dorsal to ventral side, long seta PA9, pore-like sensillum PA19, short seta PA20, very long seta PA21, short seta PA22. Pore-like sensilla PA23–25 close to ventral mandibular articulation; PA23 and PA24 on posterior part of outer margin, PA25 on anterior part of inner margin. PA12–15 on midlength of lateral surface; very long setae PA12 and PA13 posterior to rather long seta PA14; pore-like sensillum PA15 ventral to PA12–14. PA16, PA17, and PA26 closely situated on anterior fourth of lateroventral face, seta PA16 lateral to PA17, pore-like sensillum PA17 between PA16 and short seta PA26, PA26 posteromesal to PA17. Very long seta PA18 and pore-like sensillum PA30 situated lateroventrally on posterior third, PA18 anterolateral to PA30. PA27–29 on midwidth between lateral face and gular sulcus; porelike sensillum PA27 and seta PA28 on anterior third, pore-like sensillum PA29 on posterior third.

Antenna ( Fig. 3A ): Antennomere 1 with 5 porelike sensilla (AN1–5). AN1 situated dorsolaterally on posterior third; AN2 dorsally on distal fifth; AN3–5 subapical, AN3 laterodorsal, AN4 and AN5 on inner face, AN4 dorsal, AN5 ventral. Antennomere 2 with 2 pore-like sensilla (AN6 and additional sensillum) situated dorsally on subapical (AN6) and proximal (additional sensillum) parts of sclerite. Setae AN7, AN8, AN10, AN11, and sensorium (SE1) on anterior part of antennomere 2, AN9 absent. Short seta AN7 and minute seta AN8 on intersegmental membrane between palpomeres 2 and 3, behind SE1; very long seta AN10 and short seta AN11 closely situated on inner face, AN11 on border between sclerite and intersegmental membrane, AN10 anterior to AN11. Sensorium SE1 stout, almost as long as antennomere 3, sclerotized or pigmented in basal third. Antennomere 3 with group of apical sensilla (gAN) in apical membranous area; gAN variable in length and shape.

Mandibles ( Fig. 3B, C ): Asymmetrical. Right mandible: Short seta MN1 situated laterally on posterior third. Pore-like sensilla MN2–4 forming oblique row on median part of dorsal surface; MN2 between MN3 and MN4, MN4 on lateral face. Minute seta MN5 on lateral face, anterior to MN4. MN6 undetectable, but by comparison with older instars, MN6 may be present subapically on incisor area. Four pore-like sensilla on apical part. Left mandible: MN1–4 similar to right mandible; MN5 subapically on lateral face. Three minute sensillumlike structures on subapical part.

Maxilla ( Fig. 3D–F ): Cardo with 1 ventral seta ( MX 1). Inner face of stipes with a row of 8 stout setae, possible MX 7 at base, remaining 7 forming gMX2. Part of gMX2 on right maxilla with subapical tooth (often undetectable due to direction of seta). Pore-like sensilla MX 2–4 on ventral face; MX 2 on median part, MX 3 on apical fourth of inner face, MX 4 on subapical part of lateral face of sclerite. Long seta MX 5 and short seta MX 6 situated laterally and subapically on sclerite. Dorsal surface of palpomere 1 with 1 short seta ( MX 16) situated basally on inner face. MX 12–14 located ventrally on anterior part of sclerite; MX 12 pore-like on lateral part, very long seta MX 13 and rather long seta MX 14 on midwidth. Pore-like sensillum MX 15 situated ventrally behind gAPP. MX 17 absent. Four setae of variable length and shape (gAPP) present apically on inner face of intersegmental membrane between palpomeres 1 and 2. Palpomere 2 with 2 pore-like sensilla ( MX 18, MX 19) and 1 minute seta ( MX 27). PA18 on lateral face, PA19 on inner face of intersegmental membrane between palpomeres 2 and 3; MX 27 located proximally on lateral face. Palpomere 3 with 2 pore-like sensilla ( MX 20 and MX 22), and long ( MX 21) and very long ( MX 23) setae. MX 23 on midlength of lateral face; MX 20–22 on ventral face, MX 20 on lateral part, MX 21 on inner part, MX 22 on median part. Palpomere 4 with 1 short seta ( MX 24), and digitiform ( MX 25) and pore-like ( MX 26) sensilla; MX 24 situated proximally on inner face; MX 25 and MX 26 dorsally on lateral ( MX 26) and median ( MX 25) parts. Apical membranous area of palpomere 4 with several minute setae (gMX).

Labium ( Figs. 2C , 3G, H ): Submentum with 2 pairs of setae (LA1, LA2); LA1 long on median part, LA2 short on median part of anterior margin ( Fig. 2C ). Mentum ( Fig. 3H ) with 2 pairs of sensilla (LA3, LA4) situated ventrally on lateral face, LA3 seta, LA4 pore-like. Prementum and its adjacent membranous area with 3 pairs of sensilla (LA5–7). LA5–7 on median part; LA5 very minute, between LA6 and LA7, LA6 very long seta on sclerite, mesal to LA5, LA7 pore-like sensillum. Digitiform sensillum LA12 on membrane between palpomeres. Palpomere 1 with a minute seta (LA13) proximally and pore-like sensillum (LA14) on intersegmental membrane between palpomeres 1 and 2. Palpomere 2 with 1 pore-like sensillum (LA15) on midlength ( Fig. 3G ); several minute setae of variable shape (gLA) on apical membranous area.

Description of Second Instar. General morphology and chaetotaxy of L2 very similar to L3. Frontal lines clearly visible. Head capsule covered with microtrichiae and with granulate posterolateral face, but both finer and weaker than in L3.

Description of Third instar. General Morphology. Body ( Fig. 1A ) light yellowish white, moderately flat, maggot-like in shape, widest in posterior half.

Head capsule ( Fig. 4A ): Oval; cervical sclerite transverse, indistinct. Frontal lines almost undetectable, coronal line absent. Surface of head capsule densely covered with microtrichiae, granulate posterolaterally ( Fig. 1B, C ). Large stemma on each anterolateral corner of head capsule. Posterior tentorial pits present on median part close to submental sulcus ( Fig. 5B ). Clypeolabrum asymmetrical; borderline between nasale and epistome undetectable ( Fig. 5C ). Anterior margin of head capsule without teeth, rounded apically with round incision on left side close to mesal line; margin around the incision area bearing fine and dense fringe ( Fig. 5C ).

Antenna ( Fig. 6A ): Three-segmented; surface smooth. Antennomere 1 longest and widest, antennomere 3 shortest and narrowest. Approximate ratios of length of antennomeres 1–3 as follows: 1.0:0.6:0.3 (n = 1). Antennal sensorium present, long.

Mandibles ( Fig. 6B–D ): Stout, asymmetrical, left mandible slightly longer than right one. Left mandible without inner tooth, narrow in apical half, distinctly widened in basal half; incisor area finely and weakly serrate. Right mandible with 1 inner tooth present on apical third, inner tooth serrate; distinctly widened at basal third.

Maxilla ( Fig. 7 ): Six-segmented, longer than antenna, asymmetrical. Cardo moderately large, irregularly shaped. Stipes asymmetrical, much longer than palpomeres 1–4 combined. Left stipes ( Fig. 7A, B ) wider than right one, dorsal surface less sclerotized and pigmented except for median part, bearing fine and densely arranged hair-like cuticular projections on inner and lateral parts and anterior margin. Right stipes ( Fig. 7C, D ) with dorsal surface well-sclerotized and pigmented, bearing fine and densely arranged hair-like cuticular projections on anterolateral corner of sclerite, and fine and minute cuticular projections behind hair-like projections. Maxillary palpus short, 4-segmented; palpomeres 1–3 wider than long, 4 longer than wide. Approximate ratios of length of palpomeres 1–4 as follows: 1.0:0.6:0.6:0.7 (n = 1). Palpomere 1 widest; dorsal surface bearing small cuticular projections medially. Inner process reduced. Palpomere 2 wider than palpomeres 3 and 4, incompletely cylindrically sclerotized dorsally but often very narrowly connected dorsally. Palpomere 4 narrowest.

Labium ( Figs. 5B , 8 ): Reduced excluding submentum. Submentum ( Fig. 5B ) fused to head capsule, large, transverse, with a pair of deep lateral notches slightly asymmetrical. Submental sulcus indistinct. Mentum ( Fig. 8 ) very short, transversely subrectangular. Hypopharyngeal lobe present, subtriangular, projecting into left side, bearing dense pubescence. Prementum reduced as transverse and irregularly shaped sclerite on medial ventral face. Ligula reduced. Labial palpus moderate in length; palpomere 1 short, palpomere 2 moderately long.

Thorax ( Figs. 1A , 4B–E ): Membranous parts very densely covered with microtrichiae. Prothorax wider than head capsule ( Fig. 1A ). Proscutum formed by 1 large plate subdivided by fine sagittal line; anterior and posterior margins weakly sclerotized, bearing extremely fine surface microstructures; proscutal plate bearing several sensilla of variable length and very densely covered with microtrichiae ( Figs. 1D , 4B ). Prosternal sclerite ( Fig. 4C ) subquadrate, transverse, bearing several sensilla, with 1 pair of patch bearing microtrichiae; anterior part less pigmented. Mesonotum ( Fig. 4B ) with 1 pair of large dorsal sclerites attached mesally; mesonotal plate bearing several sensilla and covered with microtrichiae on anterior part. Mesothoracic spiracles projecting laterally, forming small, fingerlike projection ( Fig. 4B ). Metanotum ( Fig. 4B ) with 1 pair of dorsal sclerites attached mesally; sclerites moderate in size, smaller than mesonotal sclerites, bearing few sensilla, and sparsely covered with microtrichiae posteriorly. Legs ( Fig. 4E ) reduced, extremely short, 2-segmented but borderline between membrane and sclerite mostly undetectable, bearing setae of variable length and hair-like cuticular projections.

Abdomen ( Figs. 1A, E , 4F ): Ten-segmented, broad; membranous parts densely covered with microtrichiae. Segments 1–7 similar to each other, with 1 pair of weakly tuberculate spiracles laterally. Spiracular atrium ( Figs. 1E , 4F ) with dorsal plate very large, suboval, covered with microtrichiae and numerous sensilla ( Fig. 1E ). Posterior edge of dorsal plate sinuate, with triangular posterior projection medially; sinuation and size of triangular projection variable. 1 pair of large spiracles present in spiracular atrium. Procercus moderate in size, inner face partly sclerotized, narrowing apically. Segment 9 trilobed, with a pair of 1-segmented, sclerotized urogomphi bearing a very long seta. Lateral lobe larger than procercus, partly sclerotized. Acrocercus absent. Median lobe broader than lateral lobe, dorsal surface sclerotized medially.

Chaetotaxy. Frontale ( Fig. 5A, C ) with 1 secondary pore mesal to PA12. Parietale ( Fig 5A, B ) with 1 very minute sensillum close to PA2, 1 minute sensillum close to PA5, 1 short seta between PA4 and PA7. 1 secondary seta between PA7 and PA12/ 13, undetectable from PA12–14. 1 secondary seta between PA8 and PA9, 1 secondary seta mesal to PA9. 1 secondary sensillum close to PA19. 1 rather short secondary seta ventral to PA11. Mandibles with 2–3 short secondary setae present basally on lateral face, 1 small seta on anterior third.

Biology. Adults and larvae were collected from leaf litter ( Fig. 1G, H ) (Hoshina and Satô 2006a).

Remarks. Megasternini are the most genus-rich group within the Hydrophilidae (Short and Fikáček 2011, 2013). This indicates that the adults of Megasternini are morphologically highly diverse. However, their larval morphology is similar among the genera ( Archangelsky 2016). Larval morphology of Armostus is especially similar to that of Cercyon and Cryptopleurum . Differences in larval morphology among Armostus and the other two genera are mostly ambiguous, but there is a significant difference: the incompletely cylindrically sclerotized maxillary palpomere 2 ( Figs. 3E , 7A, C ), which is unique to A. ohyamatensis among the known larvae of Hydrophilidae . Although maxillary palpomeres sometimes can be incompletely sclerotized in hydrophilid larvae, this character state has always been observed in palpomere 1 in other genera (e.g., Archangelsky 1997; Minoshima and Hayashi 2011).

Larval chaetotaxy of megasternine genera remains largely unknown. Cercyon is the only genus for which the chaetotaxy has been described ( Fikáček 2006; Fikáček et al. 2008; Archangelsky 2016). Based on our study, the chaetotaxy of Armostus and Cercyon are similar, but several differences are observed. A group of setae, gFR1, seems to differ within the genus, composed of seven setae in Armostus ( Fig. 2D ) but seven or eight in Cercyon ( Archangelsky 2016) . Fikáček et al. (2008) treated gFR as the setae on the anterior margin only, thus they might report the number as five. Homology of the pores FR11 and FR13 is uncertain, and I assumed the pore close to FR12 as FR13 based on its position ( Fig. 2D ). In this context, FR11 is absent in Armostus , whereas it is present in Cercyon ( Fikáček et al. 2008; Archangelsky 2016). However, the FR11-like (secondary) pore appears in L2 and L3, anterior to “FR13” on the right side; homology of the pore is unclear. PA12–14 are closely aggregated in Cercyon but rather loosely distributed in Armostus ; they are therefore homologized in Armostus ( Fig. 2A ). This character is similar to that of Pelosoma ( Archangelsky 2016) . The maxillary setae (gMX) are in an irregular, longitudinal row in Armostus ( Fig. 3D, E ), whereas they comprise two groups (basal four and distal three) in Cercyon quisquilius (Linnaeus) ( Archangelsky 2016) .

On the other hand, Armostus larvae share the potential synapomorphies of Megasternini proposed by Archangelsky (2016) except for the: arrangement of PA12–14 (see above); presence of an additional pore on the base of antennomere 2 ( Figs. 3A , 6A ); basal setae of gMX2 on the left maxilla arranged in an oblique row (fewer setae than Cercyon ) ( Figs. 3D , 7A ); gMX composed of 5–9 setae (7 in Armostus )); and reduced legs ( Fig. 4D, E ). In addition, Archangelsky (2016) mentioned the stipital asymmetry in the tribes Megasternini and Sphaeridiini , which is also observed in Armostus ( Figs. 3D, E , 7 ). Besides Megasternini and Sphaeridiini , asymmetrical stipes are observed in the rygmodine genera Andotypus Spangler, 1979 and Austrotypus Fikáček, Minoshima, and Newton, 2014 ( Fikáček et al. 2014). However, the asymmetry is only apparent in the outline of the stipes in Rygmodinae , whereas it is seen in the outline, shape of the sclerite, and arrangement of cuticular hairs in Megasternini .

Figures

Fig. 1. Larvae and collecting localities of Armostus ohyamatensis. A) Third instar habitus, ventral and dorsal views, B) Third instar, posteromesal surface of head capsule, dorsal view (FR1 = frontale setae, FR2 = frontale pores, PA7 and PA8 = parietale setae), C) Third instar, posterolateral surface of head capsule, dorsal view D) Third instar, anteromesal surface of prothorax, dorsal view E) Third instar, dorsal plate of spiracular atrium (t = trachea), F) First instar, dorsal plate of spiracular atrium, G) Collecting locality Loc.#2016-8, H) Collecting locality Loc.#2016-11 (same as Loc.#2017-23). A–E = larva YNME#16-1; F = larva YNME#16-16.

Fig. 2. Armostus ohyamatensis, first instar YNME#16-16, head capsule. A) Dorsal view, B) Arrangement of sensilla PA1–7 on left side, C) Ventral view, D) Anterior margin, dorsal view. gFR = group of sensilla on frontale; FR = frontale; LA = labium; PA = parietale. Fine surface structures are omitted.

Fig. 3. Armostus ohyamatensis, first instar YNME#16-16, head appendages. A) Antenna, dorsal view, B–C) Mandibles, dorsal view, D) Left stipes, dorsal view, E–F) Right stipes, dorsal (E) and ventral (F) views, G–H) Labium, dorsal (G) and ventral (H) views. AN = antenna; gAN = group of the apical antennal sensilla; gAPP = group of sensilla on inner appendage of maxilla; gLA = group of the apical sensilla on labial palpus; gMX2 = group of sensilla on inner face of stipes; LA = labium; MN = mandible; MX = maxilla; SE = sensorium.

Fig. 4. Larvae of Armostus ohyamatensis. A) Third instar YNME#16-15 (broken parts modified), head, dorsal view, B) Third instar, thorax, dorsal view, C) Third instar, prosternal sclerite, ventral view, D) First instar YNME#16- 16, anterior leg, ventral view, E) Third instar, anterior leg, ventral view, F) Third instar YNME#16-1, spiracular atrium, dorsal view. l = lateral lobe of spiracular atrium; m = median lobe of spiracular atrium; p = procercus; sp = spiracle; t = trachea. Fine surface structures are omitted.

Fig. 5. Armostus ohyamatensis, third instar YNME#16-1, head capsule. A) Dorsal view, B) Ventral view, C) Anterior margin, dorsal view. gFR = group of sensilla on frontale; FR = frontale; LA = labium; PA = parietale. Fine surface structures are omitted.

Fig. 6. Armostus ohyamatensis, third instar YNME#16-1, head appendages. A) Antenna, dorsal view, B–C) Mandibles, dorsal view, D) Inner margin of right mandible, dorsal view, different direction from C. AN = antenna; gAN = group of the apical antennal sensilla; MN = mandible; SE = sensorium.

Fig. 7. Armostus ohyamatensis, third instar. A) Left maxilla, dorsal surface, B) Left maxilla, ventral surface, C) Right maxilla, dorsal surface, D) Right maxilla, ventral surface. All drawn from dorsal view. gAPP = group of sensilla on inner appendage of maxilla; gMX2 = group of sensilla on inner face of stipes.

Fig. 8. Armostus ohyamatensis, third instar YNME#16-1. A) Labium, ventral view, B) Labial palpus, ventral view.