Guyanobius adocetus Spangler, 1986

Archangelsky, Miguel, Rodriguez, Georgina & Torres, Patricia Laura Maria, 2022, Testing the monophyly of Chaetarthriinae (Coleoptera, Hydrophilidae) and the phylogenetic position of Guyanobius with larval characters, Arthropod Systematics & amp; Phylogeny 80, pp. 229-242 : 229

3.1. Third instar larva of Guyanobius adocetus Spangler, 1986

Diagnosis.

The following combination of characters distinguishes Guyanobius larvae from any other known hydrophilid larvae.

Larval morphology.

Head capsule subquadrate (Fig. 1 ); frontal lines inversely bell-shaped, converging towards occipital foramen but not coming together, coronal line absent; clypeolabrum symmetrical, nasale bearing five sharp teeth (Fig. 2 ), lateral ones slightly shorter; lateral lobes of epistome symmetrical, not projected farther than nasale, bearing a sharp spine projecting mesally; posterior tentorial grooves close to midline, subapical. Cervical sclerites present. Antenna short, first and second antennomeres subequal in length, basal one slightly wider; third antennomere short and narrow; first and second antennomeres bearing sharp cuticular spines on dorsal surface. Mandibles symmetrical, with three inner teeth, basal one smaller. Maxilla with large stipes, longer than palpus, dorsal face with sharp cuticular spines, apically with a stout spine on inner margin; first palpomere slightly longer, wider than long, incompletely sclerotized and bearing small cuticular spines on dorsal and inner faces, remaining palpomeres subequal in length. Labium stout, submentum wide, mentum subtrapezoidal, much wider than prementum, with strong cuticular projections on dorsal face; prementum wider than long with cuticular spines on anterior corners; palpi with few small cuticular spines, basal palpomere the shortest; ligula much longer than palpi, strongly sclerotized. Prothoracic plate large, covering most of pronotum, with sagittal line, sternal sclerite subrectangular, with sagittal line; meso- and metathorax with pleural areas strongly lobed, with one pair of narrow subrectangular tergites, those of metathorax narrower. Legs short, five-segmented. Abdominal segments poorly sclerotized, with one pair of small oval sclerites dorsally and pleural areas strongly lobed; segment eight covered by a large dorsal plate; pleura and lateral margins of abdominal tergites bearing stout asperities.

Chaetotaxy.

Frons with six secondary setae on each side along inner margin of frontal lines; gFR1 with eight setae, six stout, dorsal setae and two smaller setae ventrally, below two dorsal innermost setae; gFR2 with four stout setae, bifid apically; FR1 short and stout; pores FR15 not closely aggregated. Parietale with setae PA13 and PA14 closely aggregated; seta PA16 short; PA26-28 forming a triangle. Antenna with AN9 absent; SE1 as long as A3. Mandible with MN1 rather long, on basal fifth; minute seta MN5 closer to pore MN4 than to apex; MN2-4 forming a triangle. Maxilla with seta MX7 slender; setae MX8-11 stout and bifid apically; two secondary setae on ventral side near seta MX5; seta MX24 very long. Labium with 18 secondary setae on mentum along outer and anterolateral corners; seta LA5 rather long; seta LA10 at base of ligula; pore LA11 sub-basal. Morphometric measures are detailed in Table 1 .

Description of chaetotaxy.

Head capsule (Figs 3 - 5 ). Frontale with 30 primary sensilla and 12 secondary setae: two stout setae at midlength close to frontal lines (FR1); two pores (FR2) and two short setae (FR3) closer to midline on distal half; two pairs of setae (FR5 stout and short, FR6 slender and rather long) and one pore (FR4) close to base of antennal socket; short seta (FR7) on inner margin of antennal socket; distal area of frontale in front of antennal socket with three setae (FR9 short, FR10 long, FR12 minute) and two pores (FR11 and FR13); central area behind nasale with on pair of rather long setae (FR8) and one pair of pores (FR15); nasale dorsally with six stout setae intercalated between teeth and two short setae ventrally, below dorsal two innermost setae (gFR1); each epistomal lobe with four stout and apically bifid curved setae (gFR2). Each parietale with 30 primary sensilla and 17-19 secondary sensoria on dorsal and lateral surfaces (16-18 setae and on pore). Dorsal surface with a basal longitudinal row of four minute setae (PA1, PA2, PA4, PA5) and one pore (PA3); one sub-basal pore (PA6) close to but not touching frontal lines; four setae arranged in an oblique row behind stemmata (PA7 rather long and slender, close to frontal line, PA12 and PA13 short and stout, PA14 rather long and slender on outer face); one stout and short seta (PA8) close to frontal line on distal third of parietale; one pore (PA10) between two innermost stemmata; three long setae (PA9, PA20, PA21) and one pore (PA19) distal to stemmata; one short seta (PA11) between PA14 and PA20; secondary setae arranged as follows: 3-4 along frontal lines, 6 behind stemmata, one between middle stemmata, two distal to stemmata and one on outer face; one secondary pore between seta PA9 and pore PA19. Ventral surface with three pores (PA23, PA24, PA25) and one long seta (PA22) on anterolateral corner, behind mandibular acetabulum; three setae (PA26 and PA18 long, PA16 short) and three pores (PA15, PA17, PA30) along outer margin; one rather short seta (PA28) and two pores (PA27, PA29) forming a triangle closer to midline; three minute secondary setae on outer face close to pore PA15. Antenna (Fig. 6 ). A1 with five pores, three dorsal (AN1 at midlength closer to outer margin, AN2 subapical closer to inner margin, AN4 distally on membrane of inner margin) and two ventral on distal margin (AN3 on outer margin, AN5 on inner margin). A2 with one dorsal pore (AN6) on distal fourth, two minute apical setae on outer margin (AN7, AN8) close to base of SE1, and two apical setae on inner margin (AN10 long, AN11 very short); AN9 absent. A3 bearing a group of at least four short setae, one pore and two long setae (gAN); SE1 as long as A3. Mandible (Fig. 9 ). Bearing five primary sensilla and six or seven secondary setae; three dorsal pores on retinacular area arranged in a triangle (MN2 and MN4 on outer margin, MN3 at base of distal retinaculum); one rather long seta (MN1) on outer margin at basal fifth of mandible and one minute seta (MN5) on outer margin at distal quarter; pore MN6 not found; two secondary short sub-basal setae on outer margin and four or five more secondary setae near pore MN2, two or three minute behind MN2, one minute and one rather short between pores MN2 and MN4. Maxilla (Figs 10-11 ). Cardo with a rather long seta (MX1); stipes with an inner row of five setae (MX7-11), MX7 slender, remaining ones stout and bifid apically; ventrally with three pores (MX2 at basal third, MX3 at midlength on inner margin, MX4 at distal third on outer margin) and four long setae on outer margin (MX6 subapical, MX5 close to MX4 and two secondary setae). MP1 dorsally with one basal hair-like seta (MX16) and one pore at base of appendage (MX17); ventrally with two long subapical setae (MX13, MX14) and two pores (MX12 on outer margin and MX15 at base of appendage); inner appendage with two long setae and two short sensoria (gAPP). MP2 with two pores, one ventral and apical (MX18) and one dorsal on membrane connecting with MP3 (MX19); minute seta MX27 basal on outer margin. MP3 ventrally with two rather long setae (MX21 on inner margin, MX23 on outer margin) and two pores (MX20, MX22). MP4 with one basal long seta (MX24) on inner margin and two subapical pores on outer face (MX25 digitiform and dorsal, MX29 ventral); a group of at least seven short sensoria constitute gMX. Labium (Figs 4 , 7-8 ). Submentum with two pairs of setae, one long (LA1), the other very short, on anterior margin (LA2). Mentum ventrally with two rather long setae (LA3) and two pores (LA4) close to anterolateral angle; distal and outer margins with nine pairs of stout secondary setae. Prementum ventrally with two pairs of setae (LA6 long and subapical, LA5 short, basal) and one pair of pores on distal margin (LA7); dorsally with one pair of pores on disc (LA8) and one pair of minute seta-like sensilla (LA9) on membrane connecting with labial palpi. Ligula with three pairs of sensilla, one pair of very long basal setae (LA10) and two pairs of pores (LA11 sub-basal and ventral, LA12 subapical and dorsal). LP1 with one minute seta (LA13, ventral) and one distal pore (LA14, dorsal) on membrane connecting with LP2; LP2 dorsally with one subapical pore on outer face (LA15); distally with a group of at least six or seven sensoria (gLA).

3.2. Cladistic analysis

The unconstrained MP analysis produced one most parsimonious tree (549 steps); the tree with support values is shown in Fig. 12 . Chaetarthriinae appears as non-monophyletic, with Chaetarthria branching early on. The remaining genera of Chaetarthriinae cluster in one clade but including Paracymus + Tormus ( Hydrophilinae : Laccobiini ). The tribes Chaetarthriini and Anacaenini were not recovered as monophyletic. Guyanobius appears with low support as the sister group of the Anacaenini (( Pseudorygmodus ( Crenitis Crenitulus )), and forms a clade with Paracymus and Tormus , which diverge earlier into two successive branches.

Figure 15 shows a detailed diagram of the Chaetarthriinae clade with the synapomorphies mapped on it. The complete most parsimonious tree with all synapomorphies is shown in Supplementary file 5.

To test if the resultant topology of the unconstrained analysis was not affected by the inclusion of Paracymus + Tormus , we performed a constrained MP forcing Chaetarthriinae monophyly. This analysis generated two most parsimonious trees (554 steps) with no differences in the divergence patterns for the subfamily; a detail of the Chaetarthriinae clade is shown in Figure 13 . In this case, Guyanobius appears nested within Anacaenini , forming a cluster with Crenitis + Crenitulus .

The unconstrained BI with homoplasy as a partitioning scheme does not recover neither the subfamily nor the tribes as monophyletic (Fig. 14 ). Chaetarthria appears as a basal taxon, sister to all taxa included, and Guyanobius is revealed to be nested within Anacaenini . Guyanobius clustered with Crenitis + Crenitulus with a rather high support (pp=0.94) and Pseudorygmodus appears as sister group of all of them.

Figures

Figure 15. Detail of the tree that includes Chaetarthriinae with synapomorphies mapped. Characters in red indicate unique transformations; characters in white indicate homoplasious transformations.

Figures 1 - 2. Guyanobius adocetus, third instar larva. (1) head capsule, dorsal view; (2) detail of clypeolabrum, dorsal view.

Figures 3 - 4. Guyanobius adocetus, third instar larva, head capsule. (3) dorsal view; (4) ventral view. Scale bar: 0.2 mm.

Figures 5 - 8. Guyanobius adocetus, third instar larva. (5) detail of clipeolabrum, dorsal view; (6) left antenna, dorsal view; (7) labium, dorsal view; (8) labium, ventral view. Scale bars: 0.05 mm.

Figuress 9 - 11. Guyanobius adocetus, third instar larva. (9) right mandible, dorsal view; (10) left maxilla, dorsal view; (11) left maxilla, ventral view. Scale bars: 0.05 mm.

Figures 12 - 14. Unconstrained-MP analyses, most parsimonious tree (549 steps) with Bremer and Bootstrap values of support (12); Constrained-MP analyses, forcing Chaetarthriinae monophyly (13); unconstrained-BI analysis with homoplasy as partitioning scheme and mapped posterior probabilities (pp).