Gymnetis pudibunda Burmeister, 1866

Ibarra Polesel, Mario G., Valle, Nestor G., Cave, Ronald D. & Damborsky, Miryam P., 2022, Descriptions of the larva and pupa of Gymnetis pudibunda Burmeister, 1866 (Coleoptera: Scarabaeidae: Cetoniinae: Gymnetini), with notes on natural history and a key to the known larvae of New World Gymnetini, Journal of Natural History 56 (13 - 16), pp. 969-987 : 971-979

Gymnetis pudibunda Burmeister, 1866

Third instar

( Figures 1–14 )

Description. Dorsal body length 79.2–82.3 mm, width 11.7–12.4 mm ( Figure 8 ), weight 3.6– 4.3 g (live specimens). Cranium ( Figure 1 ): Head capsule width 3.7–4.1 mm. Colour orangebrown. Surface mostly smooth, with few pits. Epicranial, frontal and clypeofrontal sutures distinct. Frons ( Figure 1 ): Surface with 2 posterior setae, each anterior angle with 1 long seta; anterior setae and exterior setae absent. Remaining cranial surface with 2 anterior epicranial setae; 6–8 dorsoepicranial setae on each side, 4–6 exterior epicranial setae and 3–6 posterior epicranial setae on each side. Stemmata absent. Clypeus ( Figure 1 ): Trapezoidal, lateral margins slightly convergent. Postclypeus well sclerotised, with 2 anterior setae and 2 exterior setae on each side. Preclypeus weakly sclerotised, with about 28 sensillae, setae absent. Labrum ( Figure 1 ): Symmetrical, lateral margins rounded, anterior border trilobed, clithra present; anterior margin with about 18–24 sensillae. With 2–4 posterior setae, 2 paramedian setae, 3–4 exterior setae on each side and 12–14 anterior labral setae; margins without setae. Epipharynx ( Figure 2 (a,b)): Haptomeral region without haptomeral process nor heli. Corypha with 6–10 long, stout setae; acroparia with 8–10 long, stout setae on each side. Zygum transverse, slightly convex, with an arcuate row of 10–14 spinelike setae and 6–10 stout setae irregularly placed behind. Acanthoparia with 7–9 short setae; right chaetoparia with 42–60 setae and left chaetoparia with 36–48 setae. Pedium without sensilla. Dexiotorma well developed, narrow, elongate, transversely extended. Laeotorma shorter than dexiotorma, with poorly developed pternotorma. Dexiophoba and laeophoba absent. Sense cone well developed, with 2 lateral sensillae on each side and above a group of 18–22 short, slender setae. Sclerotised plate absent; crepis ill defined. Antenna ( Figures 1 and 3 (a–d)): Four antennomeres, 2–4 subequal in length. Antennomeres 1, 2 and 3 about 1.6, 0.9 and 0.7 times longer, respectively, than terminal antennomere ( Figure 3 (a)). Antennomere 3 with distal process. Terminal antennomere with 6 sensory spots: dorsal surface with 2 sensory spots ( Figure 3 (a,b)); ventral surface with 1 sensory spot ( Figure 3 (c)) and 3 sensory spots at apex ( Figure 3 (d)).

Mouthparts. Left mandible ( Figure 4 (a–c)): Scissorial region with 3 well-developed teeth, teeth S1 and S2 separated by an acute scissorial notch, teeth S2 and S3 separated by broad postincisor notch. Dorsal surface with 2 long, stout setae on dorsal carina at level of S2–S3 notch and tooth S3. Scrobis with 1–3 long, stout setae and 2–6 slender, short setae. Dorsomolar area with tuft of about 8 setae. Preartis distinct, concave. Ventral surface with small, elongate-oval stridulatory area, formed by approximately 16–20 subparallel ridges, ridges closely spaced at the base and widening strongly towards the middle. Ventromolar area with tuft of 8–11 setae. Three molar lobes present, 1st molar lobe (M1) large, not subdivided and below 2 acute lobes. Acia absent. Brustia with 10–16 stout, long setae. Postartis large, rounded. Ventral process distinct, blunt, asperate. Right mandible ( Figure 5 (a–c)): Form falcate. Scissorial region with 3 well-developed teeth, teeth S1 and S2 separated by an acute scissorial notch, teeth S2 and S3 separated by broad postincisor notch. Dorsal surface with 2 long, stout setae on carina at level of S3 and S4. Scrobis with 2–4 long, stout setae and 3–6 slender, short setae. Dorsomolar area with tuft of 5–10 setae. Preartis distinct, concave. Ventral surface with small, elongate-oval stridulatory area, formed by approximately 18–22 subparallel ridges, ridges closely spaced at the base and widening strongly towards the middle. Ventromolar area with tuft of 8–11 setae. Calx large, molar crown with 3 large lobes. Brustia with 6–10 stout, long setae. Postartis large, rounded. Ventral process distinct, blunt, asperate. Maxilla ( Figure 6 (a–c)): Galea and lacinia fused, forming a mala. Uncus well developed, approximately 0.7 times length of apical maxillary palpomere ( Figure 6 (a)). Lacinia with large uncus at apex and subterminal uncus fused at its base; 1 spine-like seta on dorsal area of large uncus ( Figure 6 (b)). Dorsal surface of mala with about 28 setae. Maxillary palpus with 4 palpomeres; palpomeres 1, 2 and 3 about 0.4, 0.6 and 0.5 times, respectively, length of palpomere 4; palpomere 3 with lateral seta. Stridulatory area with row of 4–6 curved, acute teeth and a distal truncate process ( Figure 6 (c)). Labium ( Figure 7 (a–c)): Hypopharyngeal sclerome asymmetrical, medially concave. Right side with well-developed, truncate process, forming a right angle on inner side ( Figure 7 (a)); basal region an unsclerotised circular area with 2–4 long setae. Left side with well-developed basal process ( Figure 7 (b)). Glossa with 24–32 long setae, 30–36 spine-like setae; basal area with row of 12–18 short, spine-like setae. Left lateral lobe with 14–20 slender, moderately long setae; right lateral lobe with 10–14 slender setae. Left margin with row of about 12 setae in a line, directed towards centre of sclerome ( Figure 7 (a,c)). Labial palpus with 2 segments.

Thorax ( Figure 8 ): Pronotum with numerous sparse, long setae; sclerotised area with 2–3 long, stout setae on superior-posterior region, 4–7 long setae on inferior-posterior region, 7–10 long setae on anterior region, 2–4 short setae on central region. Prothoracic spiracle 0.65–0.79 mm long, 0.55– 0.49 mm wide ( Figure 9 ); spiracular plate yellowish brown, closed C-shaped, spiracular bulla rounded, slightly elevated ( Figure 13 ); distance between 2 lobes of spiracular plate much less than dorsoventral diameter of bulla. Plate with 32–40 microscopic pores in dorsal radius ( Figure 14 (a)), 22–30 in lateral radius ( Figure 14 (b)), 30–34 in basal radius ( Figure 14 (c)); pores entirely margined, irregularly shaped. Prescutum, scutum and scutellum of meso- and metathoraxes with numerous long and short setae. Legs: Pretarsus cylindrical, rounded apically ( Figure 10 ), bearing 11–14 long setae; prothoracic pretarsus short (≈ 0.37 mm) relative to mesothoracic (≈ 0.48 mm) and metathoracic (≈ 0.57 mm) pretarsi. Legs gradually increasing in length from 1st to 3rd pair ( Figure 11 ). Coxa, trochanter, femur and tibiotarsus of all legs with numerous long, stout setae.

Abdomen ( Figure 8 ): Abdominal spiracles subequal in size ( Figure 9 ). Spiracles on segments I–VIII of similar height (I: 0.72 mm; IV: 0.75 mm; VIII: 0.77 mm) and progressively wider (I: 0.53 mm; IV: 0.57 mm; VIII: 0.67 mm). Distance between 2 lobes of spiracular plate considerably less than dorsoventral diameter of bulla. Bulla irregularly oval, slightly convex. Abdominal segments (I–IX) with numerous long, slender setae ( Figure 8 ). Dorsum of segments VII and VIII each with 2 annulets (prescutum and scutum); segments IX and X fused. Raster ( Figure 12 ): Palidia monostichous, closed anteriorly, open posteriorly; each palidium a row of 16–21 mid-size, apically flattened pali. Septula oval, length 5.3 times width. Tegilla composed of numerous evenly distributed short, thick setae and fewer long, slender setae. Chaetotaxy of ventral anal lip similar to that of tegilla, but with more numerous long setae. Dorsal anal lip with dense, reddish, mid-size setae and some long setae. Anal slit transverse, bordered by dense rows of stout, mid-size setae.

Female pupa

( Figure 15 (a–d))

Description. Length 27.1, 23.5 mm, greatest width 13.8, 14.2 mm ( Figure 15 (a)), weight 1.6, 1.4 g (live specimens). Form adecticous, exarate, body suboval, slightly elongate, stout. Colour yellowish brown. Entire body with very fine velvety microtrichia. Head: Mouthparts directed ventrally ( Figure 15 (c)); antenna, labrum, mandibles, maxillae and palps discernible; antennal tecae expanded, stout, with rounded apices. Compound eyes sunken, partially covered by anterior edges of pronotum. Thorax: Pronotal disc convex, with welldefined margins ( Figure 15 (c)), subheptagonal, widest posteriorly, with 2 large protuberances near basal angles, basal margin slightly bisinuate, centre of base projecting posteriorly as in adult. A narrow, median, longitudinal sulcus extending from apex to near base. Meso- and metascutella acute, projecting posteriorly. Thoracic spiracle present in cavity formed by anterior and middle legs, hypomeron and pterothecae. Mesometasternal process large, with rounded apex, protruding between procoxae and mesocoxae ( Figure 15 (b)). Metasternum wider than long. Pterothecae free, closely appressed, curved ventrally around body ( Figure 15 (c)); posterior pterotheca reaching abdominal sternite V. Legs: Protibia with 3 short teeth on external apical border and tuberculiform apical spurs. Meso- and metatibiae each with 2 well-developed inner spurs. Metafemur covered by pterotheca. Tarsomeres and pretarsus distinct; protibial pretarsi at level of mesofemur, mesotibial pretarsi at level of metafemur, metatibial pretarsi at level of abdominal sternite VIII. Abdomen: Tergites I–VIII well defined, convex, segments III and IV widest. Dioneiform organs absent. Tergolateral tubercles II–VI distinct, with very fine, velvety microtrichia, inner side of each tubercle a longitudinal fold. Spiracles I–IV light brown, tuberculiform, with ring-like, sclerotised peritreme; spiracle I partially covered by pterotheca. Spiracles V– VIII closed, tuberculiform, surrounded by fine rugae. Sternites II–VI distinct; sutures between segments VII and X incomplete, segments partially fused. Terminal tergite without urogomphi; fleshy lobes with apical, velvety, white-gold vestiture extending ventroapically. Surface of sternite IX roughened apically, genital ampulla convex and wide, genital pore distinct ( Figure 15 (d)).

Male pupa

( Figure 16 )

Description. Length 24.1–25.2 mm, greatest width 13.3–14.1 mm, weight 1.6– 1.3 g (live specimens). Same as female except sternite IX anteriorly convex. Terminalia with welldefined anterior genital ampulla, formed by 2 large, semicircular lobes ( Figure 16 ); posterior genital ampullae rounded, with a distally impressed line. Sternite IX slightly exposed, roughened apically.

Life cycle and natural history

( Figures 17–24 )

Larvae of G. pudibunda were successfully reared in containers with a sawdust base with pieces of rotten wood on the surface. Some areas of the containers were spray-moistened, while others were kept drier. In all cases, the larvae preferred the moister areas of the substrate. The larvae were supplied with balanced food for cats in croquettes, which was consumed mainly by third instars.

Laboratory rearing showed that G. pudibunda in north-eastern Argentina has a univoltine life cycle. The three adults captured in early December (two females and one male) were held in a glass container and fed small pieces of banana ( Figure 17 ). The eggs ( Figure 18 ) were observed after a few days (mid-December). At the end of January and the beginning of February, most larvae were already in the second instar. By mid- March, all larvae became third instar. Larvae formed pupal chambers of sawdust and excrement in July and August when they reached a weight of 3.3–4.2 g. Pupal chamber size ranged 20.2–22.8 mm wide and 12.4–14.5 mm long. By the end of August, all larvae had formed their pupal chamber and pupated in late August and mid-September. In the pupal chamber, the larval exuvia were found collapsed (accordion-shaped) next to the ventral region of the abdomen of the pupa ( Figure 19 ). Emergence of adults ( Figure 20 ) occurred in October and November. Under laboratory conditions, the duration of the second instar was ≈ 45 days (n = 20), third instar 104–142 days (n = 14), pre-pupa 22–45 days (n = 8), and pupa 26–33 days (n = 5).

Esteban Abadie [personal communication, August 2017, in Ratcliffe (2018)] reported dead specimens of G. pudibunda outside the nests of the leafcutter ant Acromyrmex lundii (Guérin-Méneville) ( Hymenoptera : Formicidae ) in Buenos Aires, Argentina. Similarly, live larvae of the cetoniine Neocorvicoana reticulata (Kirby, 1819) were found inside debris dumps of A. lundii by Ibarra-Polesel et al. (2017), which suggests that the immature stages of G. pudibunda may develop inside the debris dumps of these ants. The finding of various species of Cetoniinae in termite and ant nests is reported in several studies ( Ritcher 1966; Micó et al. 2001; Morón and Arce 2002; Orozco and Pardo-Locarno 2004; Puker et al. 2012; Di Iorio 2014, among others). Puker et al. (2014) provided a complete review of the association of cetoniine beetles with ants.

In north-eastern Argentina, G. pudibunda adults are captured in fruit traps in wooded areas such as gallery forests ( Figure 21 ) and quebracho forests ( Figure 22 ). Di Iorio (2014) reported adults of G. pudibunda feeding in baited banana traps, on the sap from galleries made by larval and adult Megaplatypus mutatus (Chapuis) ( Coleoptera : Curculionidae : Platypodinae ) in boles of Salix humboldtiana Willdenow (Salicaceae) , and on the pollen of a male cone of Cycas revoluta Thunberg (Cycadaceae) . Esteban Abadie [personal communication, August 2017, in Ratcliffe (2018)] collected specimens in banana traps in the biogeographic region of Espinal Pampeano (Buenos Aires province, Argentina). He also reported adults visiting flowers of Sambucus australis Chamisso and Schlechteendal (Adoxaceae) .

Cetoniine adults are known to damage fleshy soft-shelled fruits, such as the ripened fruits of Ficus carica Linnaeus (Moreacae) , Prunus domestica Linnaeus (Rosaceae) , and Pyrus communis Linnaeus (Rosaceae) ( Cordo et al. 2004; Di Iorio 2014). Montero and Seta (2015) reported several adults of G. pudibunda feeding on fruits of Prunus persica Linnaeus and Prunus armeniaca Linnaeus (Rosaceae) close to organoleptic maturity in urban gardens in the city of Rosario in Santa Fe province, Argentina. According to the three aforementioned studies, the beetle produces mechanical damage to the pericarp and mesocarp of the fruits; subsequently, a necrotic area with internal rot develops around the bite mark. The fruits usually fall before maturity, but even if they do not, they are no longer of commercial value. Adult G. pudibunda were never found on ripe fruits fallen on the ground ( Di Iorio 2014).

Intraspecific variation in elytral colour pattern of laboratory-reared adult G. pudibunda was observed ( Figure 23 ). Three of the specimens in Figure 23 are the first generation of breeding females and male; the other nine are progeny from a second generation of breeding. We do not rule out the idea that the colour pattern may vary even more markedly among individuals from other populations. According to Ratcliffe (2018), this intraspecific variation in the colour and pattern of the pronotum and elytra of Gymnetis species is the result of genetic or environmental influences, which makes it difficult to circumscribe the species. Melanistic specimens in which virtually all colours and patterns are attenuated or absent, as well as specimens with greatly reduced vittae or blemishes, contribute to additional challenges in characterising and identifying species. Gymnetis pudibunda is one of these challenging species.

Temporal distribution. The adult activity period of G. pudibunda in Argentina begins in November and lasts until March; peak activity occurs in December and January ( Di Iorio 2014).

Distribution. According to Ratcliffe (2018), G. pudibunda is known from Argentina, Bolivia, Brazil and Paraguay ( Figure 24 ).

Figures

Figure 23. Range of variation in pale elytral markings in Gymnetis pudibunda. The asterisk (*) indicates three field-collected specimens (first generation), and no asterisk indicates laboratory-reared specimens (second generation).

Figure 24. Distribution of Gymnetis pudibunda (image from Ratcliffe (2018) and used with the permission of the University of Nebraska State Museum).

Figures 1–3. Gymnetis pudibunda, third instar. (1) Cranium, frontal view; (2) epipharynx, in (a) lateral (b) and apical views; (3) antenna: lateral view (a), terminal antennomere in (b) dorsal, (c) ventral, and (d) apical views.

Figures 4–7. Gymnetis pudibunda, third instar.(4) Left mandible, in (a) dorsal,(b) lateral,and (c) ventral views; (5) right mandible, in (a) ventral, (b) lateral, and (c) dorsal views; (6) left maxilla, (a) dorsal view, (b) apex of mala showing unci, and (c) stridulatory teeth on maxilla; (7) labium, (a) dorsal view, (b) hypopharyngeal sclerome, and (c) lateral view.

Figures 8–12. Gymnetis pudibunda, third instar. (8) Habitus, lateral view; (9) thoracic spiracle (Th) and abdominal spiracles from first (Ab I) to last (Ab VIII) segment; (10) pro-, meso- and metathoracic pretarsi, dorsal view; (11) pro-, meso- and metathoracic legs, lateral view; (12) last abdominal segments, ventral view.

Figures 13, 14. Gymnetis pudibunda, third instar. (13) Thoracic spiracle; (14) microscopic pores in the (a) dorsal, (b) lateral, and (c) basal areas of the plate.

Figures 15, 16. Gymnetis pudibunda, pupa. (15) Female:(a) dorsal,(b) ventral, and (c) lateral views, and (d) ventral view of apex with genital ampulla; 16) male: ventral view of apex with genital ampulla.

Figures 17–22. Gymnetis pudibunda. (17) Adults feeding on banana; (18) eggs; (19) pupa within the pupal chamber, with the larval exuvia next to the ventral region of the abdomen; (20) adult within the pupal chamber, with pupal and larval exuviae; (21) riverside forest habitat; (22) quebracho forest habitat.