Megasoma actaeon (Linnaeus, 1758)

Ratcliffe, Brett C. & Morón, Miguel-Angel, 2005, Larval Descriptions Of Eight Species Of Megasoma Kirby (Coleoptera: Scarabaeidae: Dynastinae) With A Key For Identification And Notes On Biology, The Coleopterists Bulletin 59 (1), pp. 91-126 : 117-124

Megasoma actaeon (L.), 3rd instar ( Figs. 81–93 )

Description based on one third instar larva reared from eggs laid by female collected in French Guyana (1992) and one cast skin of lab-reared, third instar larva (1997) (UNSM).

Description. Head: Maximum width of head capsule 15.1 mm. Surface of cranium dark reddish brown, coarsely rugopunctate ( Fig. 81 ); punctures large, dense, deep. Frons on each side with 4 short, posterior frontal setae, 8 short, anterior frontal setae, and 2 long anterior angle frontal setae; remaining cranial surface on each side with 10 dorsoepicranial setae, 7–10 minute epicranial setae, and 6 long, paraocellar setae. Clypeus trapezoidal, with 1 long, posterior clypeal seta near each lateral margin. Labrum slightly asymmetrical, rounded at sides, with 2 long, setae on each side, and 18 long setae in transverse row in basal third. Ocelli vaguely indicated. Epipharynx ( Fig. 82 ) with fused zygum and epizygum forming raised, longitudinal, sclerotized tubercle on haptomerum; heli and plegmatia absent; right chaetoparia with about 75 spine-like setae and 15 sensillae; left chaetoparia with about 75 spine-like setae and 25 sensillae; acroparia with 9 long, thick setae on right side and 6 long, thick setae on left side; both acanthoparia with vaguely indicated (nearly obsolete) tormae of about 4 setae; both gymnoparia wide, weakly sclerotized; pedium wide; dexiotorma wide, elongate; laeotorma slender, elongate, with widely rounded pternotorma; dexiophoba with 15 long setae located near haptolochus; laeophoba with 10 long setae sclerotized plate of left nesium elongately subtriangular, strongly sclerotized; sense cone of left nesium represented by large, longitudinal, well-sclerotized plate with suboval anterior end and truncate posterior end; crepis vaguely indicated by shallow sulcus. Scissorial area of right mandible ( Figs. 84–85 ) with 3 scissorial teeth vaguely separated by notches and 1 small, basal, angular tooth; molar area (ventral view) with 3 poorly defined lobes, calx prominent, brustia sparse. Scissorial area of left mandible ( Figs. 83, 86 ) with 3 scissorial teeth distinctly separated by notches and 1 basal, wide, subquadrangular tooth; molar area with 1 feeble lobe, brustia sparse, acia low and rounded. Stridulatory area of each mandible with about 14 well-marked striae, striae broad basally, becoming finer apically. Mala ( Fig. 87 ) with large, apical uncus on galea and with 3 well-developed unci fused at their bases at apex of lacinia; maxillary stridulatory area ( Fig. 88 ) with short row of 4 laterally compressed, semi-oval teeth and heavily sclerotized, narrowly elongate anterior process. Labium ( Fig. 86 ) with numerous, long setae located on sides, slightly shorter setae near lateral margins, and minute sensillae at center near base; hypopharynx with right anterior process of hypopharyngeal sclerome moderately developed, raised, heavily sclerotized. 118 THE COLEOPTERISTS BULLETIN 59(1), 2005

Last antennal segment on dorsal surface ( Fig. 89 ) with 11 sensory spots; ventral surface ( Fig. 90 ) with 9 spots. Thorax: Pronotum with irregularly shaped, reddish brown, lateral sclerome with vertical row of 10 long setae, and about 110 mostly long setae near anterior margin, and about 100 long setae near posterior margin, setae not arranged in rows. Mesonotum with transverse group (briefly interrupted at middle) of about 70 long, slender setae, and 50 long, slender setae in irregular rows and a group of 75 long, slender setae on each side; metanotum with 2 transverse groups of 35 and 50 long, slender setae on each side and about 75 other long, slender setae on each side. Thoracic spiracle 2.7 mm long and 2.0 mm wide; lobes of respiratory plate contiguous, directed posteriorly. Spiracular bulla rounded, irregularly convex, prominent. Tarsal claws ( Fig. 91 ) similar on all legs, with rounded apex bearing a sclerotized tooth and with 1 long, slender seta on median side and 1 short, thick seta on outer side. Abdomen: Dorsum of segment I with

120 THE COLEOPTERISTS BULLETIN 59(1), 2005

transverse row of 40 mixed, medium sized and long, slender setae and 1 row of about 70 long, slender setae and 40 sort, spine-like setae. Dorsum of segment II on prescutum with 1 transverse

group of about 100 long, slender setae mixed with about 80 short, spine-like setae; scutum with about 16 long, slender setae mixed with about 110 short, spine-like setae; postscutum with about 36 long, slender setae mixed with about 120 short, spine-like setae. Dorsa of segments III–VI each with 1 transverse group of about 40 long, slender setae mixed with about 160 short, spine-like setae on prescutum; scutum with 6 long, slender setae mixed with about 140 short, spine-like setae; postscutum with about 30 long, slender setae mixed with about 210 short, spine-like setae. Dorsum of segment VIII with anterior transverse row of 8 long, slender setae and posterior transverse row of about 44 long, slender setae and about 80 long, slender setae on sides surrounding spiracle and about 120 short, spine-like setae generally distributed, and with a transverse row of about 100 long, slender setae (mostly on sides) near posterior border. Dorsum of segment IX with 2 transverse rows of 14 long, slender setae each, about 75 short, spine-like setae widely distributed, and about 80 long, slender pleural setae. Dorsum of segment X with about 30 medium sized, slender setae widely distributed, about 150 short, spine-like setae widely distributed and about 150 long, slender setae on posterior border. Venter of abdominal segments I–VI each with about 80 long, slender setae. Venter of segments VII–VIII each with about 50 long slender setae. Venter of segment IX with about 30 long, slender setae either side of middle. Raster lacking pallidia and septula, with about 30 medium sized, slender setae on campus; teges formed by about 65 short, slender setae. Barbula formed by 60 long, slender setae. Lower anal lip with about 90 long, slender setae. Upper anal lip with about 120 short, spine-like setae. Spiracles of segments I–VIII ( Fig. 92 ) progressively larger (2.2–2.8 mm long, 2.0– 2.2 mm wide); lobes of respiratory plate contiguous, directed anteriorly. Spiracular bulla irregularly convex, weakly prominent; respiratory plate with about 90 minute holes across any diameter. Approximate dorsal body length 130 mm.

Remarks. Megasoma actaeon is unique among all of the known species in the genus because it has the fewest number of maxillary stridulatory teeth with only 4 (5–8 in other species) and only 2 setae on each tarsungulus. It is easily separated from the equally large and partially sympatric M. elephas by the relatively small and ‘‘sunken’’ head of M. elephas , far fewer cranial frontal setae in M. actaeon , dorsum of the first abdominal segment with a mixture of short and long setae in M. actaeon (long setae only in M. elephas ), and by the presence of about 28 long setae on the campus of the last abdominal segment in M. actaeon (only 4–6 setae in M. elephas ).

Megasoma actaeon differs from the other large (but allopatric) species in Mexico, M. occidentalis , by the presence of 8 cranial frontal setae in M. actaeon (4 frontal setae in M. occidentalis ), last antennal segment with 11 dorsal sensory spots (8 dorsal sensory spots in M. occidentalis ), and maxillary stridulatory area with only 4 teeth in M. actaeon (8 teeth in M. occidentalis ).

Distribution and Habits. Megasoma actaeon is known from Panama, the Guianas, Bolivia, Venezuela, Ecuador, Colombia, Peru, Brazil (primarily Amazonia), and northern Paraguay ( Lachaume 1985; Ratcliffe 2003). Like the other large Dynastini , the larvae probably develop in rotting logs. Adults ( Fig. 93) are nocturnal and attracted to lights. Radnai (2003) compiled the following life history information based upon six years of observations. Eggs were deposited into a suitable log or compost and were approximately 8–10 mm long and 7.5 mm in diameter when laid. After deposit, the eggs increased in size, presumably by absorption of water. Eggs increased in length by 5–7 mm, and the diameter increased by 3.5–5.5 mm. Eclosion took from 25–36 days.

Upon hatching, first instar larvae measured about 8 mm in length and 3–4 mm in diameter. A first instar larva weighed about 0.17 g whereas a third instar larva will weigh 90 g. Laboratory development time for all three instars averaged 943 days (893 days minimum, 993 days maximum). The prepupal stage took about 24 days after which pupation occurred. Pupation lasted about 38 days. The pupation chamber is comprised of compacted compost about the size of an adult’s fist. Total development time averaged 1,035 days (2.8 years). In captivity, adults lived from 100 to 151 days. Lai (2001) suggested they live this way in nature.

Megasoma occidentalis Bolívar y Pieltain, Jiménez-Asffla, and Martínez, 3rd instar ( Figs. 94–106 )

Description based on three third instar larvae collected in association with an adult female in Mexico: State of Colima, Tecomán Municipality, El Saucito, 28-VIII-70, Y. Domínguez & R. Peña, in rotten log of Cocos nucifera L., 200 m (IEXA).

Redescription. Head: Maximum width of head capsule 16 mm. Surface of cranium ( Fig. 94 ) strongly and densely punctate, dark brown to black. Frons on each side bearing 2 posterior frontal setae, 3 median lateral frontal seta, 4 anterior frontal setae, and numerous anterior frontal angular setae of which 1 is distinctly thicker and longer; remaining cranial surface bears numerous short, fine, uniformly distributed setae and 2 conspicuous, dorsoepicranial setae and 4 epicranial setae, all placed well away from epicranial suture. Clypeus with 4 long setae on preclypeus and numerous short, fine, scattered setae. Labrum subtrapezoidal, with 5 or 6 posterior setae, 2 central setae, 4 lateral setae, and numerous short, fine, scattered setae. Ocelli vestigial. Epipharynx ( Fig. 95 ) with fused zygum and epizygum forming oblique, sclerotized tubercle; heli absent; right chaetoparia with 52–55 spinelike setae and 56 fine setae; left chaetoparia with 86–90 spinelike setae and few fine setae; right acroparia with 15–17 straight, long, thick setae; left acroparia with 18–20 long, slightly curved, thick setae; acanthoparia with 9 short, curved, spinelike setae. Scissorial area of right mandible ( Fig. 96 ) with 3 teeth separated by 2 notches; molar area with 3 lobes. Scissorial area of left mandible ( Fig. 97 ) with 3 teeth and 1 truncate pre-incisor tooth; molar area with 2 lobes and long, truncate acia. Stridulatory area of each mandible well-marked by distinct striae. Galea ( Fig. 98 ) with conical uncus well-developed; lacinia with 3 conical unci fused at their bases; maxillary stridulatory area with 8 small, subtrapezoidal, truncated teeth and wide, truncate anterior tubercle ( Fig. 98 ). Labium cylindrical in form ( Fig. 99 ), with numerous long, strong setae located principally at sides; hypopharynx with right anterior lateral process of hypopharyngeal sclerome strongly developed, raised and curved, heavily sclerotized; left hypopharyngeal lateral lobe with oblique row of 7 posterior lateral setae. Dorsal surface of last antennal segment ( Fig. 100 ) with 8 sensory spots. Thorax: Pronotum with wide lateral areas, well-sclerotized and reddish brown. Mesoprescutum with 1 anterior row of short setae and 4 rows of posterior large setae; mesoscutum with 3 lateral rows of long setae; mesoscutellum with 2 rows of long setae; metaprescutum with anterior, irregular row of short setae and 3 posterior rows of long setae; metascutum with 4 lateral rows of long setae; metascutellum with 3 rows of long setae. Thoracic spiracles ( Fig. 101 ) 2.06 mm long and 1.93 mm wide. Respiratory plate with indefinite number of irregular-shaped ‘‘holes’’ ( Fig. 102 ), lobes of respiratory plate contiguous. Spiracular bulla convex, longitudinal. Claws ( Fig. 103 ) each with 2 internal, long, thick basal setae and 2 external, long thick preapical setae. Abdomen. Dorsa of segments I–VI each with 3–5 transverse rows of short, spine-like setae; dorsa of segments I–II each with 2–3 transverse rows of long, spinelike setae; dorsa of segments III–VI each with transverse row of long, spinelike setae; dorsum of segment VII with 8–9 transverse rows of short, spinelike setae, 1 anterior row of long, fine setae, 1 subposterior row of long, fine setae, and 1 posterior row of long, fine setae; dorsum of segment VIII with 7–8 transverse rows of short, spinelike setae, some scattered anterior long, fine, setae, 1 subposterior row of long, fine setae, and 1 posterior row of long, fine setae; dorsum of segment IX with anterior scattered, short, spinelike setae, 1 anterior row of long, fine setae, and 1 posterior row of long, fine setae; dorsum of segment X with scattered short, spinelike setae and long, fine setae (pleural lobes of all abdominal segments densely covered with setae). Venter of abdominal segments I–IX each with 1 median, transverse row of long fine setae. Raster ( Fig. 104 ) with 22–27 long, fine setae in campus; teges with 40–44 scattered, long, fine setae; lower anal lip with 78–86 fine setae; upper anal lip with numerous mixed short, spinelike setae and long, fine setae; setae of barbula sparse. Dorsolateral region of segment X (dorsally adjacent to outer extremity of anal slit) with light, cribrate, subtriangular depression ( Fig. 105 ). Spiracles of segments I–VIII progressively larger (2.0–3.0 mm long and 1.8–2.0 mm wide). Approximate dorsal body length 90–100 mm.

122 THE COLEOPTERISTS BULLETIN 59(1), 2005

( Fabaceae ), Puerto Vallarta, Jalisco, Mexico. Photo by G. Nogueira.

Remarks. Larvae of M. occidentalis are characterized by 9 spines on acanthoparia; right laterocentral part of anterior border of labrum not sinuous; ocelli vague; maxilla with 8 stridulatory teeth; last antennal segment with 8 dorsal sensory spots; anterior abdominal spiracles smaller than posterior spiracles; more than 50 ‘‘holes’’ in respiratory plate; 2–12 rows of setae on abdominal dorsa; setae of barbula sparse; campus with 22–27 setae.

Distribution and Habits. Adults of M. occidentalis ( Fig. 106 ) have been collected on the slopes of Pacific Ocean from southern Sinaloa (Mazatlán) to Oaxaca (Pochutla) México (19 8 40 9 N) (Ratcliffe and Morón 1997; Morón and Gómez-Anaya 2002). The habitat for M. occidentalis is tropical deciduous forests and tropical plantations located between sea level and 700 meters in elevation. Larvae were also found inside the rotten stem of a coconut palm in plantations near Tecomán, Colima ( Morón 1977). Adults of M. occidentalis are frequently attracted to lights in high numbers in some Mexican localities, but searching for larvae in rotten logs and stumps has not revealed them. It 124 THE COLEOPTERISTS BULLETIN 59(1), 2005

may be possible that the life history and habits of this species are similar to those of

Figures

Figs. 81–92. Megasoma actaeon, third instar. 81) Head, frontal view; 82) epipharynx; 83–84) mandibles, dorsal view, 83) left and 84) right; mandibles, ventral view, 85) right and 86) left; 87) right maxilla and hypopharynx, dorsal view; 88) detail of maxillary stridulatory area; 89) distal segment of antenna, dorsal view; 90) distal segment of antenna, ventral view; 91) left middle tarsal claw, anterolateral view; 92) second right abdominal spiracle. Fig. 81, bar ¼ 5 mm. Figs. 82–87, 89–90, 92, bar ¼ 1 mm. Figs. 88, 91, bar ¼ 0.5 mm.

Figs. 94–105. Megasoma occidentalis, third instar. 94) Head, frontal view; 95) epipharynx; 96–97) mandibles, ventral view, 96) right and 97) left; 98) labium and hypopharynx, dorsal view; 99) left maxilla dorsal view; 100) distal segment of antenna, dorsal view; 101) thoracic spiracle; 102) structure of holes of respiratory plate; 103) left prothoracic claw, lateral view; 104) venter of segment X of abdomen; 105) last abdominal segments, lateral view. Bar ¼ 1 mm for all figures except Fig. 102.

Fig. 106. Megasoma occidentalis, adult male, on fruit of Delonyx regia (Bojer) Raf.