Megasoma thersites LeConte

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 : 99-104

Megasoma thersites LeConte , 3rd instar ( Figs. 29–42 )

Description based on one third instar larva collected in México: Baja California Sur, 11 mi N Cabo San Lucas, 26-IV-1985, from soil dug for post hole, R.L. Westcott (UNSM).

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Description. Head. Maximum width of head capsule 7.4 mm. Surface of cranium ( Fig. 29 ) dark brown, punctate, punctures large, deep. Frons moderately punctate, each side with 3 posterior frontal setae, anterior frontal setae lacking, 1 exterior frontal setae, and 6 anterior angle frontal setae. Remaining cranial surface with 2 dorsoepicranial, short setae, 46 epicranial setae, and 3

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paraocellar setae on each side. Clypeus trapezoidal, with 2 lateral setae on each side and 4 posterior clypeal setae. Labrum slightly asymmetrical, subrectangular, with row of 15 anterior labral setae, widely interrupted at middle, 7 posterior setae, central setae absent, and 5–6 lateral setae. Ocelli not apparent. Epipharynx ( Fig. 30 ) with fused zygum and epizygum forming weakly raised, subconical, sclerotized tubercle on haptomerum; heli and plegmatia absent; right chaetoparia with 50 spine-like setae and 45 small setae and sensillae toward gymnoparia; left chaetoparia with 35 spine-like setae and 40 small setae and sensillae; acroparia with 9 straight, long, thick setae toward right side and 10 straight, long, thick setae toward left side; right acanthoparia with 12 short, curved, spine-like setae or insertion pits; left acanthoparia with 13 short, curved, spine-like setae or insertion pits; pedium wide and epitorma not defined; dexiotorma long, sinuate, with rounded inner side; laeotorma short, with rounded inner side and subquadrate pternotorma; dexiophoba sparse, formed by 11 setae; laeophoba between haptolachus and inner side of laeotorma formed by 32 setae; sclerotized plate of right nesium large, subpyriform; sense cone of left nesium represented by large, wide, longitudinal, well-sclerotized

plate with rounded ends and row of 5 sensillae near base; crepis not defined. Scissorial area of right mandible ( Fig. 31 ) widely abraded, without evidence of teeth or notches; molar area with 3 lobes, calx prominent, brustia sparse. Scissorial area of left mandible ( Fig. 32 ) deeply abraded, without evidence of teeth or notches; molar area with 2 wide lobes, brustia sparse, long acia with rounded apex. Stridulatory area of each mandible well-marked by 22–24 fine striae. Mala ( Fig. 33 ) with apical uncus of galea well-developed and 3 conical unci fused at their bases at apex of lacinia (2 of these abraded); maxillary stridulatory area ( Fig. 34 ) with 7 small, subtrapezoidal, rounded teeth and wide, truncate anterior process. Labium ( Fig. 35 ) with numerous long, strong setae located on sides and short, spine-like setae at middle and proximal border; hypopharynx with right anterior lateral process of hypopharyngeal sclerome strongly developed, raised and curved, heavily sclerotized; left hypopharyngeal lateral lobe with patch of 20 posterolateral short setae and a line of 9 minute, thin setae. Last antennal segment on dorsal surface ( Fig. 36 ) with 7 sensory spots; ventral surface with 6 spots ( Fig. 37 ). Thorax. Pronotum with wide, irregular, reddish brown, lateral scleromes and 140 long, slender setae near anterior border and 44 medium sized, slender setae near posterior border, setae not arranged in definite rows. Mesonotum with 3 transverse groups of 20–90 long, slender setae and 4 groups of 18–48 medium sized, slender setae; metanotum with 4 transverse groups of 35–80 long, slender setae, 3 groups of 10–22 medium sized, slender setae and 1 group of 20 short, spine-like setae, setae not arranged in rows. Thoracic spiracles 0.66 mm long and 0.81 mm wide; lobes of respiratory plate nearly contiguous, directed posteriorly. Spiracular bulla rounded, convex, irregular, scarcely prominent. Tarsal claws ( Fig. 38 ) similar on all legs, with acute apex, each bearing 2 internal, long, thick, basal setae and 2 external, long, thick, preapical setae. Abdomen. Dorsum of segment I with 2 rows of 30–34 short, spine-like setae, 4 rows of 20–46 medium sized, slender setae, and 2 rows of 36–38 long, slender setae, and more than 120 long setae on each side. Dorsa of segments II–VI each with 85–100 long setae mainly on sides and 90–120 spine-like, short setae mixed with long setae; at middle third these setae form more or less regular row of 20–24 long setae. Dorsum of segment VII with more than 200 long, slender setae, more than 100 short, spine-like setae on each side, and transverse, anterior row of 36 slender, long setae. Dorsum of segment VIII with 1 row of 26 long, slender setae, more than 100 short, spine-like setae, and 80 long setae on each side. Dorsum of segment IX with 2 rows of 20–30 long, slender setae and 50 long slender setae and more than 80 short, spine-like setae on each side. Dorsum of segment X with approximately 200 short spine-like setae and 40 long, slender setae on each side. Venter of abdominal segments I–II each with 5 rows of 40–45 long, slender setae and 2 rows of 30–35 medium sized, slender setae; venter of segments III–V each with 1 row of 20 long, slender setae, and 2 rows of 25–30 medium sized, slender setae; venter of segments VI–VIII each with 80 long and medium sized, slender setae on each side; venter of segment IX with 70 long and medium sized, slender setae on each side. Raster without pallidia and septula, with 20 long, slender setae on campus, teges formed by 10 medium sized, slender setae; cuticle slightly rugose. Barbula formed by 20–25 long, slender setae. Lower anal lip with 140 long and medium sized mixed, slender setae. Upper anal lip with more than 100 short, spine-like setae and long, slender setae mixed, mainly towards sides. Spiracles of segments I–VIII progressively shorter (0.7– 0.6 mm long; 1.0– 0.8 mm wide); lobes of respiratory plate separated by distance less than 1/3 diameter of convexity of bulla ( Fig. 39 ), directed anteriorly. Spiracular bulla convex, rounded, weakly prominent; respiratory plate with about 28 irregularly shaped holes ( Fig. 40 ) across any diameter. Approximate dorsal body length 85 mm.

Second-instar Larva. Based on one third instar larva reared from egg laid by female collected in México: Baja California Sur, San José del Cabo , 2-X- 1999, 580 m, fixed 5- IV-2000, G. Nogueira ( IEXA) .

Description. Similar to third-instar larva except as follows: maximum width of head capsule 5.0 mm. Right chaetoparia with 38 spine-like setae and 34 small setae and sensillae; left chaetoparia with 24 spine-like setae and 32 small setae and sensillae. Scissorial area of both mandibles with 3 teeth clearly separated by notches. Prothoracic spiracles 0.23 mm long, 0.35 mm wide. Abdominal spiracles progressively smaller (0.16– 0.13 mm long, 0.34– 0.29 mm wide). Approximate dorsal body length 26 mm.

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collected in México: Baja California Sur, San José del Cabo , 2-X- 1999, 580 m, fixed 15- XII-1999, G. Nogueira ( IEXA) .

Description. Similar to second-instar larva except as follows: maximum width of head capsule 2.7–3.0 mm. Sclerotized plates on sides of pronotum absent. Eclosion spine on each side of metanotum small, conical, surrounded by 3 short, spine-like setae. Thoracic spiracles 0.13 mm wide, 0.04 mm long. Abdominal spiracles similar in size, 0.14 mm wide, 0.02 mm long, without bulla. Approximate dorsal body length 4.5–6.0 mm.

Remarks. Larvae of M. thersites are similar to those of M. vogti and M. punctulatus but may be distinguished by 3 postfrontal setae on each side, 6 anterior angle frontal setae, 7 setae near the posterior border of the labrum, pternotorma subquadrate, 7 dorsal sensory spots on the last antennal segment, acanthoparia with 12–13 setae, and lobes of the respiratory plate of the spiracles narrowly separated.

Distribution and Habits. Adults of M. thersites have been collected in many localities in the state of Baja California Sur, México such as Todos Santos, La Paz, Cabo San Lucas, and San José del Cabo at elevations from sea level to 600 meters ( Hardy 1972). Males and females are attracted to lights during September and October, but they are also commonly found in the soil or on branches of ‘‘palo verde,’’ ( Cercidium peninsulare Rose , Caesalpinaceae) where adults feed on the cambium ( Fig. 41 ) by ‘‘gumming’’ away the bark layer ( Fig. 42 ).

Figures

Figs. 29–40. Megasoma thersites, third instar. 29) Head, frontal view; 30) epipharynx; 31–32) mandibles, ventral view, 31) right and 32) left; 33) distal part of right maxillary mala; 34) detail of maxillary stridulatory area; 35) right maxilla and hypopharynx, dorsal view; 36–37) distal segment of antenna, 36) dorsal view and 37) ventral view; 38) left posterior tarsal claw, external view; 39) second right abdominal spiracle; 40) microstructure of respiratory plate (5003). Figs. 29–32, 35, bar ¼ 1 mm. Figs. 33–34, 36–39, bar ¼ 0.5 mm.

Fig. 41. Megasoma thersites on palo verde, Cercidium microphyllum Rose & I. M. Johnst. Photo by Barney Streit.

Fig. 42. ‘‘Gumming wound’’ made by adult Megasoma thersites on palo verde tree. Photo by Barney Streit.