Mesosemia thymetus thymetina A. Butler, 1869

Nielsen, Gregory J. & Kaminski, Lucas A., 2018, Immature stages of the Rubiaceae-feeding metalmark butterflies (Lepidoptera: Riodinidae), and a new function for the tentacle nectary organs, Zootaxa 4524 (1), pp. 1-32 : 12-13

Mesosemia thymetus thymetina A. Butler, 1869

Natural history. Adults ( Figs. 5–8 ) were commonly seen between 9:00 to 16:00 h. along semi-shaded forest trails and males engaged in perching (territorial) behavior along these trails with a peak of activity between 11:00 to 15:00 h. Males were the most numerous sex observed; during a 17 month segment of a survey females only accounted for 7% of 245 sightings. Adults were absent or very infrequent during the dry season months and recolonized the study area during the first few months of the rainy season. This species is a continuous breeder with overlapping generations. Immatures were recorded on four host plant species: Psychotria caerulea , Ps. poeppigiana Müll. Arg. , Palicourea gracilenta (Müll. Arg.) Delprete & J. H. Kirkbr , and Pa. racemosa (Aubl.) Borhidi. Gallard (2017) also mentions Ps. poeppigiana as a host for this species in French Guiana. Eleven eggs and larvae were found on greenhouse plants during the rainy season of 2017, between the months of August and November. A single larva was found in January, 2018, during the dry season. Eggs are laid singly on the leaf underside, larvae chew a hole around the micropylar area to hatch and the remaining exochorion is not eaten ( Fig. 74 ). In the early instars (first to third), larvae fed on the underside of the leaf. First they cut a roughly semicircular channel through the leaf tissues and veins ( Fig. 76 ), probably preventing this portion of leaf from receiving plant defense chemicals. Then they continued to feed in this isolated area biting through the leaf layers but leave the adaxial epidermis intact leaving small translucent circles ( Fig. 75 ). Early instar larvae had functional tentacle nectary organs (TNOs) that secreted drops of an opaque and viscous liquid when molested manually or by ants ( Fig. 77 ). However, the larvae were not tended by ants, i.e., larvae do not establish stable symbiotic interactions with ants. The third and fourth instar larvae fed on the leaf underside, eating all leaf layers, not leaving the transparent windows as the previous instars. In the last instar larvae fed and rest primarily on the upper side of older, mature leaves. The development time from egg to adult was 45–50 days. Larvae underwent five instars and pupation occurred off the host plant. Adults emerged in the afternoon between 13:00 and 15:30 h.

Description of immature stages. Egg ( Figs. 72–74 ). Embryonic development 9 days (n = 4); diameter 0.58 mm; height 0.41 mm (n = 4). Dark plum color after oviposition, turning a golden brown several days later. General shape spherical with a flat bottom. Exochorion with raised border hexagons with small spines on the rib intersections; hexagon cells of varying sizes and arranged in rows of 9–10 from the base to the micropylar area.

First instar ( Fig. 75 ). Duration 3 days (n = 4). Head capsule width 0.29 mm (n = 4). Newly hatched larvae just over 1 mm in length and reached 2 mm after three days of feeding. Head light tan with black stemmata; body whitish with green dorsally colored by the intestinal content. The single pair of dark brown dorsal setae on each segment; the lateral setae white and number approximately four per segment.

Second instar ( Figs. 76–77 ). Duration 3–4 days (n = 4). Head capsule width 0.46 mm. Maximum length 4 mm (n = 4).Head light tan with black stemmata. Body light green, last three abdominal segments pale with lateral lobes pale green. Dorsal setae dark reddish-brown; lateral setae white; spiracles white. General morphology similar to the first instar, but with more numerous and enlarged setae. Spiracles on A8 were twice the diameter of those on A7. Tentacle nectary organs (TNOs) on segment A8 were functional and everted ( Fig. 77 ).

Third instar ( Fig. 78 ). Duration 3–4 days (n = 4). Head capsule width 0.69 mm (n = 4). Maximum length 7 mm Body countershaded, darker green dorsally and lighter green laterally. Dorsal setae dark brown and lateral setae white. Tegument sprinkled with tiny white setae. Each segment, from T2 to A7, had 2 pairs of white spots located at the anterior and posterior border of the segment at the edge of the dorsal dark green stripe.

Fourth instar ( Fig. 79 ). Duration 5 days (n = 4). Head capsule width 1.1 mm (n = 4). Maximum length 11 mm. General morphology and color similar to fifth instar.

Fifth (last) instar ( Figs. 80–81 ). Duration 6–7 days (n = 4). Head capsule width 1.6 mm (n = 3). Maximum length 20 mm, Head beige, body color pattern countershading more subtle with less contrasting tones of green; medium dark green dorsally and lighter gray-green laterally. Body more slender than previous instars with lateral lobes reduced. Tegument with small white setae distributed over the entire upper body surface. White dorsal spots reduced to pairs on the posterior segmental border and in some individuals connected by a thin white bar. Dorsal setae dark reddish brown with distally pointed tiny spines and of varying lengths. Lateral setae numerous and white, forming a protective barrier or skirt. Spiracles white. TNOs subjacent to the spiracles on segment A8. In the prepupa phase larvae acquired a brownish pink aspect with pinkish transversal bands.

Pupa ( Fig. 82 ). Duration 12–19 days (n = 8). Length 10.6–12.6 mm, width at widest point on A2 3.8 mm (n = 7). Ratio of length/width in dorsal view was 1:0.33. Body color mottled brown with a pair of black spots dorsally on T3. Wing pads and head dark brown to black; dorsal anterior portion of T1 and A2 segments black. Prominent dorsal crest on A2 segment, with the following abdominal segments sloping downwards towards the cremaster. T2 segment domed dorsally. Body ventrally flat and smooth from the cremaster to the wing pads. Longest setae are on the A2 crest and on the T1–T2 dorsal tubercles. Numerous posteriorly pointing short setae on dorsal abdominal segments. The proximal one third of each seta was pilose and the remainder a thin smooth shaft. Eyes setose. Silk girdle position variable, crossing either A1 or A2.

Figures

FIGURES 1–24. Adults of six Mesosemiina species included in this work, in dorsal and ventral views, respectively. 1–4, Mesosemia cippus, male (1–2) and female (3–4) from Villavicencio, Meta, Colombia; 5–8, M. thymetus thymetina, male (5–6) and female (7–8) from Villavicencio; 9–12, M. walteri, male (9–10) and female (11–12) from Villavicencio; 13–16, Leucochimona lagora, male (13–14) and female (15–16) from Villavicencio; 17–20, L. icare matatha, male (17–18) and female (19–20) from Jundiai, São Paulo, Brazil; 21–24, Semomesia croesus lacrimosa, male (21–22) and female (23–24) from Villavicencio.

FIGURES 72–82. Immature stages of Mesosemia thymetus thymetina. 72, recently laid egg; 73, egg at day 7; 74, hatched egg shell; 75, first instar; 76, second instar, note channel cut in the leaf (arrows); 77, second instar, showing everted TNO in response to a Monomorium floricola ant approach; 78, third instar in premoult; 79, fourth instar; 80, fifth (last instar); 81, prepupa; 82, pupa in lateral, dorsal and ventral views, from the top to bottom, respectively.