Bombyx incomposita (van Eecke, 1929)

Lin, Rung-Juen, Lin, Yu-Chi & Hsu, Yu-Feng, 2021, Systematic position of Bombyx incomposita (Lepidoptera: Bombycidae), with notes on its immature biology and hostplant association, Zootaxa 5052 (1), pp. 91-104 : 95-99

Morphology of immature stages of Bombyx incomposita

Egg ( Figs. 5 , 16, 17 ). Approximately 1.95 mm in diameter, 0.86 mm in height; flat-shaped, yellow when laid, then turning to pale pink upon hatching.

First instar larva ( Figs. 6, 7 , 18 ). Body length x = 7.18 ± 0.63 (SD) mm (n = 11). Head: rounded, rather flat in front, hypognathous. Six stemmata on each side of head; stemmata 1–4 and 6 arranged in a loose circle, stemma 5 away from the others and ventrally located near the antennal scape; all stemmata nearly equal in size ( Fig. 19 ). Head capsule surface smooth, slightly glossy; color black. Body: Body surface bearing long setae, in form of radiating setae on convex verruca. T2 and T3 both conspicuously humped. T1–T3 and A1 whitish, A2 and A3 deep grey, A4– A10 with ground color orange mottled with complex black markings, middorsal horn on A8. Spiracles black. A3– A6 prolges of same size, caudal prolegs on A10 enlarged. Crochet on ventral prolegs and caudal prolegs uniordinal arranged in lateropenellipse. Chaetotaxy: Head: all 17 primary setae present, anterior (A1, A2, and A3), stemmatal (S1, S2, and S3), substemmatal (SS1, SS2, and SS3), lateral (L1), posterodorsal (P1 and P2), frontal (F1), adfrontal (AF1 and AF2), and clypeal (C1 and C2) setae; S3 shorter than S1 and S2; AF1 and AF2 longer than C1 and C2; L1 equal to A3 ( Figs. 19, 20 ). Body: most of primary setae on verrucae. Prothorax (T1): prothoracic shield lightly sclerotized along anterior margin and surround XD, SD, D1, and D2 setae. XD consisted of six setae, SD consisted of three setae, seta D1 and D2 solitary ( Fig. 21 ); L consisted of five setae; SV consisted of five setae. Meso- and metathroax (T2 and T3): verruca D1+D2 consisted of twelve setae on segment of T2 and T3; SD1 consisted of six setae; seta L3 solitary above L1+L2, verruca L1+L2 consisted of five setae; SV consisted of five setae. Abdomen: segments A1–A8 with verruca D1 consisted of five setae, but segments A1 and A2 verruca D1 consisted of seven setae; seta D2 isolated posterior to D1; D1 longer than D2; SD1 consisted of four setae, SD2 absent; L1 posterior to seta L2; SV consisted of eight setae, while segments A1 and A2 SV consisted of five setae; seta V present. A8 with D verruca enlarged ( Figs. 22, 23 ). A9 with only one D verruca and one L seta, SD verruca absent. L group absent on A10. D1+SD verruca on anal shield, consisted of 5 pairs of setae on A10. Paraproctal group (PP) on the posterior margin of the anal lobe, PP consisted of 4 pairs of setae.

Second instar larva ( Fig. 8 ). Body length: approximately 9.5 mm. Head black. Body covered with wax, T2 and T3 both conspicuously humped. T1–T3 and A1 whitish, A2 and A3 deep grey, A4–A10 with orange color and black spots, dorsal scoli stout, a pair of small horns on A2–A7, a middorsal horn on A8, a pair of small bumps on A9–A10. Spiracles black. Legs black, prolegs brown.

Third instar larva ( Fig. 9 ). Body length: approximately 14.5 mm. Head black. Body covered with wax, T2 and T3 both conspicuously humped. T1–T3 and A1 whitish, A2 and A3 deep grey, A4–A10 with orange color and black spots, dorsal scoli stout, a pair of small horns on A2–A7, a middorsal horn on A8, a pair of small bumps on A9–A10. Spiracles black. Legs black, prolegs brown.

Fourth instar larva ( Figs. 10, 11 ). Body length x = 28.05 ± 2.03 (SD) mm (n = 3). Head black. T2 and T3 both conspicuously humped. T1–T3 and A1 whitish, A2 and A3 deep grey, A4–A10 with orange color and black spots, dorsal scoli stout, a pair of small horns on A2–A7, a middorsal horn on A8, a pair of small bumps on A9–A10. Spiracles black. Legs black, prolegs brown.

Fifth instar larva ( Figs. 12, 13 ). Body length: approximately 45.5 mm. Head capsule surface smooth, slightly glossy; ground color deep grey with black spots. A Reddish orange triangle on frontoclypeus ( Fig. 14 ). T2 and T3 both conspicuously humped. T1–T3 and A1 whitish, A2–A10 ground color deep grey with reddish orange spots, A2–A7 dorsal filaments short, black, orange basally, a middorsal horn on A8 ( Fig. 15 ). A pair of small orange bumps present dorsally on A9 and A10. A small orange bump present subventrally on A3–A9. Spiracles black. Legs dark orange, prolegs and anal proleg black with dark orange spots.

Cocoon. Silk yellow, cocoon length: x = 29.02 ± 2.40 (SD) mm in length, x = 13.19 ± 0.64 (SD) mm in width (n = 4).

Hostplant associations. Morus australis (Moraceae) , an introduced, cultivated tree in Southeast Asia, was accepted as a larval hostplant of Bombyx incomposita in the present study. A species of mulberry tree, M. macroura is native in Malaysia ( Berg et al. 2006), possibly representing the primary larval hostplant of B. incomposita .

Biology. Eggs were laid in clusters, with each egg separated from other eggs, attached on the underside of the larval host plant. The first instar larvae emerged from the lateral side of egg, without completely consuming the chorion. The larvae only fed on the leaves. When not feeding or molting, the larvae usually resided on the twigs. The final instar larvae spun its cocoon between two or three leaves before the pupal stage. The duration from egg to adult took about 60 days.

Morphology of genitalia. Male genitalia ( Figs. 24–27 ). Uncus broad, stout, bifurcate, with caudal end slightly enlarged, clavate; gnathos heavily sclerotized, forming a disc protruding posteriad with caudal mardin rounded; saccus digitate, with anterior end enlarged. Valva broad basally; ampulla elongate, tapering caudally, with distal end pointed; sacculus prominent, ovate. Juxta as a pair of triangular sclerite, fused with sacculus ventrally. Phallus elongate, slender, with aedeagus 3X length of phallobase, attenuate posteriad, but slightly enlarged at caudal end. Female genitalia ( Fig. 28 ). Corpus bursae ovate, with ductus seminalis joining at posterior end near ductus bursae; signum absent. Ductus bursae thick, short, approximately as long as length of corpus bursae, membranous but posterior portion forming sclerotized, funnel-like antrum. Sterigma heavily sclerotized, forming a dome with jagged edge posteriorly, with lateral sclerotized walls extending caudally.

Diagnosis. Bombyx incomposita is similar to B. huttoni in wing pattern, but can be distinguished by male, female genitalia, and body coloration of larva. Uncus is broader in B. incomposita than in B. huttoni . The sterigma is less jagged in B. incomposita than B. huttoni . Larva bears a pair of slender dark horns on A2 to A7; the thorax is whitish; the abdomen is deep grey with orange spots.

Figures

FIGURES 5–15. Immature stages of Bombyx incomposita: 5, eggs; 6, first instar in lateral view; 7, first instar in dorsal view; 8, second instar; 9, third instar; 10, fourth instar in lateral view; 11, fourth instar in dorsal view; 12, fifth instar in lateral view; 13, fifth instar in dorsal view; 14, head capsule of fifth instar in frontal view; 15, A8 middorsal horn of fifth instar. Scale bars = 1 mm (Figs. 5–7) and 5 mm (Figs. 8–15).

FIGURES 16–23. Egg and the first instar of Bombyx incomposita under scanning electron microscopy: 16, general view of egg; 17, surface structure of chorion; 18, general view of larva in lateral; 19, head in lateral view; 20, head in anterior view; 21, T1 shield in lateral view; 22, lateral view of A3–A10; 23, D group on A8.

FIGURES 24–30. Genitalia and wing venation of Bombyx incomposita: 24, male genitalia; 25, phallus (posterior on the right, anterior on the left); 26, tergum VIII; 27, sternum VIII; 28, female genitalia; 29, forewing; 30, hindwing. Scale bars = 2 mm and 10 mm (Figs. 29–30).