Comparative morphology of the egg of the castniid palm borer, Paysandisia archon (Burmeister, 1880) (Lepidoptera: Castniidae)

Abstract

Paysandisia archon (Burmeister, 1880) is an attractive neotropical castniid moth whose presence in Europe was recently reported. Its larvae are endophagous, feeding inside the trunks and branches of several species of palm trees (Arecaceae). The present paper deals with the morphology and biometry of the egg of this moth, comparing them with those of other castniid species. The egg is a typical castniid egg, fusiform, upright sensu Döring, light cream or creamy pink when freshly laid, 4.69 ± 0.37 mm long and 1.56 ± 0.11 mm wide. Larvae emerge by gently splitting the chorion along one of the longitudinal ridges, on the half closer to the micropyle. SEM, TEM and LSCM photographs showing ultrastructural details of the egg are shown for the first time. The micropylar rosette (c. 54 μm in diameter) has generally 14–17 cells; in its centre lies the micropylar pit (c. 6 μm in diameter) which bears 12–16 micropylar canal openings (= micropyles) around its periphery. The pathways followed by those canals through the chorion have been figured. Eggs sampled in the wild (so laid by several females) were found to have a slightly variable number of ridges: most bore seven ridges (68.87%), although a significant portion (30.46%) bore eight and 1 egg (0.67%) bore only six; this against the currently accepted rule of five‐ridged eggs for Castniini (i.e. Neotropical castniids) to which Paysandisia archon belongs. It has also been found that the same female specimen has the capability of producing six‐, seven‐ or eight‐ridged eggs. Five types of egg irregularities affecting the longitudinal ridges are also figured and described. Transverse striae on the egg of P. archon are about 122. Aeropyles (c. 4 μm in diameter) occur on the ridges, at the intersections between the latter and two contiguous (left and right of the ridge) transverse striae, amounting to c. 854 on a seven‐ridged egg and to c. 976 on an eight‐ridged egg. Occasionally minute aeropyles ('microaeropyles') (c. 1.96–3.13 μm in diameter) also occur on transverse striae located close to both egg poles. The chorion of P. archon shows the typical ditrysian fine structure with very thin basal layer (C‐1), 0.3–0.2 μm thick, gas‐filled trabecular layer (C‐2), c. 0.9 μm thick, and lamellar layer (C‐3), its thickness varying between 18.5 and 13 μm due to the bumpy external surface of the chorion. Aeropylar canals, that penetrate layer C‐3, connect the air‐containing inner chorionic meshwork (the trabecular layer C‐2) with the surrounding air; their outer part forms a big bulbous cavity (which opens to the outside through the small opening seen in external SEM images) and, underneath, a narrow canal follows, leading into the trabecular layer (C‐2).