Morphological Survey of the Maxillae in Last Stage Larvae of the Suborder Ditrysia (Lepidoptera): Palpi

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Abstract
Sensilla on the distal segment of the maxillary palpus of 36 species of ditrysian larvae, representing 21 families, were examined with the scanning electron microscope. Variability of the sensilla in external appearance and location on the segment among families was recorded and a sensillar terminology developed. Externally, three kinds of sensilla were recognized: eight sensilla basiconica on the apex, one sensillum digitiformium on the dorsoposterior wall, and several sensilla campaniformia on the wall adjacent to the digitiform sensillum. In all species the sensilla basiconica occur in three morphological groups: three apical sensilla (A1, A2, and A3), three lateral sensilla (L1, L2, and L3), and two medial sensilla (M1 and M2). Apical sensilla have rounded apices, and M and L sensilla are pointed or papilliform. The A sensilla are either multiporous, wrinkled, and short, with cuticular projections encompassing a terminal pore; or uniporous, smooth-walled, and tall, without recognizable projections. The first type is typical of families with small endophagous larvae, while the second is more common in families with large exophagous larvae. In some species the apical sensillum, A2, is distinctly styloconic. The digitiform sensillum (SD) is usually subapical. Epicuticular pores on the sensillum digitiformium are absent but a subapical pore occurs in some species, and in one species, both a subapical and basal pore are seen. These pores are probably ecdysial pores (molting scars). In some species four sensilla campaniformia (SC) are present. SC1 is the most prominent and consistent, being lateral to the sensillum digitiformium and taking the form of a large, circular depression. SC2 is an indentation or circular depression below SC1. A medial SC3 is sometimes present. Frequently, an SC4 is near the base of the sensillum digitiformium. Actias luna (L.) has two SC4s, each a prominent bulbous swelling at the base of the digitiform sensillum. Ecdysial pores are sometimes observed on the sensilla campaniformia.