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Abstract
Cultures of Manduca sexta Johanssen in our laboratory were found to have larvae with missing or deformed mouthparts or antennae. Hypothesizing that these developmental deformities were caused by crowded rearing conditions, we reared larvae in four different population densities and recorded the incidence (% of larvae affected) and types of chemoreceptor deformities. Results showed that the incidence of these deformities was directly proportional to larval population density. Deformities of the maxilla and palp were the most frequent, followed by those of the antenna, epipharynx and maxillary styloconica. Life history traits of larval mass, food consumption, and rate of development were inversely related to larval density for both normal and deformed larvae. We discuss possible causes and mechanisms of these deformities and of changes to life history traits.
Highlights
Larvae of oligophagous insects such as the tobacco hornworm, Manduca sexta Johanssen, rely primarily on their gustatory chemoreceptors for food selection
Chemosensory Organ Deformities: Types and Incidence A larva with a normal set of chemosensilla and larvae with a missing or deformed epipharynx, antenna, maxilla, maxillary palp, and styloconica were selected for scanning electron microscopy (SEM) (Fig. 1)
In larvae from group cultures, such deformities occurred frequently, and experiments showed that the incidence of missing or deformed chemosensilla was directly proportional to population density
Summary
Larvae of oligophagous insects such as the tobacco hornworm, Manduca sexta Johanssen, rely primarily on their gustatory chemoreceptors for food selection. The main gustatory organs border the chewing mouthparts (mandibles). Dorsal to these mandibles and covering the upper portion of the oral cavity is a bi-lobed flap, the epipharynx (Fig. 1A, arrow); in each lobe is an extremely small sensory organ that is in continuous contact with the oral contents during feeding. Bilateral ablation of all of these sensilla essentially eliminates the animal’s ability to discriminate between host and non-host plants; for example, chemosensory-ablated larvae will eat Canna generalis, a plant so deterrent that normal larvae would prefer to starve rather than eat it (Waldbaur and Fraenkel 1961, Hanson and Dethier 1973, de Boer and Hanson 1987, de Boer 1991). Our research on the sensory basis of food selection has traditionally employed laboratory-reared larvae that were screened to exclude those with missing or deformed chemosensory organs described above. We discuss possible mechanisms and compare results with those from other species
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