Sensing the Plant Surface Prior to Feeding and Oviposition: Differences in External Ultrastructure and Function Among Tarsi of Heliconius erato

Abstract

Adult foretarsi of Heliconius erato Linnaeus (Lepidoptera, Nymphalidae) are reduced in size and are not used for walking. Foretarsi of the female have specialized sensilla that are presumably used to identify the host plant, by drumming. The mid- and hind tarsi also bear sensilla in both sexes, but these have not been described in detail, nor has their chemosensory function been determined. We described and compared the tarsi of H. erato under light and scanning electron microscopy. Behavioral experiments showed that differences in the shape, number, and size of sensilla were related to feeding and oviposition behaviors. Two types of sensillum (chaeticum and trichodeum) were found in similar numbers and size on the mid- and hind tarsi of both sexes. Sensilla on the female foretarsi act in host-plant site selection, strongly affecting oviposition rates when isolated. Male foretarsi lack sensilla, which may have been selected against due to the absence of function and thus lost. Sensilla on the mid- and hind tarsi are involved in sugar detection in both sexes, responding to an effective dose of sucrose (ED) near 0.01 M, and therefore might be used to identify food resources when the butterflies settle on flowers.

Morphology and function of ovipositorial and tarsal sensilla of female Asian citrus psyllid

“…The morphology and the types of ovipositorial and tarsal sensilla found in this study are quite similar to those reported previously by (Cruz & Eizaguirre 2016) in Sesamia nonagrioides, Papilio hospiton (Sollai et al 2010) and Heliconius erato (Silva et al 2017). In our study, we focus on the ovipositorial and tarsal sensilla most likely involved in selection of a suitable host for oviposition.…”

“…Generally, the sensilla, small hair like projections mainly distributed over the cuticular walls, are involved in mechanoreception and chemoreception (Carle et al 2014) located on the antennal flagellum. However several studies regarding ovipositorial and tarsal sensilla of major agricultural pests including Bactrocera dorsalis and Bactrocera diaphora (Zhang et al 2012), Sesamia nonagrioides (Cruz & Eizaguirre 2016), Spodoptera littoralis (Seada et al 2016) and Heliconius erato (Silva et al 2017) has revealed that beside olfaction it plays an important role in host and oviposition site selection.…”

“…Sensilla trichoid (ST) are widely believed to have chemosensory roles and sometimes could function as mechanoreceptors; they are usually linked to pheromone detection and host location (Silva et al 2017;Zhang et al 2012). Sensilla trichoid (ST) are distributed on the whole ovipositor; sensilla chaetica (ChS) are mainly distributed at the end of the ovipositor back plate, and sensilla basiconica (SB) are mainly distributed at the end of the ovipositor back plate.…”

“…It is reported that sensilla chaetica (ChS) can feel the air flow and vibration of the host leaf surface (Zhang et al 2012). The large sensilla chaetica (ChS) on the tarsus of Heliconius erato have a typical form of mechanical sensors, which are mainly used to determine the position and motion of the oviposition (Silva et al 2017). These have a spiral structure and no holes on the surface.…”

“…As a basic unit of the insect sensory system, the sensilla is a part of somatic specialization of the body surface. Sensilla trichoid (ST), especially on the anterior tarsal joints, have the important functions of tasting and smelling (Silva et al 2017;Zill et al 2010). Sensilla trichoid (ST), especially on the anterior tarsal joints, have the important functions of tasting and smelling (Silva et al 2017;Zill et al 2010).…”