Fine Structure and Optical Features of the Compound Eyes of Adult Female Ceratosolen gravelyi (Hymenoptera: Agaonidae)

TL;DR

This study explores the eye structure of female Ceratosolen gravelyi wasps to understand how vision helps them find their host fig trees.

Contribution

The paper provides the first detailed structural and optical analysis of the compound eyes of Ceratosolen gravelyi wasps.

Findings

The wasps' eyes have 228–263 ommatidia with specific optical features suited for diurnal activity.

The eye structure supports efficient light collection and processing of visual cues during host interactions.

The findings suggest vision works alongside smell and touch in guiding wasps to their host trees.

Abstract

Fig wasps and fig trees depend on each other to survive, and wasps locate their host trees with their sense of smell. However, the role that vision plays in locating host trees remains unclear. Here, we examined the eye structure of female Ceratosolen gravelyi wasps (which only pollinate Ficus semicordata) using scanning/transmission electron microscopy. Their oval-shaped eyes have 228–263 tiny eye units (ommatidia). Each unit has a clear lens, a four-part light-focusing structure, and light-sensing cells wrapped in pigment cells. The retinula cells form a rhabdom with nine cells per unit. Eight photoreceptors (R1–R8) form the rhabdom from the cone base to the basal matrix, and a ninth cell replaces R8 in the apical third of the rhabdom. Various optical measurements revealed that their eyes collect light efficiently. These features are consistent with their daytime activity, suggesting that their eyes process visual cues when approaching figs. This indicates that vision likely works with smell and touch to guide wasps to the right trees. Our findings thus enhance our understanding of how senses work together to mediate fig-wasp interactions.

Pollinating fig wasps (Agaonidae) engage in an obligate mutualism with Ficus species, which is mediated by host-specific chemical cues. However, the role of visual perception in host recognition remains poorly understood, particularly because of a lack of structural studies of their compound eyes. We investigated the ocular morphology of female Ceratosolen gravelyi (exclusive pollinator of F. semicordata) using scanning/transmission electron microscopy. The oval apposition eyes contain 228–263 ommatidia, which are asymmetrically distributed between the left and right eyes. Each ommatidium comprises a biconvex corneal lens overlying a tetrapartite eucone crystalline cone; proximal cone cells reveal an interlaced labyrinth. Pigment cells encapsulate each ommatidium, and numerous pigment granules and mitochondria are present in both pigment and retinular cells. Nine retinular cells comprise a unit, with eight photoreceptors (R1–R8) forming the rhabdom from the cone base to the basal matrix; a ninth cell replaces R8 in the apical third of the rhabdom. Optical metrics, including F-number (1.1°), acceptance angle (10.0°), and ommatidial sensitivity (0.26 µm2/sr), indicate diurnal activity in bright environments. These adaptations suggest that their eyes are critical for processing visual cues during host interactions, which advances our understanding of multimodal sensory integration in fig–wasp mutualism.

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