A Tape-Mounting Protocol for High-Resolution Z-Stack Imaging of Drosophila Compound Eyes

Summary

The primary goal of this protocol is to enable non-destructive imaging and quantitative analysis of Drosophila compound eyes. It combines a tape-mounting strategy with stereomicroscopic Z-stack acquisition and Extended Depth of Focus reconstruction to produce high-resolution, artifact-free images.

Abstract

Here, we present a non-destructive mounting and imaging protocol for high-resolution phenotypic analysis of the Drosophila compound eye. Accurate phenotyping of the Drosophila compound eye is often limited by its convex geometry and mechanical fragility during handling. To address this, we detail a versatile, non-invasive imaging workflow that enables high-resolution visualization from multiple perspectives. The protocol utilizes a custom adhesive mounting strategy on an inverted-tape platform, securing flies by the body and wings while leaving the head completely untouched. Crucially, this flexible configuration supports distinct mounting orientations: a lateral position to capture the full convexity of individual eyes, and a frontal position to assess whole-head symmetry. This method also facilitates the arrangement of high-density arrays, allowing for the rapid processing of 20–30 samples per session. By integrating reflected-light stereomicroscopy with Z-stack acquisition and Extended Depth of Focus (EDF) reconstruction, the workflow generates uniformly sharp, quasi-3D composite images. This approach preserves natural pigmentation and produces high-fidelity, contrast-rich images suitable for quantitative organ-scale phenotypic analysis.

Introduction

The Drosophila melanogaster compound eye is a classical and powerful model for studying organ growth, patterning, and robustness during development. Composed of approximately 800 highly ordered ommatidia, the fly eye provides a sensitive and quantifiable readout of genetic perturbations affecting proliferation, differentiation, and tissue organization^1,2,3. Genetic manipulation of eye development has historically yielded fundamental insights into conserved signaling pathways. Seminal studies demonstrated that ectopic expression of the transcription factor eyeless is sufficient to induce eye formation in non-retinal tissues, establishing the eye as a paradigm for organ specification and plasticity⁴. Subsequent work identified the D. melanogaster Hippo signaling pathway as a central regulator of eye size and cell number through coordinated control of proliferation and apoptosis^5,6. In parallel, Hedgehog and BMP signaling were shown to regulate morphogenetic progression, growth precision, and spatial patterning within the developing retina^7,8.

However, capturing high-fidelity images of the adult compound eye presents significant optical challenges due to its convex, hemispherical geometry. Standard bright-field microscopy is limited by a shallow depth of field, making it impossible to simultaneously focus on the dorsal-most point (pole) and the marginal edges (equator) of the eye in a single exposure⁹. This limitation often results in images where significant portions of the retinal surface are blurred, hindering the application of high-resolution image analysis and machine learning tools designed to quantify variegated phenotypes. To fully characterize the retinal surface, the entire curvature must be visualized without optical artifacts. Therefore, an imaging method that preserves native eye morphology while enabling high-resolution visualization of the entire curved retinal surface is required.

Furthermore, traditional sample preparation methods for microscopy often compromise the structural integrity of the fly head. Conventional mounting techniques frequently rely on adhesives that can obscure surface details or involve physical manipulation with tweezers that applies pressure to the head capsule. As noted in studies of retinal morphogenesis, the structural integrity of the eye is paramount. Direct pressure or puncture causes hemolymph leakage, leading to internal pressure loss and the subsequent collapse or "denting" of the eye surface. These mechanical artifacts can mimic neurodegenerative phenotypes or genetic defects, leading to false-positive results in large-scale screens. The goal of this protocol is to provide a non-destructive mounting and imaging workflow for high-resolution visualization of intact Drosophila compound eyes.

Here, we present a rapid and reproducible imaging workflow that enables high-resolution visualization of intact Drosophila compound eyes and whole-head morphology without direct contact with the head. In this method, adult flies are immobilized using a flexible adhesive mounting strategy that secures the body and wings, allowing for stable positioning in either a lateral orientation—to capture the full curvature of a single eye—or a frontal orientation—to assess whole-head symmetry and inter-ocular features. This versatile mounting strategy minimizes mechanical damage and preserves native eye geometry in three dimensions. When combined with reflected-light stereomicroscopy, Z-stack acquisition, and extended depth-of-focus reconstruction⁹, this approach generates uniformly sharp, high-quality images from multiple perspectives, suitable for both qualitative inspection and quantitative phenotypic analysis. This method is particularly suitable for studies of genetic perturbations, developmental phenotypes, and high-throughput screening of eye morphology in Drosophila.

Protocol

1. Sample collection and synchronization

NOTE: To ensure experimental comparability, strict control of eclosion timing is required. Under standard culture conditions at 25 °C, eclosion typically occurs 10 to 12 days after egg laying. Pupae should be closely monitored during this window, as the darkening of the pupal case indicates imminent eclosion.

1. Direct tapping method (Standard method)
   1. Gently tap the standard 24 x 95 mm plastic Drosophila culture vial (sealed with a foam plug) against a soft pad to dislodge adult flies to the bottom.
   2. Rapidly remove the plug and align the mouth of a fresh collection vial with the culture vial.
   3. Invert the setup so the culture vial is on top. Gently tap the walls to transfer the flies into the collection vial.
      NOTE: This method is not recommended if the standard cornmeal-agar food medium is moist, as newly eclosed flies are easily trapped and killed. To minimize mortality, initially collect flies into an empty 24 x 95 mm vial. After anesthesia (Step 3), transfer the flies onto the inner wall of a 24 x 95 mm vial containing fresh food medium.
2. Negative geotaxis transfer method (Anti-adhesion method)
   1. Remove the plug from a fresh collection vial and invert it on the benchtop to prevent stray flies from entering.
   2. Hold the culture vial with the opening facing downward. Flies will climb upward toward the food side, away from the opening.
   3. Once the opening is clear of flies, rapidly uncork the culture vial and align it with the inverted empty 24 x 95 mm collection vial on the bench.
   4. Gently tap the connection point against the benchtop to drop the flies into the collection vial.
      NOTE: This method significantly reduces mortality caused by food adhesion but is more time-consuming than direct tapping.
3. CO₂ anesthesia method (High-efficiency method)
   1. Invert the culture vial so the plug faces downward.
   2. Carefully insert the needle of the CO₂ anesthesia gun between the plug and the vial wall.
   3. Gently engage the foot pedal and hand valve to release 100% CO₂ at a low, gentle flow rate.
   4. Once flies are anesthetized and have fallen onto the plug, place the culture vial horizontally. Remove the plug, align with an empty collection vial, and gently tap the culture vial to transfer the anesthetized flies into the collection vial.
      NOTE: Excessive airflow may blow flies onto the food surface, causing adhesion and death; strictly maintain a gentle CO₂ flow sufficient to anesthetize the flies without physically displacing them. Monitor flies closely for early signs of recovery, such as subtle twitching or movement of the wings and legs.
4. Labeling and maintenance
   1. Label the collection vial with the sex, collection date, eclosion date, genotype, and culture conditions.
   2. Sex Separation: If imaging flies older than 10 days, separate males and females into different food-containing culture vials to prevent mating and the emergence of an F2 generation, which could interfere with the experimental environment.
      NOTE: Flies can be maintained in these food-containing vials under former culture conditions (e.g., 25 °C) until they reach the desired age for imaging.

2. Preparation of the mounting plate

1. Obtain a clean standard glass microscope slide.
2. Cut a strip of white paper masking tape approximately 2 cm longer than the slide.
3. Place the tape on the slide with the adhesive side facing upward (non-adhesive side against the glass). Ensure the tape overhangs the slide by approximately 1 cm on both ends.
4. Secure the overhanging ends by wrapping two additional ~5 mm strips of tape tightly around the back of the slide.
5. Ensure that the adhesive surface is flat, taut, and free of wrinkles.
6. Labeling: Mark orientation indicators (e.g., "Top/Up" and "Bottom/Down") or sample identifiers on the non-adhesive ends of the tape to facilitates orientation and sample tracking under the microscope.
   NOTE: When selecting flies for imaging, avoid choosing individuals with significant body size disparities. Even with identical magnification, body size variations can result in non-comparable compound eye surface areas.

3. Euthanasia and mounting

1. Euthanasia
   1. Transfer the flies into a sealed and empty collection vial.
   2. Place the sealed vial in a -80 °C freezer for 15–20 min.
      ​CAUTION: Never puncture or squeeze the fly with the sharp tips of fine tweezers. Even minor hemolymph leakage causes a rapid loss of internal hydrostatic pressure, leading to immediate denting and irreversible collapse of the compound eye, which destroys imaging quality. 
2. Mounting for lateral view (Single compound eye)
   1. Positioning: Under a standard stereomicroscope (e.g., Nikon SMZ745), use fine-point tweezers to orient the euthanized fly into a lateral (side-lying) position.
   2. Transfer: Using fine-point tweezers, gently grasp the fly by the wing (maintaining the lateral posture) and transfer it to the prepared adhesive mounting plate.
   3. Fixation: Adhere one side of the fly's body to the sticky tape. Spread the wings and adhere them flat against the tape to increase stability.
   4. Appendage Management: During mounting, ensure legs do not obstruct the compound eye. If the forelegs block the field of view, use tweezers to carefully pull them away from the eye and firmly adhere them to the tape (e.g., extending them downwards or tucking them under the thorax).
   5. Securing: Tilt the tweezers and use the side of the tip (not the sharp point) to gently press the body against the tape to ensure a secure bond.
   6. Fine Adjustment: Check if the head is horizontal. If the eye is not on a single focal plane, use tweezers to micro-adjust the body angle.
3. Mounting for frontal view (Whole head)
   1. Positioning: On the anesthesia pad, orient the fly into a ventral-side-down (prone) position.
   2. Transfer: Use tweezers to grip one wing and lift the fly. Transfer it to the adhesive mounting plate.
   3. Initial Fixation: Gently spread both wings outward (left and right) and press them firmly onto the adhesive surface to anchor the fly.
   4. Body Stabilization: Gently press down on the thorax to ensure full contact with the adhesive plate. Use tweezers to lightly grip the posterior abdomen (tail) and pull gently to straighten the body axis before adhering the abdomen to the tape.
   5. Head Orientation: Adjust the angle of the head using fine tweezers to ensure the frontal plane (face) is parallel to the adhesive surface. Both compound eyes should be level and visible in the same focal plane.
4. Arrangement
   1. Mount multiple flies in parallel rows or a grid-like array on the adhesive surface. Ensure that individual flies do not overlap to prevent Z-stack interference, though close packing is permitted to maximize throughput (e.g., 20–30 flies per slide). Align all flies with their heads facing the same direction (as indicated by the labels in step 2.6) to facilitate systematic imaging.
   2. Prepare separate sample arrays for lateral and frontal imaging using different sets of euthanized flies from the same cohort. Once a fly is adhered to the tape, it cannot be safely removed or repositioned without causing severe mechanical damage to the specimen; therefore, multiple views cannot be sequentially imaged from the same individual.
      NOTE: It is highly recommended to image the samples immediately after euthanasia and mounting to prevent tissue desiccation.

4. Image acquisition

1. Motorized stereomicroscope imaging system startup and initialization
   1. Power on the PC, monitor, and microscope unit in sequence.
   2. Launch the LAS X software.
   3. When the "Initialize/Self-test" prompt appears, ensure the space under the objective is clear of obstacles and click Confirm. Allow the microscope to complete the full Z-axis calibration movement.
      NOTE: The following steps are optimized for a Stereomicroscope with the LAS X software system.
2. Sample placement and lighting
   1. Transfer the prepared mounting plate to the base platform of the motorized stereomicroscope, manually positioning it to center the first sample in the array under the objective of the digital camera.
   2. Turn on the Transmitted Light (base) and Incident Light (top) sources. For compound eye imaging, primarily use Reflected Light. Adjust the aperture to ensure uniform illumination.
3. Software configuration
   1. Set the acquisition mode to Z-stack. In the project settings, set the multi-dimensional movement priority to 'Z-first' to ensure continuous and stable motor drive operation.
   2. Set the imaging mode to Reflected Light.
   3. Enable Extended Depth of Focus (EDF) to allow for the synthesis of a fully focused image.
   4. In the Image Advanced Settings, enable High Dynamic Range (HDR). This is highly recommended to mitigate the specular highlights and overexposure commonly caused by the highly reflective, chitinous surface of the compound eye.
4. Positioning and white balance
   1. Adjust the magnification knob to the appropriate level (e.g., a zoom factor of 16x for the lateral single-eye view, and 12.5x for the frontal whole-head view), ensuring the specific region of interest occupies 1/2 to 2/3 of the field of view.
   2. Manually position the mounting plate to bring the fly's thorax into the center of the field of view.
   3. Perform white balance calibration using the thorax to correct color temperature.
5. Exposure and imaging parameters (Reference values)
   Exposure: 60–80 ms.
   Gain: 0–5 (Keep low to minimize noise).
   Intensity: 60%–80%.
   Z-Stack Step Size: Standard: 8–12 µm.
   NOTE: For smooth, healthy compound eyes (e.g., w¹¹¹⁸ × GMR-GAL4), step sizes up to 15 µm may be sufficient, as the continuous and regular curvature requires fewer optical sections. However, for rough or uneven eye phenotypes characterized by abrupt topological changes, smaller step sizes (as low as 5 µm) are required to capture fine structural details and significantly improve EDF reconstruction accuracy.
6. Z-Stack definition and capture
   1. For Lateral View: Set the Z-stack range from the orbital rim (Start) to the center ommatidia (End) using a step size of 8–12 µm.
   2. For Frontal View: Adjust the focus knob until the outer contour of the compound eyes is just in focus (Start). Adjust focus in the opposite direction until the central ocelli (yellow simple eyes) on the top of the head are just in focus (End). Click Set End.
   3. Click Stop Live to freeze the preview.
   4. Click Start Acquisition. The system will automatically capture the defined focal planes and synthesize a high-resolution, fully focused image.
7. File saving:
   1. Upon completion of the EDF reconstruction, click the Save icon in the software interface (or navigate to File > Save As).
   2. Select the destination directory, input the designated file name, and click Save.
   3. Note that each acquisition generates three distinct image files: a. Raw/channel image (grayscale);b. Color preview image; c. EDF composite image (with the "_Composite" suffix)
      ​NOTE: Both the raw/channel image and the EDF composite image should be retained for analysis and documentation

Results

The complete experimental pipeline, from sample collection to image acquisition, is summarized in Figure 1. The efficiency of the modified protocol is demonstrated through the creation of high-density sample arrays. As shown in Figure 2A, 20–30 anesthetized flies can be arranged in a grid-like pattern on a single inverted-tape platform. Orientation markers written on the non-adhesive ends of the tape ensure consistent head directionality, which facilitates rapid...

Discussion

The Drosophila compound eye serves as a powerful phenotypic readout for genetic screening, where observable changes in eye size, roughness, or pigmentation^2,3,8often indicate perturbations in fundamental signaling pathways such as Hippo^5,6 or Hedgehog⁷. However, the efficiency of such screens has long been bottlenecked by the limitations ...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Carbon dioxide (CO2) gas cylinder	Local supplier	N/A
CO2 anesthesia blow gun	T&L	DG-08
Digital microscope camera	Leica	DMC6200
Fine tweezers	VETUS	MOS-167B
Foam plugs	Dongpeng	N/A
Image acquisition software (LAS X)	Leica	3.7.1
Image processing software (FIJI/ImageJ)	NIH	Open source
LED Dome Illuminator	Leica	LED5000 HDI
LED Gooseneck illuminator	cossim	LG-10
Motorized stereomicroscope	Leica	M205 FCA
Objective lens	Leica	1.0x PlanApo NA0.2
Pneumatic foot pedal valve	DELIXI	FV-320
Standard Drosophila culture vials	LAIBOER	2424951
Standard Drosophila food medium	N/A	Cornmeal-agar-molasses
Standard glass microscope slides	Citotest	10127101P-G
Standard stereomicroscope	Nikon	SMZ745
White paper masking tape	UBWIN	25mm width