Image Spotlight

Figure 1. Cryo-EM image of ladybird tarsal claw and distal leg structures. The sample was prepared using the PP3010 preparation system, sublimed for 2 minutes at 90 °C, and coated with Ir (5 mA, 45 seconds). Imaged using a Thermo Fisher Apreo ChemiSEM.

Cryo-EM unlocks a unique perspective into specimens where micro- and nanostructures may otherwise be challenging to view using light or confocal microscopy. The ability to prepare a sample via rapid freezing, allows imaging of the sample in its near-native state without the need for time-consuming dehydration techniques such as critical point drying and freeze drying.

This month, we decided to explore the intricate morphology of a ladybird using the PP3010 cryo preparation system. The sample was mounted on a flat specimen shuttle using Tissue Tek:colloidal graphite (50:50), and plunge frozen in slush liquid nitrogen. The sample was transferred to the PP3010 preparation chamber, sublimed for 2 minutes at 90 °C, and coated with Ir (5 mA, 45 seconds). Imaging was performed using a Thermo Fisher Apreo ChemiSEM.  

Several distinct anatomical features of the ladybird were examined in detail, each revealing unique aspects of the insect morphology. 

Figure 2. Cryo-EM image of ladybird antenna. The sample was prepared using the PP3010 preparation system, sublimed for 2 minutes at 90 °C, and coated with Ir (5 mA, 45 seconds). Imaged using a Thermo Fisher Apreo ChemiSEM.

In insects, antennae function as sensory structures, responsible for detecting chemical and mechanical changes. In addition, they help to secure prey, and play a key role in the nervous system. The ladybird antennae consist of nine flagellomeres that are cylindrical and gradually increase in thickness from the base to the tip (flagellomeres F8 and F9 can be seen in Figure 2 left). The images reveal the “scaly” surface texture of the antenna and six types of sensilla (sensory receptors).1

Figure 3. Cryo-EM image of ladybird tarsal segments (left) and lanceolate setae (upper right) and pointed tarsal setae (lower right). The sample was prepared using the PP3010 preparation system, sublimed for 2 minutes at 90 °C, and coated with Ir (5 mA, 45 seconds). Imaged using a Thermo Fisher Apreo ChemiSEM.

Analysis of the legs revealed the distinct morphology of the tarsal setae. Ladybirds possess adhesive tarsal setae that promote efficient attachment to flat surfaces. These setae are approximately 15-30 μm in length and have tips ranging from 0.2-0.5 μm which can either be pointed, discoid, lanceolate or spatula in shape.2 Interestingly, discoid setae are only observed in male beetles - revealing our specimen to be female.

Figure 4. Cryo-EM image of ladybird eye. The sample was prepared using the PP3010 preparation system, sublimed for 2 minutes at 90 °C, and coated with Ir (5 mA, 45 seconds). Imaged using a Thermo Fisher Apreo ChemiSEM.

Ladybirds have compound eyes, composed of repeated hexagonal units known as ommatidia. Figure 4 highlights the structure of the eyes, while revealing short hairs (sensilla) distributed across the eye. These sensilla function as mechanoreceptors - detecting touch, airflow or vibrations near the surface of the eye.3 

Together, these observations demonstrate how cryo-EM can bridge the gap between morphology and function, showing that visualising structures at the micro/nano-scale is crucial for deepening our understanding of anatomical biology of insects. 

References

1. Hosoda N, Nakamoto M, Suga T, Gorb SN, Evidence for intermolecular forces involved in ladybird beetle tarsal setae adhesion. Scientific Reports, 2021, 11(1), 7729.
2. Hao YN, Sun YX, Liu CZ, Functional morphology of antennae and sensilla of Hippodamia variegata (Coleoptera: Coccinellidae), PLOS ONE, 2020, 15(8), e0237452.
3. Liang, Z.-L., Zhang, T.-H., Muinde, J., Fan, W.-L., Dong, Z.-Q., Wu, F.-M., Huang, Z.-Z., Ge, S.-Q. Ultrastructure and Spectral Characteristics of the Compound Eye of Asiophrida xanthospilota (Baly, 1881) (Coleoptera, Chrysomelidae), Insects 2024, 15, 532.