Goniometric investigation of scattering from insect wings in near infrared

Abstract

In recent decades, there has been significant attention on the decline in insect populations, including moths and hover flies. To tackle this issue, innovative methods such as entomological LiDAR technology have revolutionized the remote identification of free-flying insects. The distinct spectral reflection from moth and hover fly wings enables precise differentiation between species and sexes. While a comprehensive parameter space is crucial for accurate identification at the species and family levels, it’s important to note that some parameters overlap. Consequently, incorporating more parameters would improve the identification accuracy of insect species.

This ex-vivo study investigates wing surface irregularities in moth and hover fly species by analyzing their short-wave infrared angular scattering properties in both the goniometric and spectral domains. These distinct wing surface irregularities can enhance the potential for discriminating between insect species.

The estimation of surface roughness involved fitting a Bidirectional Reflectance Distribution Function (BRDF) model to the angular scattering lobe. Subsequently, a short-pass function was applied to spectrally fit the surface roughness curve. The reflectance signals from moth and hover fly species were also effectively explained using a short-pass function. Notably, the retrieved wing surface heterogeneity and steepness remained consistent and offer valuable insights into the irregularities of wing surfaces.

The study reveals that both the angular scattering lobe and wing surface roughness increase toward visible wavelengths across all species. This information is valuable for researchers when selecting wavelengths for surface roughness studies. Additionally, the ventral side of the wings exhibits greater roughness compared to the dorsal side. This finding has practical implications for groundbased LiDAR technology, particularly when studying insect species with subdued spectral features, such as the ventral side of moth wings. Minimal surface roughness is associated with specular reflectance, resulting in lidar signals that exhibit numerous harmonic overtones. Overall, the results affirm that insect wings become more specular and smoother as wavelengths shift toward the infrared range.

Popular Abstract

Quantifying surface roughness of moth and hover fly wings

The global decline in insect populations is a significant concern due to its adverse impact on agricultural food production. Regrettably, the primary driver of this decline is intensified agriculture, specifically through the use of pesticides and fertilizers. While many insects, including moths and hover flies, play a crucial role in pollinating crops, others can cause damage or transmit diseases. Accurate species-level identification of insects is essential for monitoring biodiversity. However, traditional methods like traps often pose challenges, such as being labor-intensive, time-consuming, and requiring specialized expertise that is not always readily available, hindering efficient and comprehensive monitoring efforts.

While the reflection of light from insects provides valuable information for remote identification, there is an issue of overlapping characteristics among different species. Consequently, additional data is necessary to enhance the accuracy of identifying free-flying insects remotely. Notably, the rough surface texture found on the wings of most insects, including moths, can be leveraged to improve identification methods.

This thesis seeks to validate the angular sactter model proposed by Li et al., 2022 and demonstrate its application in estimating the surface roughness of moths’ and hover flies’ wings. The model leverages the measured angular backscattering of photons from the wing surface to quantify its roughness. In this model, imagine a ping-pong-sized ball bouncing off a road paved with cobblestones. The ping-pong ball represents a photon of a specific wavelength, and the cobblestones are the wing surface features. If the cobblestones are larger than the ping-pong ball, the ball will bounce off randomly. However, if the cobblestones are smaller, the bouncing follows the laws of reflection. The angular model quantifies how much the cobblestones scatter the ping-pong-sized ball as roughness.

An interesting result was seen in the clear wings of hover flies. The surface roughness increases at off resonance wavelength (i.e. destructive interference) and decreases at resonance wavelength (i.e. constructive interference). Since off resonance is mainly due to the light probing the vein tube of the wing more than the wing membrane, it implies that clear-winged insects such as hover fly with many wing veins can be differentiated from those with less wing veins using this angular model. In the case of moths, the ventral side of the wings are rougher than the dorsal side and distinct surface roughness properties are observed among the examined species. This result has a practical implications for ground based entomological light detection and ranging (LiDAR) technology. The conclusion is that the angular model proposed by Li et al., 2022 holds true, and it has been used to show that surface roughness increases towards short wavelengths.

• author: Robertson, Emmanuel Kotu
• supervisor: Mikkel Brydegaard; MEng Li; Hampus Månefjord
• organization: Combustion Physics; Department of Physics
• course: PHYM03 20241
• year: 2024
• type: H2 - Master's Degree (Two Years)
• subject: Physics and Astronomy
• language: English
• id: 9161462
• date added to LUP: 2024-06-13 09:02:45
• date last changed: 2024-06-13 09:02:45