This is an essay question example answer from the Sensory Ecology (31132) module within the University of Bristols BSc Zoology. This answer was graded a 68 (high 2:1). The essay revolves around insects and their visual systems in relation to their natural environment, with the question; 'While stu...
QUESTION: While studying two different species of butterfly, you discover that one
possesses a homogeneous trichromatic visual system while the other is tetrachromatic,
with extensive intraretinal variation and sexual dichromatism. How can such differences be
explained when both species live in the same light environment?
Introduction
Butterfly compound eyes contain hundreds or thousands of ommatidial units, each topped with a
facet lens (van der Kooi et al., 2021). Each ommatidia holds nine rhabdomeric photoreceptor cells,
with their microvillous projections (rhabdomeres) fusing to form a central rhabdom (Frentiu and
Briscoe, 2008). The visual pigments, found within the rhabdomeres, are formed via a light-sensitive
11-cis-3-hydroxyretinal chromophore covalently binding to an opsin protein; opsins are seven
transmembrane-domain G protein-coupled receptors (Sondhi et al., 2021). The absorbance
spectrum of each visual pigment is based on the interaction between the chromophore and the
amino acid sequences of the opsin, inducing a confirmational change in the opsin and initiating a
biochemical cascade that sends neural signals to the brain (Frentiu and Briscoe, 2008).
Distinguishing wavelengths of light for colour perception requires at least two classes of
photoreceptors with distinct spectral sensitivities (Sondhi et al., 2021). Most insects have three
independent colour channels (Briscoe et al., 2008); peak sensitivities occur in the ultraviolet (UV,
300-400nm), blue (B, 400-500nm) and long wavelength (LW/green, 500-600nm) parts of the visible
light spectrum (Stavenga and Arikawa, 2006). Although, some species express up to five different
opsins within the retina and extend their spectral sensitivities even further via use of filtering
pigments (Kelber, 2016). Graphium Sarpedon has 15 different spectral sensitivities across 6
photoreceptor classes (Chen et al., 2016).
Butterflies use visual cues for a variety of tasks, including foraging, oviposition site location, and
conspecific identification (Briscoe et al., 2003). Any one of these tasks may apply a selective
pressure on colour vision to become better adapted to a specific sensory niche. In this essay,
various explanations for the differing visual systems of these butterflies residing in the same light
environment will be considered.
Tri vs tetrachromacy
Trichromatic butterflies have three independent channels of colour, whereas tetrachromats have
four (Briscoe et al., 2003). Copious studies identified blue as the preferred floral colour for most
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, butterflies (Tang et al., 2012); this may be true for the trichromat as they possess the required
receptors (UV and B for floral patterns, G to distinguish leaves). Conversely, the tetrachromat may
have a preference for red flowers, and has experienced an opsin gene duplication event to gain
red-sensitivity. Duplications of B and LW opsins have occurred independently across different
families (Frentiu and Briscoe, 2008), with many using a duplicated LW opsin to see in red up to
780nm (Sondhi et al., 2021). As most photosynthesising leaves have a peak reflection ~540nm,
red-sensitivity helps butterflies to distinguish red flowers from leafy backgrounds (Kelber, 1999).
Different butterfly species place priority on different sensory cues when foraging (Tang et al.,
2012). Flower foragers, like Vanessa indica and perhaps the tetrachromat, place greater
importance on visual cues (Ômura and Honda, 2005). Whereas tree sap foragers, like Kaniska
canace and likely the trichromat, prioritise olfaction as the minimal visual cues are often insufficient
for locating food (Omura et al., 2000). Species with higher reliance on visual cues are likely to
experience greater selective pressures on their visual systems; this may explain some differences
between these species.
Most adult female butterflies lay eggs on leaves to ensure offspring have immediate food sources
upon hatching. Species differ in both selectivity of oviposition host-plant sites and location methods
(Wiklund, 1984). Research suggests additional red-sensitive visual pigments allow better
discrimination of suitable leaves for ovipositing (McCulloch et al., 2016). Young green leaves may
be distinguished from older less nutritious yellow leaves through opponent red-green channels,
giving information about the chlorophyll:red pigment ratio (Kelber, 1999). The tetrachromat may be
selective over its hostplants, utilising a red receptor to select the youngest leaves to boost offspring
survival and fitness (van der Kooi et al., 2021). The trichromat may be less selective of oviposition
sites, requiring a lesser degree of colour discrimination; Melanargia galathea drop eggs randomly
over tall grasses (Wiklund, 1984). Or perhaps chemical cues are more heavily utilised; before
laying, Papilionidae drum their forelegs on leaf surfaces to detect specific chemicals with their
chemosensilla (Osaki et al, 2011).
Intraretinal variation
All butterflies possess three types of ommatidia, all of which contain LW-sensitive receptors but
differ in their ratio of B and UV- sensitive receptors (Kelber, 2016). Ommatidia types are
heterogeneously distributed across the retina, with areas of non-random regionalisation in many
species. As lighting is not uniform, regionalisation of certain visual pigments across the retina may
occur to match the spectral sensitivities in that field of view with the visual task (Temple, 2011). For
example, most have a dorsal rim area containing UV-sensitive receptors specifically aligned to
detect polarised skylight for use in navigation (Stavenga and Arikawa, 2006). The trichromat will
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