DOI: 10.5061/dryad.v41ns1s5k
title: Data from: No evidence for the melanin desiccation hypothesis in a larval Lepidopteran

description: Water regulation is an important physiological challenge for insects due
 to their small body sizes and large surface area to volume ratios.
 Adaptations for decreasing cuticular water loss, the largest avenue of
 loss, are especially important. The melanin desiccation hypothesis states
 that melanin molecules in the cuticle may help prevent water loss, thus
 offering protection from desiccation. This hypothesis has much empirical
 support in Drosophila species, but remains mostly untested in other taxa,
 including Lepidoptera. Because melanin has many other important functions
 in insects, its potential role in desiccation prevention is not always
 clear. In this study we investigated the role of melanin in desiccation
 prevention in the white-lined Sphinx moth, Hyles lineata (Lepidoptera,
 Sphingidae), which shows high plasticity in the degree of melanin
 pigmentation during the late larval instars. We took advantage of this
 plasticity and used density treatments to induce a wide range of cuticular
 melanization; solitary conditions induced low melanin pigmentation while
 crowded conditions induced high melanin pigmentation. We tested whether
 more melanic larvae from the crowded treatment were better protected from
 desiccation in three relevant responses: i) total water loss over a
 desiccation period, ii) change in hemolymph osmolality over a desiccation
 period, and iii) evaporation rate of water through the cuticle. We did not
 find support for the melanin desiccation hypothesis in this species.
 Although treatment influenced total water loss, this effect did not occur
 via degree of melanization. Interestingly, this implies that crowding,
 which was used to induce high melanin phenotypes, may have other
 physiological effects that influence water regulation. There were no
 differences between treatments in cuticular evaporative water loss or
 change in hemolymph osmolality. However, we conclude that osmolality may
 not sufficiently reflect water loss in this case. This study emphasizes
 the context dependency of melanin’s role in desiccation prevention and the
 importance of considering how it may vary across taxa. In lepidopteran
 larvae that are constantly feeding phytophagous insects with soft
 cuticles, melanin may not be necessary for preventing cuticular water
 loss.
description: # No evidence for the melanin desiccation hypothesis in a larval
 Lepidopteran ## Sarah Britton and Goggy Davidowitz ### Description of data
 The data can be found in three files. ####
 "Desiccation\_Data\_F22.csv" This file contains the raw for the
 primary experiment that was conducted (where we measured total water loss
 and change in osmolality). Below is a description of the variables. * ID =
 Individual larval ID numver * Devo_Day = Day of development when
 measurement was taken for that individual (Day 0 = day egg was laid). All
 larvae were in 5th isntar. * Treatment = From crowded (C) or solitary (S)
 experimental treatment * Treatment_Binary = Converted to binary for path
 analysis. Crowded = 0, Solitary = 1 * Day in = Date of measurements *
 Chamber_Humidity = humidity range in chamber * Avg_Temp = avereage
 temperature in chamber in degrees C * before_mass = larval mass before
 going in chamber (grams) * before_mass_mg = mass in mg * SA = surface area
 in mm^3 * after_mass = larval mass after desiccation in chamber (grams) *
 frass_mass = mass of frass (if any) found in cup (grams) * delta_mass =
 after mass - initial mass * percent_mass_change = (after mass - initial
 mass)/ initial mass * before_osmo = osmolality before going into chamber *
 after_osmo = osmolality after desiccaton chamber * delta_osmo = after -
 before osmolality Note that only a subset of individuals was used for
 osmolality measurements so the osmolality columns (before_osmo*,
 *after_osmo, and delta_osmo) will have empty cells. Osmolality
 measurements were taken for a subset of individuals so some individuals do
 not have values in the osmolality columns. ####
 "Desiccation\_Photo\_Data\_F23" This file contains the ImageJ
 data used to quantify melanin. Variables with "gray" represent
 gray values while variables with "percent" represent percentage
 of melanic pixels. R, G, and B refer to the red, green, and blue channels
 of the multispectral image. While all data are included here, note that we
 only used date from the green channel. Thus, the two variables of interest
 are "darkness_G" and "percent_G." Darkness was
 calculated as 29-gray value (see text for explanation) ####
 "Vapometer\_Data\_S24" This file contains data from the second
 experiment where we measured evaporative water loss across the cuticle.
 Below is a description of the variables. * larval_ID = Individual larval
 ID number * Treatment = From crowded or solitary experimental treatment *
 Treatment_Binary = Converted to binary for path analysis. Crowded = 0,
 Solitary = 1 * date = Date of measurements * ambient_temp = temperature in
 room during measurements in degrees C * ambient_humidity = humidity in
 room during measurements * mass = larval mass in grams * body_temp =
 larval body temperature in degrees C * reading_1...reading_8 = replicate
 readings from vapometer (g/m^2/hour) * avg_reading = average of all
 vapometer readings (g/m^2/hour) * readings_stdev = stadard deviation of
 readings (g/m^2/hour) * range = range of readings (g/m^2/hour) * coef_var
 = coefficient of variation for readings The ImageJ data for this dataset
 can be described the same way as above. Note that most individuals had 7
 vapometer readings, but a few had 6 or 8 readings. Thus, there are some
 empty cells. The average reading was taken for all available readings.  
 ### Description of Code All statistical analyses were conducted in R. The
 R file "Hyles_desiccation" can be used to replicate all analyses
 with the above data files.