Abstract：
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.
摘要：
由于昆虫的小体型和大表面积与体积之比，水分调节是昆虫的重要生理挑战。适应减少角质层水分流失，最大的损失途径,尤其重要。黑色素干燥假说指出，角质层中的黑色素分子可能有助于防止水分流失，从而提供保护免受干燥。这个假设在果蝇物种中有很多经验支持，但在其他类群中大部分仍未测试，包括鳞翅目.因为黑色素在昆虫中具有许多其他重要功能，它在防止干燥中的潜在作用并不总是很清楚。在这项研究中，我们调查了黑色素在白纹狮身人面像蛾的干燥预防中的作用，Hyleslineata（鳞翅目，Sphingidae),在幼虫后期的黑色素色素沉着程度上显示出很高的可塑性。我们利用了这种可塑性，并使用了密度处理来诱导广泛的角质层黑色素化；孤立的条件会导致黑色素色素沉着低，而拥挤的条件会导致黑色素色素沉着高。我们在三个相关响应中测试了拥挤处理中的更多黑色素幼虫是否更好地防止干燥：i）干燥期间的总失水，ii)在干燥期间血淋巴渗透压的变化，和iii)水通过角质层的蒸发速率。在该物种中，我们没有发现黑色素干燥假说的支持。虽然处理影响了总失水，这种作用并不是通过黑化程度发生的。有趣的是,这意味着拥挤，用于诱导高黑色素表型，可能有影响水调节的其他生理效应。在表皮蒸发失水或血淋巴渗透压变化方面，治疗之间没有差异。然而,我们得出的结论是，在这种情况下，渗透压可能不足以反映水分流失。这项研究强调了黑色素在干燥预防中的作用的背景依赖性以及考虑其在分类单元中的变化的重要性。在鳞翅目幼虫中，它们不断喂养具有软质角质层的植食性昆虫，黑色素可能不是防止角质层水分流失所必需的。
