Department of Biology, University of Central Arkansas, Conway, Arkansas, United States of America.
Department of Biological and Agricultural Engineering, University of Arkansas, Fayetteville, Arkansas, United States of America.
PLoS One. 2019 Jul 10;14(7):e0218994. doi: 10.1371/journal.pone.0218994. eCollection 2019.
Unlike most notodontids, Theroa zethus larvae feed on plants that emit copious latex when damaged. To determine how the larvae overcome this defense, we filmed final instars on poinsettia, Euphorbia pulcherrima, then simulated their behaviors and tested how the behaviors individually and combined affect latex exudation. Larvae initially scraped the stem, petiole, or midrib with their mandibles, then secreted acid from their ventral eversible gland (VEG) onto the abraded surface. Scraping facilitated acid penetration by disrupting the waxy cuticle. As the acid softened tissues, the larvae used their mandibles to compress the plant repeatedly, thereby rupturing the latex canals. Scraping, acid application, and compression created withered furrows that greatly diminished latex exudation distal to the furrows where the larvae invariably fed. The VEG in notodontids ordinarily serves to deter predators; when attacked, larvae spray acid aimed directly at the assailant. Using HPLC, we documented that the VEG secretion of T. zethus contains 30% formic acid (6.53M) with small amounts of butyric acid (0.05M). When applied to poinsettia petioles, the acids caused a similar reduction in latex outflow as VEG secretion milked from larvae. VEG acid could disrupt latex canals in part by stimulating the normal acid-growth mechanism employed by plants to loosen walls for cell elongation. Histological examination of cross sections in poinsettia midribs confirmed that cell walls within furrows were often highly distorted as expected if VEG acids weaken walls. Theroa zethus is the only notodontid caterpillar known to use mandibular scraping and VEG acid to disable plant defenses. However, we document that mandibular constriction of petioles occurs also in other notodontids including species that feed on hardwood trees. This capability may represent a pre-adaptation that facilitated the host shift in the Theroa lineage onto latex-bearing plants by enabling larvae to deactivate laticifers with minimal latex contact.
与大多数 Notodontidae 不同，Theroa zethus 幼虫以受损时会分泌大量乳液的植物为食。为了确定幼虫如何克服这种防御机制，我们对一品红（Euphorbia pulcherrima）上的末龄幼虫进行了拍摄，然后模拟它们的行为，并测试了这些行为单独和组合时如何影响乳液的渗出。幼虫最初用下颚刮擦茎、叶柄或中脉，然后用腹侧可外翻的腺体（VEG）将酸分泌到磨损的表面上。刮擦通过破坏蜡质表皮来促进酸的渗透。随着酸软化组织，幼虫用下颚反复压缩植物，从而破坏乳胶管。刮擦、酸的应用和压缩会产生干枯的皱纹，大大减少了皱纹下游离乳管的乳液渗出，而幼虫总是在这些皱纹处进食。Notodontidae 中的 VEG 通常用于吓阻捕食者；当受到攻击时，幼虫会将酸喷向攻击者。我们使用 HPLC 记录到，T. zethus 的 VEG 分泌物中含有 30%的甲酸（6.53M）和少量的丁酸（0.05M）。将这些酸应用于一品红叶柄上，会导致与从幼虫中挤出的 VEG 分泌物相似的乳液流出减少。VEG 酸可能会通过刺激植物正常的酸生长机制来破坏乳胶管，这种机制用于植物为细胞伸长而松动细胞壁。对一品红中脉横切面的组织学检查证实，如预期的那样，在皱纹内细胞壁经常高度扭曲，如果 VEG 酸削弱了细胞壁。Theroa zethus 是已知唯一一种使用下颚刮擦和 VEG 酸来破坏植物防御的 Notodontidae 毛毛虫。然而，我们记录到，其他 Notodontidae 物种包括以硬木为食的物种，也会在叶柄上发生下颚收缩。这种能力可能代表了一种预先适应，通过使幼虫在与乳胶接触最小化的情况下使乳管失活，使幼虫能够在 Ther oa 谱系中转移到分泌乳胶的植物上。
