Date Published: January 26, 2017
Publisher: Public Library of Science
Author(s): Jiří Skuhrovec, Robert Stejskal, Filip Trnka, Andrea di Giulio, Wolfgang Arthofer.
The last instar larva and pupa of Eucoeliodes mirabilis (A. Villa & G. B. Villa, 1835) (Curculionidae: Ceutorhynchini) are described using drawings and SEM images and are compared and keyed with already described larvae of 58 other ceutorhynchinae taxa. The larval body has an effective combination of morphological adaptations that assist a unique biological defensive strategy. All larval stages of E. mirabilis feed ectophytically on leaves of Euonymus europaeus L. (Celastraceae), and the larval body is covered with a thick faecal shield. The fixation of this protective shield on the larval back is performed by a peculiar dorsal microsculpture composed of a dense carpet of microtrichia on the thorax and abdomen, which serves effectively as a velcro system. Because of this strategy, macrosetae on the larval and pupal body of E. mirabilis are completely reduced. Larvae of E. mirabilis also have distinct morphological adaptations for protecting the spiracles against intrusion of faeces and avoiding occlusion of the tracheal system: a) microtrichia around spiracles are slightly shorter, distinctly stronger and are arranged with high-density and in clusters and b) spiracles are protected by an external safety valve. This strategy of E. mirabilis larvae is unique, although somewhat similar to that of Criocerinae and Blepharida-group leave beetles (Galerucinae) (both Coleoptera: Chrysomelidae), but with distinctly different morphological adaptations.
The endless adaptations and counter-adaptations among three trophic levels (plants, herbivores and their enemies) are responsible for the amazing variety of defensive strategies recorded for both plants and their herbivorous insects . To avoid the attacks of predators and parasitoids, herbivorous insects have developed many types of defensive strategies that vary from avoiding detection using visual camouflage to deceiving enemies by mimicking unpalatable species . One of the most astonishing defensive strategies may be the faecal ecology of insects, which has received some attention . Immature stages of five clades of leaf beetles, Cassidinae, Criocerinae, Cryptocephalinae, Lamprosomatinae and Galerucinae, use faeces as a material for building domiciles and protective shields/coats against their enemies [4–20]. The shields provide a mechanical defence and may also include chemicals that originated from the host plant or derivatives of these compounds produced by the larvae [21–23].
Our observations confirm that the larvae of E. mirabilis feed exclusively ectophytically on leaves. The appearance of E. mirabilis larvae on leaves is remarkable, with the protective faecal shield covering the entire body. Although most similar to the strategy adapted by the Criocerinae, the defensive shield as a strategy is unique in E. mirabilis, and the larvae have clearly different morphological adaptations. Macrosetae on larval and also pupal bodies of E. mirabilis are completely reduced, and the vesture is covered with a high density of microtrichia, which serve as velcro. Because of the high density of microtrichia on the dorsal vesture, the faecal shield is most likely removed only during moulting. As a system to protect spiracles against incursion of faeces into the tracheal system, the density of microtrichia around all spiracles is high, and the microtrichia are slightly shorter, distinctly stronger, and arranged in clusters. All spiracles are also protected by a safety valve, which decreases the space in which faeces could enter. All these mentioned morphological adaptations are unique and used in combination to use faeces as a defensive strategy in E. mirabilis. Finally, the generic keys for identification of larvae and pupae based on useful preliminary descriptions is also presented. All these new data about biology and also morphology of E. mirabilis are very useful in next studies of immature stages in Polyphaga beetles, but there are also still some missing gaps (e.g. chemical composition of host plant tissues, and feces; description of shield construction; morphology of younger instars, etc.).