Luminescent system of Lucihormetica luckae supported by fluorescence lifetime imaging
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The presence of bioluminescent system in cockroaches (Zompro and Fritzche 1999) and in Lucihormetica luckae in particular has been contested in reply by Merritt (2013), based on the absence of direct evidence of bioluminescence in the original article (Vršanský et al. 2012).
Main counterarguments were based on (1) a single anecdotal reference to the presence of luminescence among cockroaches, (2) lack of the manifestation of the difference in fluorescence spectra recorded from lanterns and the rest of the body and (3) lack of luminescence among larvae in cultures.
The first and last objections in the comment of Merritt (2013) can be true as a single observation reports alight Lucihormetica verrucosa larvae, and, in culture, the adult apparently loses the capability of bioluminescence after some time period (M. Bartos, personal communication 2012). Our effort since the publication of our original article, when we have collected about a hundred of anecdotal records to observations of luminescence of diverse species of Lucihormetica spread throughout the northern half of South America, including two records from Ecuador and one record likely corresponding to a closely related species of L. luckae from Brazil (Curuça River, B. Pawlikowska, personal communication 2013), is also irrelevant because such massive evidence should result in a measured data of bioluminescence. Nevertheless, it is important to note that many of these observations were reported by trustworthy researchers and could not be ignored. Also, in the course of the present research, we discovered Lucihormetica with four luminescent lanterns—an addition to 107 species (40 indigenous) recently reported from continental Ecuador (Vidlička 2013a, b; unpublished) and also a brand new group of luminescent cockroaches.
Focus on plausibility of bioluminescence in L. luckae holotype thus lies on autofluorescence (AF) measurements. It is necessary to highlight that we agree with the author that, principally, there is no way to prove the bioluminescence in the species that might be extinct, except for finding a living alight individual.
Additional support comes from the sophisticated morphology of the lanterns (Fig. 1d; Vršanský et al. 2012, Fig. 1g, h, k, l), covered with thin, transparent, film-like reflector with ridges for light dispersion.
Based on the evidence gathered so far, we think that there exist enough arguments to conclude that alight cockroaches do exist and might represent a remarkable example of frequency-dependent evolution. Extreme rarity (eleven of 15 species are known based on a single individual) is much more significantly influencing their evolutionary success, than their fitness, to prevent the predators to check whether under the luminescent dots are not present edible cockroaches, instead of toxic click beetles.
We thank Vladimír Jánsky (Natural History Museum, Bratislava) for technical support. This work is supported by the United Nation Educational Scientific and Cultural Organisation project Amba (supporting grant of International Scientific and Technical Co-operation of the Slovak Academy of Sciences). This work was supported by the Slovak Research and Development Agency under the contract No. APVV-0436-12 and by Slovak Agency for the Structural Funds of EU under the contract ITMS:26240120018 (project NanoNet2).
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