Structure and function of the eversible glands of the aquatic firefly Luciola leii (Coleoptera: Lampyridae)

Abstract

When attacked or otherwise disturbed, larvae of the aquatic firefly Luciola leii display fork-shaped glands laterally from the meso-and metathorax as well as each abdominal segment. Glandular eversion is accompanied by a strong pine oil-like odour, thanatosis and glowing from paired larval light organs. Observations by SEM and TEM revealed that there are numerous, almost spherical protuberances, measuring 9 µm in diameter on the surface of each gland. Each protuberance is connected to a well developed secretory cell via a 0.1 µm thick and 0.2 µm long stalk and bears three to six 7 µm long spines on its apex. The convoluted glandular walls measure 0.2 µm. The cytoplasm of the secretory cells is characterized by the presence of numerous mitochondria and an extensive system of cisternae and tubular endoplasmic reticulum. Preliminary GC-MS analysis of the glandular secretion revealed two volatile terpenoids: terpinolene and γ-terpinene. Choice and no-choice bioassays involving fish and ants as well as other predators demonstrate that the secretions serve as an effective deterrent against a range of ecologically relevant enemies. The larval postural adjustments, light emission, everted glands, and glandular release of distasteful or repellent compounds, appear to function as a multi-modal, aposematic larval defence. The tandem evolution of glandular secretions and conspicuousness in larval fireflies could partially explain their successful radiation into both aquatic and terrestrial habitats.

Structure and function of the eversible organs of several genera of larval firefly (Coleoptera: Lampyridae)

“…This is consistent with earlier studies (Lloyd 1973;Blum and Sannasi 1974;Sivinski 1981;Ohba and Hidaka 2002), in which predators including raccoon dog (Nyctereutes procyonoides), bat (Myotis macrodactylus), dragonfly (Anotogaster sieboldii) and goby (Rhinogobius brunneus) exhibited avoidance responses to a range of Luciola and Pyrocoelia larvae. This defence mechanism is likely to be particularly effective against predators which hunt using predominantly olfactory cues, such as ants and ground beetles (Carabidae) (Fu et al 2007) lizards and some amphibians (De Cock 2004) and Insectivora (e.g. mice, see Underwood et al 1997) and would complement the colouration and larval flashing that, at least in L. noctiluca, have been shown to act as aposematic cues to visually guided diurnal and nocturnal predators (De Cock and Matthysen 1999.…”

“…Each larva was attached with double-sided sticky tape to an aluminium stub and sputter-coated with gold to a thickness of about 200 nm. The specimens were observed under a JSM-6390 LV SEM at an accelerating voltage of 25 kV (Fu et al 2006a(Fu et al , 2007Mora et al 2001).…”

“…After postfixation in 2% cold osmium tetroxide in the same buffer, the organs were dehydrated through a graded acetone series and embedded in araldite. Semi-thin sections for light microscopy were stained with Delafield's haematoxylin and eosin; ultra-thin sections for TEM, double-stained with uranyl acetate and lead citrate for a few minutes each, were examined in a Hitachi H-7650 TEM microscope at 75 kV accelerating voltage following Fu et al (2007) and MeyerRochow and Liddle (1988).…”

“…The controlled group consisted of 20 fourth instar larvae kept in Petri dish with water and without any form of stimulation. The elicited behaviours were recorded on video under red light (Krall et al 1999;Fu et al 2007). To observe the responses elicited by threats on terrestrial larvae of Pyrocoelia, Diaphanes and L. noctiluca, we placed 20 larvae in Petri dishes and tested them using the same methods described above.…”

“…However, the studies on these eversible organs were not complete. Fu et al (2007) studied the external morphology and function of the eversible organs of the aquatic firefly L. leii in detail. Observations by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) revealed that there are numerous, almost spherical protuberances on the surface of each organ.…”