Glow-worms and fireflies despite their diversity in morphology and behaviour have many shared aspects of their biology. A brief overview of the principle components of their biology are presented below. It is hoped that detailed information on individual species will be created in the future therby providing an online guide to bioluminescent beetle taxa.

LIFE CYCLE AND PHYSIOLOGY

A few days after mating, a female lays her fertilized eggs on or just below the surface of the ground or at the base of grasses. The eggs are usually smooth and spherical and are often pale in colour but can also be luminescent with the incubation period ranging from 13 to 27 days. Some larvae, such as Photinus species, spend the majority of time underground. Other species such as Lampyris and Photuris are terrestrial and some larvae are semi aquatic (Luciola). All lampyrid larvae are considered predaceous; Lampyris noctiluca feeds on snails, Phosphaenus hemipterus feeds on earthworms, Rhagopthalmus predates millipedes and Pterotus species mainly eat slugs. Larvae undergo four to five molts and usually take from a few months to two years to reach pupation. The pupae have free appendages that are not glued to the body (exarate) and are generally white with some yellow or pink pigmentation. Most pupae are luminescent when disturbed and the duration of pupation can be from one to three or more weeks.

Adult fireflies tend to be brown and soft-bodied, often with the elytra (modified, hardened forewings protecting the hindwing) more leathery than in other beetles. Though the females of some lampyrid species are similar in appearance to males, some are short-winged (brachypterous), wingless (apterous) or even larviform (Lampyris noctiluca). These females can often only be distinguished from the larvae by the presence of compound eyes or more developed lanterns.

The adult fireflies during their brief life seek out members of the opposite sex by using luminescemce as a form of sexual communication. The light emission can range from steady glows through to complex pulse and flash systems. These have been studied by biologists for many years and are often found to be species specific and have been used in Japan as a form of classification forming the basis of evolutionary studies.

COMMUNICATION IN THE LAMPYRIDAE

The function of bioluminescence in the adult beetle is primarily to locate other individuals of the same species for mating. As early as 1884 the use of bioluminescence in beetles was identified as a form of sexual communication (Emery, 1886). Sexual communication in the Lampyridae can be roughly divided into three groups.

The first group contain species that do not employ luminescence as a form of sexual attraction but rely on pheromones to locate mates. These species usually exhibit diurnal behaviour such as those in the genus Ellychnia and Lucidota; it has been suggested that Lucidota atra was evolutionarily prompted to become diurnal by the opportunity to avoid being hunted by other fireflies (Gronquist et al. 2006).

The second group contains species where females elicit a continuous signal, a glow, which is recognised by a flying male which then approaches. This was described as signal system I by Lloyd (1971). These species include the genera Lampyris and Pyrocoelia amoung others.

In the third group, described by Lloyd (1971) as signal system II, a flying male emits a pulsating signal, a flash, a female recognises this and responds to him with a signal, and then, the male recognises her signal and approaches her. In this system both the flash specific interval of the male and the timing of the female’s flash response are species specific. These systems of communication were expanded upon by Ohba who studied the communication systems of 39 Japanese firefly species and divided nocturnal fireflies into four types. These were later increased in number to six types based on morphological and behavioural studies (Ohba, 1983). These six classifications are summarised here. It is evident that these groups only cover Japanese species and a number of other communication systems exist such as the mimicking Photuris species. In this additional group target males are attracted to what appears to be a suitable mate, and are then eaten. For this reason the Photuris female is sometimes referred to as the "femme fatale".

LANTERN MORPHOLOGY

Bioluminescence is localized to particular regions of the fireflies body referred to as the lanterns. Externally lanterns can be identified as white regions primarily found in the terminal abdominal segments and are composed of specialized cells known as photocytes. The lantern can vary in gross external morphology as well as internal physiology (For a detailed review see Buck, 1948).

The physiology of the lantern can very dramatically between lampyrid species, but are, in general, localised masses of photocytes of one or more with specific tracheal and nervous supplies.

The simplest form of structure is found is some of the female light organs of Lamporhiza splendidula as well as the larvae of Phausis delarouzeei whereby the organs are compact masses of small transparent polyhedral photocytes with a specific tracheal supply which progressively tapers becoming more branched and forming a root-system through the photogenic tissue.

A more complex form of light organ includes an additional layer of one to four cells on the inner surface of the photogenic tissue. This layer is known as the “reflective layer” and is composed of white opaque cells that are dramatically different to the photocytes. This type of organ has been found in the larval lanterns of Photuris pennsylvanica, Luciola cruciata, Pyrocoelia species, Lamprohiza splendidula and Phosphaenus hemipterus. This type of lantern was also found in the adult female glow-worms as well as the larvae of Lampyris noctiluca.

Other lanterns differ from the former in that they include “tracheal end cells”. This additional complexity has been studied in most detail in the genus Photuris. The Photuris lantern is composed of a flat slab of tissue with the dorsal layer composed of reflector cells. The ventral, or photogenic layer is where the light originates. The photocytes are arranged in 'rosettes' around channels or 'cylinders', each of which contains branches of both the tracheal system and the lantern nerve.

The photocytes contain the principle components of bioluminescence, the enzyme luciferase and the substrate luciferin. These specialised cells are also packed full of mitochondria which are subcellular organelles which provide the ATP for the bioluminescent reaction.

References

Buck JB (1948) The anatomy and physiology of the light organ in fireflies. Annals New York Academy of Sciences 49, 397–483.
Gronquist M, Schroeder F, Ghiradella H, et al. (2006) Shunning the night to elude the hunter: diurnal fireflies and the “femmes fatales”. Chemoecology 16, 39-43.
Lloyd JE (1971) Bioluminescent communication in insects. Annual Review of Entomology 16, 97-112. Ohba N (1983) Studies on the communication system of Japanese fireflies. Scientific Report of the Yokosuka City Museum 3, 1-62.