Bioluminescent Insects

Introduction

Bioluminescence is an example of chemiluminescence or light emission by chemical reaction in a living organism. ‘Bioluminescence’ is derived from Greek- ‘bio’ means living and ‘lumen’ means ‘light’. It is an enzyme-catalysed reaction, a process of conversion of light energy into chemical energy.

The Greeks and Romans were the first to discover glowing organisms and sea phosphorescence. Aristotle described them as ‘cold light’. The first book on Bioluminescence was published by Conrad Gesner in 1555 and later in the nineteenth century, Raphael Dubois extracted two key components- ‘luciferin’ and ‘luciferase’ of this biochemical reaction. Luciferin (from Latin of Lucifer- ‘light bringer’) are a group of heterocyclic compounds which is oxidized by luciferase to produce oxyluciferin and energy in the form of light.

Process of Bioluminescence

Light is produced from a specially evolved photogenic organ, mostly derived from fat body, but in some Diptera from the distal ends of Malpighian tubule. These organs are usually found in both sexes, or sometimes restricted to females and immature stages, seen in fireflies. They can be distributed anywhere in the body, but in adults found on the ventral side of thorax or abdomen.

Each light organ is made up of photocytes which remain scattered beneath the epidermis and covered by dorsal layer cells. The cuticle is transparent in texture. Photocytes are cylindrical, organized at right angles to the cuticle. Each cylinder consists of trachea and nerves. These tracheae break off into branches of tracheoles which innervate between the photocytes. Root of tracheoles remains enclosed within a tracheal end cell, whose inner membrane forms a folding with tracheoblasts. The nerves of photocytes terminate into an undivided process at the junction of plasma membrane of the tracheal end cell and tracheoblasts. Each photocyte cell consists of many photocyte granules, which form a cavity connecting with the cell cytoplasm via its neck.

Photocyte granules contain luciferin molecules, which are involved in the light reaction. The dorsal layer cells also have granules, called urate granules, which form a reflecting layer.

The precursors bioluminescence is -

• Luciferin, a low molecular weight compound secreted by photocyte cells, can be an aldehyde, a complex polypeptide or a protein.

• Luciferase, the enzyme for reaction.

• Luciferin binding proteins.

• Photoprotein

• Scintillon- ATP and Mg2+.

Luciferin is activated by ATP in the presence of Mg2+ and luciferase to produce adenyl luciferase. This compound is oxidized in the presence of luciferase to form adenyloxyluciferin in the excited state. Adenyloxyluciferin is converted to low energy compound by producing light. The energy for this reaction is provided by oxidation, not from ATP, released in the form of light energy.

Low-energy adenyloxyluciferin inhibits further reaction by binding to luciferase, but pyrophosphate help in removing this inhibition. The photogenic organ is stimulated by nerves, acetylcholine is released at the nerve ending, which reacts with ATP and co- enzyme A to produce pyrophosphate.

The reaction depends totally on oxygen supply, so controlling oxygen supply can control the light reaction. The emitted biological light does not produce heat, hence referred to as cold light. Intensity of the light is low and it is free from ultraviolet radiation.

Examples

Natural instances of true or self-luminescence are found mostly in insect orders of Collembola, Diptera, Coleoptera, and Homoptera. Order Coleoptera is the largest bioluminescent group, best examples shown by families Lampyridae, Elateridae and Phengodidae.

• Lampyridae is known as ‘fireflies’ or ‘lightening bugs’.

• Elateridae known as ‘wireworms’ or ‘click beetles’, Pyrophorus noctilucus-the brightest insect discovered, also called ‘Jamaican click beetle’ or ‘cucujo beetle’ in West Indies. Phengodidae is known as ‘railroad worms’.

• In Collembola bioluminescence is seen in Onychiurus armatus, in Homoptera family Fulgoridae or ‘lantern fly’ like Fulgora lanternaria, in Diptera Mycetphilidae like Arachnocampa, their larvae build webs on the roof of caves, Keroplatus, and Orfelia, larvae of families Platyuridae and Bolitophilidae.

Role of Bioluminescence in Animals

Bioluminescence undoubtedly has a functional significance in arthropods, mostly it is used to attract the opposite sex for mating or may be used for luring its prey, or as a defence mechanism.

As a mating signal

Glowing light is known to act as a mating signal in fireflies. In Lampyridae, females are wingless and sessile, therefore light production becomes important for them to attract males. Light flash patterns vary between species and sexes. In a cool pleasant night, some species emit flashes after every five second, while others may wait for only one second to emit flashes, some tropical species can emit flashes in unison.

Fireflies of Photuris pyralis emit single short flashes at regular intervals, usually from males for mating, about fifty males per female. Females climb through the grass blades following the flashes within ten to twelve feet. They exchange signals five to ten times before mating.

As a predator

Most unique example of using light as a predation is found in glowworm fly of New Zealand- Arachnocampa luminosa. The females of these insects deposit their eggs on the ceilings of dark caves, the eggs on hatching produce larvae which hang down from cave walls by a sticky thread and produce light.

In the dark of the night, the cave glows bright and attracts many insects to these hanging larvae, which are preyed upon by them. These caves also called ‘luminous caves’ are popular as tourist spots in New Zealand.

As a defence

Railroad worms glow their heads continuously when the larvae walk, these sudden flashes help to frighten the potential predators.

Economic Importance of Bioluminescence

The luciferase-light producing system has been found to be useful for bio-medical research, pest management and different scientific experiments.

• Presence of ATP in living organisms have been utilized for various medical research. Injection of normal and cancerous cells with luciferin and luciferase shows different results, help in detecting metabolic energy defects in human cells. The technique has been of great use for treatment of heart diseases, muscular dystrophy, and urological ailments.

• Recently, genetic material of pink bollworm, an insect pest of cotton, has been modified by injecting green fluorescent protein (GFP) of jellyfish, Aequora victoria. The transgenic insect will start to glow from the larval stage which helps to identify them in the agricultural fields.

• Spacecrafts sent to other planets are equipped with a special electronic device to pick up alien soil and made to react with oxygen, water, luciferin, and luciferase. If the glow is televised back to Earth, it would be evidence of presence of ATP and existence of life on that planet.

• A fluorescent marker gene made up of a short DNA sequence is inserted into the gene of interest and transformed cells would help scientists to distinguish target cells and eliminate others.

FAQs

Qns 1. Where is bioluminescence found?

Ans. They are found in forestlands, dark caves, and oceans.

Qns 2. Does bioluminescence need light?

Ans. It does not require light from external source, produced inside the organism.

Qns 3. Can bioluminescence occur during the day?

Ans. It is observed only at night.

Qns 4. Does bioluminescence produce heat?

Ans. No, it is cold light