Pheromones

Years ago one early morning my (KKVs) elder daughter, then a child, approaching my bed, asked me to get up and see a large moth resting on the mosquito net. Hazy appearance of the insect, as seen across the net, suggested that it was a saturniid moth. Sleepily I asked the child to see if the feelers of the moth had long side branches with a comb like arrangement on either side. She said she could see such side branches, but they were not quite long. I prophesied that soon there would be another moth with long side branches on its feelers. In a short while the foretelling turned out to be correct. Female saturniid moths have short side branches on their antennae, while males have long ones. Females of some moths are known to produce a sex attractant, which is a very volatile chemical and which attracts males from long distances. The prophecy was based on this information. Such chemical messengers, produced and released by an individual into the environment and producing an effect in conspecific individuals (that is individuals belonging to the same species) even in very low concentrations, have been called pheromones. Such observations were already made in 1870 by the French naturalist, Jean-Henri Fabre with Saturnia pyri, the great peacock moth. Forty male moths arrived at Fabre's home, the Serignan harmas, "eager to pay their respects to their marriageable bride born that morning" (Fabre, 1989, reissue). After that it took 90 years to identify the chemicals involved.

"Pheromone" is quite a meaningful term. It is akin to "hormone". They are chemical messengers that induce a behavioural reaction or developmental effects among individuals of same species (Cardé and Millar, in Resh and Cardé, 2003). In fact pheromones have also been called ectohormones and they are also referred to as semiochemicals or chemicals that are involved in communication. External secretions by an insect, which produce such changes/effects in individuals of other species, have been called kairomones or allomones. As E. O. Wilson (1963) has said, ".hormones are.. .secreted internally to regulate the organism's own physiology, or internal environment, pheromones are secreted externally and help to regulate the organism's external environment by influencing other animals." The regulation of the external environment is brought about by influencing and modifying the behaviour and/or development of other individuals of a conspecific group or population.

Bossart and Wilson have worked on the sex pheromone of the female gypsy moth. Taking into account diffusion properties of the pheromone, they have made mathematical models, from which they have inferred that, when a mild wind is blowing, the space, with effective concentration of the sex pheromone, will be ellipsoidal, with length of several thousand metres, and horizontal width, where it is the broadest, of about 200 metres (Wilson, 1963). Butenandt has reported that the female sex pheromone of the emperor moth may attract a male from a distance of eleven kilometers (Tembhare, 1984). On perceiving an effective concentration of the female pheromone, the male flies upwind to reach the pheromone source.

In addition to the gypsy moth (Porthetria dispar), the female sex pheromone has been extensively studied in the silk worm (Bombyx mori) and the American cockroach (Periplaneta americana). Bombyx mori pheromone was isolated in 1959 and was the first to be isolated.

In some insects male sex pheromones are known, for example in the butterfly Lycorea ceres (Danaiinae) and in the giant water bug Belostoma indica (Tembhare, 1984). Male moths can become, in some species, attractive to the opposite sex and may succeed in recruiting females through their pheromone. Some male butterflies have specialized paired brushes at the tip of the abdomen, disseminating an aphrodisiac scent from them. The scent of Danaiinae has an odor that ranges from sweet to rancid. Once in Nicaragua, PJ captured a danaiine butterfly (Lycorea cleobaea) with a tiny galerucine (Monolepta sp.), attached to its hairbrushes. Probably, the beetle was attracted by the scent and remained there during the flight of the butterfly (Jolivet and Maes, 1995). Thus in this case the male pheromone of the butterfly behaved also as a kairomone. However, butterflies rely more on visual clues for mate finding. Bark beetles (Scolytidae) use pheromones to colonize host trees and to attract mates.

Another well known example of a pheromone, inducing a certain behavioural change in conspecific individuals around, is the trail marker of ants. It is a common observation that ants are seen moving in a train like row to a feeding source. The trail is formed through guidance offered by a pheromone, the trail marker. The scout ant, going out in search of food, may locate a good source of nourishment. Then during return to its colony or nest she protrudes her functionless egg laying shaft or "sting", and with its help marks a line with the trail marker. The trail marker is a secretion of the Dufour's gland, associated with her sting apparatus. The workers of the colony, intending to collect food, move along the line of the trail marker to reach the feeding source. If the Dufour's gland of an ant is dissected out, crushed, the released contents are taken on a needle tip and the needle tip is used to draw a line on a surface, the ants from the conspecific colony nearby will be seen moving along the line.

The trail markers of ants are highly species specific, that is the pheromone of a particular species will not elicit trail making response in ants of another species. Another notable feature of the trail marker of ants is that its effect is short lived. In some seconds the pheromone evaporates away.

— Fig. 34.1. Diagram to show length and horizontal spread of the space with effective concentration of the female sex pheromone of the gypsy moth, when a mild wind is blowing. Not drawn to scale (Based on a diagram by Wilson, 1963).

— Fig. 34.1. Diagram to show length and horizontal spread of the space with effective concentration of the female sex pheromone of the gypsy moth, when a mild wind is blowing. Not drawn to scale (Based on a diagram by Wilson, 1963).

The advantage of this is obvious. If the trails are not ephemeral, there will be a confusing maze of trails around an ant colony. If a "train" of ants continues to move along a certain trail for quite some time, it means the feeding source is rich, and the returning ants are repeatedly applying trail marker along the path. Cycloalexic larvae (see chapter on Round defense) make their round formation with the help of an aggregation pheromone. Procession forming saturniid larvae are known to produce a trail marker.

Another ant pheromone, evoking immediate behavioural response in fellow workers, is the alarm substance (which in fact is a mixture of chemical messengers). The alarm substance ("substances" should be more correct) is produced by the mandibular glands and certain glands associated with the sting apparatus. These glands store their secretions in reservoirs. If an ant is disturbed, say by an approaching predator, she releases the alarm substance. Ants in vicinity rush towards the disturbed fellow, and show panicky movements, including use of their sting apparatus. Such movements are believed to be of a defensive value. Like the trail marker, the alarm substance is effective for a short period, so that, after small crises have been dealt with, turmoil and excitement in the group are over. Cycloalexic larvae are also known to produce an alarm substance, when a predator is approaching the group.

The classical observations of the Austrian naturalist Karl von Frisch on language in the honey bee, verified by other bee specialists, have firmly established that worker bees, after feeding on a good source of food, communicate to the fellow workers in the colony presence, richness, distance and in some cases also direction of the feeding station by performing some rhythmic movements or "dances". It is also known that a dancing bee releases a sweet smelling substance, which conveys richness of the food source.

In addition to those insect pheromones, which produce immediate behavioural response in the recipient, such pheromones are known, which evoke developmental changes in the recipient, and these changes may eventually produce some behavioural effects. Hence the behavioural response is a consequence of the developmental changes, and is not seen immediately after reception of the pheromone. The queen in a honey bee colony produces, through her mandibular glands, a pheromone, the queen substance. This secretion is ingested by workers, and it suppresses development of ovaries in them; as a result the workers are sterile. The queen substance also prevents the workers from constructing a queen cell in the hive and from rearing a new queen. The pheromone also evokes worker like behaviour in the sterile females. In a termite colony individuals of the reproductive and soldier castes are known to produce pheromones, which prevent the juvenile workers from moulting into reproductives and soldiers. Queen ant has also been found to produce a pheromone, which prevents development of sexual maturity in workers.

In fact we have only begun to understand pheromones and their effects. There must be many more insect pheromones, producing special effects singly or in combinations. Releaser pheromones in ants, according to E. O. Wilson, are responsible for alarm, aggregation, attraction, colony formation, territory making etc. In the mandibular glands of the weaver ant ('Oecophylla hnginoda) there are 30 different chemicals. The sting chamber of honey bee produces about 20 different pheromones. These external secretions must be acting as means of chemical communication in insect populations and societies. How else the various activities in an ant or a wasp or honey bee colony take place in a very well regulated and organized manner in the almost total darkness of their nests? Wilson points out that there are several glands in a worker ant's body, and there are at least 10 different pheromones produced by ants. Further work will reveal a number of interesting details of chemical language among insects.

Pheromones are useful in pest control either to detect pests and invaders or for mating disruption. Agriculturists are using in pheromone-baited traps synthetic copies of original chemicals.

References

Fabre, J.-H. 1989. Souvenirs Entomologiques. 2 vols. Robert Laffont, Paris. Reissue : 1138 + 1182 pp.

Jolivet, P. and Maes, J.-M. 1995. A biological accident or a strange case of phoresy? Chrysomela 30: 4.

Resh, V H. and Cardé, R. T. 2003. Encyclopedia of Insects. Academic Press-Elsvier. Tembhare, T. B. 1984. Insect Morphology, Physiology, and Endocrinology. S. Chand and Co., New Delhi. Wilson, E. O. 1963. Pheromones. Scientific American 208 (5): 100-114.

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