Georgofili World

Newsletter of the Georgofili Academy

Sensory communications among insects

The biological activities of animals, including insects, plants and micro-organisms are regulated by sensory communications, which differ according to their ecology, ethology, morphology, and physiology. Sensory communications are mediated by sensory organs, which, in the insects, consist of the integument and one or more bipolar sensory neurons. Sensory communications among insects can be classified in: tactile, visual, acoustic and chemical communications. Courtship, in both solitary and social insects, is performed by direct contact between individuals and is regulated by chemical stimuli. In social insects, such as ants and termites, antennal tapping is also an essential component of courtship communication signals. The language of bees-(called "dance") consists of sounds and chemical messages mediated by the chemoreceptor sensilla, and involves harmonically the whole family. The "dance" represents a symbolic language that allows a worker bee to inform other bees about the location of the family, the source of nourishment, and its abundance and distance from the hive. Almost all of the adult insects are equipped with sensilla photoreceptors. 
Many insects communicate with visual signals. The color patterns and other markings on the wings of butterflies and moths facilitate species recognition. Some insects use bright colors, eyespots, or other distinctive patterns in order to scare away predators, to warn about their ability to sting, or to mimic the appearance of predators. Most of these signals are effective only as long as they are visible in daylight. But a few insects can generate their own light and use visual signals that can be seen at night. The luminescence is a property of the adults of some species of Coleoptera Lampiridae, Collembola, Homoptera Cicadoidea, and larvae of Diptera. Its function varies, e.g. in some larvae can be used to attract prey, in Lampiridae for mating purposes. Chemically the luminescence is produced by the oxidation of a cellular protein (luciferin) in the presence of an enzyme (luciferase). The interaction between the oxygen in the air and adenosine triphosphate (ATP) generates light energy. In several species, the light can have different colors. It can be yellow, green, blue or orange. Each species of Lampiridae emits light signals of specific duration and intervals, which differ from those of other species, thus ensuring reproductive isolation. However, the females of the genus Photuris mimic the light signal of the species of the genus Photinus in order to attract and prey on them. 
Many species of insects are capable of transmitting acoustic signals, audible (sound) or not (infra - and ultrasound) by man. This property may be of one or both sexes. The emission of sounds can have different purposes: call, alarm, communication, etc. The acoustics are one of the mechanisms of isolation of species. Orthoptera and Hemiptera Cicadidae are equipped with apparatus for the production of sounds. The Orthoptera Ensifera produce sounds by rubbing the forewings. The males of Grillidae emit 3 types of sounds: rivalry, in the presence of other males; recall, when they are alone; courtship in the presence of the female. The Orthoptera Caelifera produce sounds rubbing the hind femurs against the forewings. The Hemiptera include many species that emit sounds. The males of almost two thousand species of cicadas chirp through a flapper in the abdomen, the pie, which produces sound by vibrating under the stimulation of a specific muscle. A massive air sac, located in the abdomen, acts as a sounding board and amplifies the stridulation. The male utters his  courtship call that attracts only the females of the same species. These have auditory organs sites in the second abdominal segment. The moth Acherontia atropos (Linnaeus, 1758) emits a plaintive squeak consisting of two short repetitive sequences in very short time: a low tone sequence, due to the expansion of the pharyngeal cavity that vibrates a prominence of the palate and a high tone sequence due to the expulsion of  air  through  the short proboscis. Other groups of insects produce sounds using the substrate that they inhabit. Among these insects, the Anobiidae Xestobium rufivillosum (De Geer, 1774) has received the common name “death watch” because it produces a rhythmic percussive sound during the night. Most grasshoppers and moths detect sound with a tympanic membrane in the abdomen or in the tibiae of the front legs (e.g. crickets and katydids). Mosquitoes have antennal hairs that resonate to certain frequencies of sound. However, sound vibrations can also travel through solid objects, and some insects (e.g. some species of ants, bees, termites, and treehoppers) can sense substrate vibrations with mechanoreceptors (chordotonal organs) in their legs (Snodgrass, 1935). Chemical communication has significant importance in the biology of insects and also animals, plants and micro-organisms. The chemical substances (usually volatile compounds) that deliver behavioral messages between both individuals of the same species (intraspecific) or between representatives of different species (interspecific) are called semiochemicals. These compounds play an important role in the behavior of the insects and in their interactions with plants and other insects. The semiochemicals have been divided in different groups. 1. Allelochemicals, which are chemical compounds transmitting chemical messages between different species of life forms. These compounds are subdivided into three groups based on who "benefits" from the message: a. Allomones benefit the sender -- such as a repellent, or a defensive compound (e. g. cyanide) that deters predation or a botanical compound that benefits the plant by repelling insect’s infestation. b. Kairomones benefit the receiver -- such as an odor that a parasite uses to find its host. c. Synomones benefit both sender and receiver -- such as plant volatiles that attract insect pollinators. 2. Pheromones are substances secrete by insects and animals that carry information from one individual to another member of the same species. These include sex attractants, trail marking compounds, alarm substances, and many other intraspecific messages. The pheromone glands are widespread in insects and produce intraspecific chemical messengers, which may result in slow responses (primer pheromones, e.g. physiological regulators or inhibitors produced by fertile females and larvae of social Hymenoptera) or instant response (releaser pheromones, e.g. sex, alarm, aggregating and markers). The primer pheromones also regulate the complex life of the societies of insects, resulting in differentiation into castes with changes in morphology, physiology and behavior of individuals intended to perform various tasks. Primer pheromones trigger a change of developmental events and differ from all the other pheromones, which trigger a change in behavior. The releaser pheromones are the main communication channels of rapid responses by social and solitary insects. They trigger an immediate and persistent reaction in the recipients. Releaser pheromones are pheromones that cause an alteration in the behavior of the recipient. Some of these pheromones cause the aggregation and dispersion of other individuals. Others are used to facilitate mating between the sexes. Depending on the type of the induced effect, they are distinguished in the following categories:
- Aphrodisiac pheromones. They are involved in mating and favor approaching and copulation. In some species of Lepidoptera, they are emitted by males through feathery tufts of scales. In some cases, these feathery glands  emit odorous "antiafrodisiac " that inhibit other males from mating.
- Aggregation pheromones. They are substances that promote an increase in population density in the vicinity of the emission source. These pheromones include trail marking pheromones of bees, ants and termites; cohesion pheromones of the colonies of social insects; and aggregation pheromones of grasshoppers. 
-Dispersion pheromones. They are substances inducing migration of individuals away from the source of emission. The best known are the host marking pheromones produced by females of many Diptera Tefritidae, which mark the fruit in which they ovideposit to prevent ovideposition by other fruit flies.
- Alarm pheromones. They are compounds that stimulate escape or defensive behavior. Aphids threatened by predators emit an alarm pheromone that induces migration of other individuals from the colony. Even the bees release an alarm pheromone (isopentil acetate or IPA), produced by a gland attached to the sting that causes other workers to rush to the rescue with aggressive behavior.
- Sex pheromones. They are substances produced by virgin females in quantities of the order of nanograms. The emission of these volatile pheromones occurs intermittently during certain times of the day (most frequently at dusk and in the early hours of darkness). The odorous molecules dispersed in the air, are picked up by the male, even at a considerable distance, through the olfactory sensilla located in the antennae. The initial olfactory stimulus is "translated" into motor stimulus that induces the male to fly towards the emission source for the purpose of mating. Sex pheromones are often composed of mixtures (blends or bouquet) of substances in different ratios, depending on the species. Each component conveys a specific message. The role of sex pheromones is to attract the opposite sex of the same species for mating purposes. The specific chemical composition of sex pheromones of each species ensures reproductive isolation and prevents interspecies hybridization

(Ph: Lasiocampa quercus)


Santi Longo - longosan@unict.it



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