Infrared Receptors

It is well known that some insects are attracted by the smoke of the blazes of forest fires, for example, little flies like Microsania (Platyperidae) and Hormope%a (Empidiidae). Hormope%a is frequently associated with the beetle Melanophila acuminata (Collin, 1918). Many years ago, a Belgian entomologist, Albert Collart, studied in good detail Microsania by attracting them to fire made by burning trash in his garden. He collected them also in Congo, around bush fires. These Diptera are common around the earth, but their biology is unknown, and specially the biology of its larvae. It could be inferred that these insects are attracted by the smell of smoke, because clothes, impregnated with smoke, attracted them.

The sense organs of these flies have not been studied, but it seems that they have also special infra-red receptors, which in Melanophila (Coleoptera Buprestidae) detect forest fires, and induce the insects to get attracted irrestibly towards the heat source. It may be that smoke smell detection also has a role in inducing this behaviour in some insects.

Evans has described in Melanophila sensorial crypts situated on the side of the mesothorax, adjacent to coxal cavities and sensitive to smoke. The sensory organs can receive waves of 2.5 to 4.0 micrometers. These buprestids can perceive them from a distance of 1 km at ground level and up to 5 km in the mountains. This way Melanophila is able to reach a fire and freshly burnt trees before any other competitor. It is a common observation that these buprestids are present in a dead tree and can be seen running along still smouldering branches (Evans and Bellamy, 1996).

People capture easily its larvae under the bark of burnt Coniferae. It is believed that the beetle has helped recycling of vegetal matter in the Yellowstone Park, after the big fire of the last century. After hatching, larvae remain in the dead tree for a year or more before tunnelling outward to pupate and to emerge as adults (Hart, 1998). Klausnitzer (1981) reports that these beetles sit and copulate on wood that is too hot to be held in hand. Fire seems to have a stimulating effect on Melanophila. However, Crowson (1981) says that charred wood is not necessary for the development of Melanophila species. The beetle can be found as developmental stages also in dead and moribund conifers with no trace of fire. It is notable that Melanophila does not have bright colours, like other buprestids, it is black, the colour of charred wood on which it copulates.

The charred wood, after a forest fire, has a characteristic fauna of its own, and in that not only Diptera are attracted. Many longhorn beetles, bark beetles (Salpingidae) and even ground beetles, like Agonum quadripunc-tatum, visit burnt wood. There must be specific organs sensitive to the smoke (Evans, 1966 a).

Several authors have discussed the possible utilization by numerous insects of various depressions on body as infra-red detectors. However, most of these organs are moderately sensitive, and only the highly specialized organs of Melanophila acuminata detect infra-red radiation with a high sensitivity. Another buprestid, Merimna atrata, in Australia, is known as the fire beetle, alighting on steaming branches, even over the parts that are glowing red (Poulton, 1915).

The cephalic capsule of many weevils has a zone, which is sensitive to only red and infrared. In the alfalfa weevil, Hypera postica, this extraocular filter is coordinated with compound eyes allowing the insect to use visual markers for locating and identifying its host-plants (Meyer, 1977).

Many cavernicolous beetles have antennae sensitive to heat and humidity. Their antennae are extremely elongated. There are thermos ensitive sensilla on the maxillary palpi and the antennal club of Dorcus beetles, and in certain Hypera (Curculionidae) a part of the cephalic capsule is sensitive to infrared radiation and is especially innervated (Paulian, 1988). Experiments have been done in various laboratories using electroantennograms and electropalpograms. A minute titanium electrode is inserted into a recipient cell. Electric waves are then studied as in an electrocardiogram and we can analyze the insect sensitivity to odour, colour, heat, chemicals.

The depressions specially sensitive to infrared in Melanophila enclose 70 to 100 sensilla each, which are tiny domes, and a mass of fibrous and waxy material (Evans, 1966 b). Experiments have confirmed the extreme sensitivity of these cells, associated with the sensilla, to infrared. The dome-shaped infrared receptors respond to a slight deformation caused by the radiation, and the resulting impulses are transmitted through nerves. In some beetles, as the tenebrionid Deretus denticollis, from the island of Socotra, close to the NE of Africa, only the male possesses three hairy pits under the first three abdominal sternites (Koch, 1970). The female does not have these organs, of which the function remains unknown. Perhaps they are linked with mating. Many carabid and tenebrionid beetles show some similar depressions, in steppic and semi-desert places. Perhaps in these insects the organs serve for hygrometric detection. Sometimes, in special cases, such depressions serve for pollen transport, according to Crowson. Setiferous sex patches and analogous structures exist in 11 families of Coleoptera (Faustini and Halstead, 1982). These structures appear to have several features in common: long ridged setae, cuticular ducts, cribriform pore plates, and the production of a secretion. These structures may be concerned with the production, release and dissemination of pheromones. Presence of such sensory pits in many insects supports the hypothesis that many are capable of sensing infrared radiation (Grant, 1950).

Several years ago, the Italian entomologist, Mauro Daccordi (1980) discovered strange depressions under the abdomen of a small chrysomelid from the Cape area, in South Africa. He named the insect Gasterantodes, which means cavernous belly. Those depressions are hairy and certainly sensitive to something, probably to humidity or to heat, since the beetles have to face a long and trying period of dryness, and in this period they hide under stones and vegetation. Even plants, such as the Mesambryanthemaceae, also hide under soil, in a stone-like form, to escape extreme rough conditions.

Unexplained invaginations on body surface, with their exact function not known, have been found by Daccordi (1994) also among Australian

Chrysomelidae, like Peltoschema (Pyrgo) nigroconspersa, a species living in dry areas. Those depressions are probably hygrometric sensors or thermoreceptors in steppes or semi-deserts. According to Daccordi (personal communication), Henicotherus sp., another chrysomeline from the Atacama desert in Chile, has subelytral bladders which are also probably linked with water detection. Abdominal structures of this nature are present in certain Australian Callidemum and Strumatophyma, but only in males, and are probably linked to sexual function (Daccordi, 1994; Jolivet et al, 2004). Perhaps these organs in some cases are comparable to the sensory pits of the pit-vipers and rattle-snakes, also sensitive to infrared. The specialized structures are located in the snakes below and ahead of the eye, and they act like infrared eyes, with binocular "vision". Boas have 13 pairs of such pits.

It is sure that such depressions on the abdomen of various beetles have a function, probably concerned with the water detection, but sometimes with sex or pollen transport. It is a domain which remains to be explored. So many beetles have been described without studying their morphology and anatomy; hence it is not known how common such differentiations are among beetles. A micropeplid beetle, discovered by Lobl in Thailand, has the body covered with lamellar secretions all over. It is not wax, and it is not a way to capture humidity since this beetle lives in a permanently humid environment. Is it a protection against predators? There are more things to be discovered and studied among the immense variety of beetles (Lobl and Burckhardt, 1988).

— Fig. 13.1. Deretus denticollis, male, a tenebrionid from the desertic island of Socotra, in the NE of Africa (after Koch, 1970).

— Fig. 13.2. The same as Fig. 13.1, with the big hairy depression of the abdomen, probably glandular (after Koch, 1970).

— Fig. 13.3. The infra-red sensitive organs of Melanophila acuminata (Buprestidae), as seen when looking at the mesothorax sensory pit (after Evans, 1966).

25 m

— Fig. 13.4. Diagrammatic cross section of the infrared sense organs of M. acuminata showing the wax glands (after Evans, 1966).


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