第6回　公開シンポジウム

「甲虫類の多様性と生息環境　甲虫の種数はなぜ多いのか？」

マズール・スワボミール（ポーランド・ワルシャワ農業大学/北海道大学総合博物館客員教授）

「エンマムシの生態と環境」

　エンマムシ科甲虫は，現在，世界から約330属3800種が知られている．エンマムシにおいて，形態的・生態的進化や適応放散が起きた主な要因は，エンマムシが、腐敗物などにすむ幼虫の捕食者（主にハエ目の幼虫捕食者）へ「適応」したことにある．
　捕食性であるエンマムシは，次の４つの生活型に分類できる．１）Dendrobites（樹木生息），2) Geobites（地表生活），3) Microhisterids（小型エンマムシ生活），4) Inquilines（社会性昆虫関連生活）．
　それぞれの生息環境に適応し，独特な生態や形態を変化させてきた捕食性甲虫、エンマムシ。その生息環境ごとの多様性を紹介する．

（講演は英語，同時通訳：大原昌宏）

The family Histeridae contains more than 330 described genera and 3800 species and forms the natural and undoubtedly monophyletic group. Their external morphology, character of prey and life cycles are closely connected with the peculiartities of their biotopes and mode of life.
The main factor of the morphological and ecological evolution and adaptative radiation in Histeridae was probably the adaptation of their ancestral forms to the predation on the insect larvae (mainly Diptera) which live in decaying substances. The histerids are mainly predators of softbodied insect larvae and eggs, particularly those of cycloraphan Diptera. Hence, substrates on which flies develop in numbers are among the best places to look fore these beetles. All histerid adults can be classified as one class of life forms - the class of Histeroid predators. This class is divided into 4 subclasses:1) dendrobites, 2) geobites, 3) microhisterids, 4) inquilines; myrmecophiles and termitophiles.
Several forms which live in the decaying-tree logs are apparently the most plesiomorphous ones among the Histeridae. They have rather small (2 - 4 mm), moderately convex body, 6 - 7 uniform not-shortened and too deep elytral striae and slightly enlarged fore tibiae with numerous this spines (for example, the genus Parepierus). These histerids, including species of Epierus and Bacanius are generally found in association with older dead trees/or piles of sawdust. The adults of these species feed mainly on fungal spores.
The subclass of dendrobites is characteristic especially for the tropical and subtropical forest regions. This subclass is divided into two series. The first one includes the species with depressed body, living under bark (or sometimes in the leaf-sheaths of palms and succulents in xeric areas). The species selectively target dipteran larvae and eggs which feed on fermenting phloeom and cambium of certain hardwoods. This series contains members of the subfamily Histerinae (tribe Hololeptini, Platysomatini, many Exosternini) and Dendrophili- nae. These taxa tend to be dorsoventrally flattened and include species of Hololepta, Platysoma, Platylomalus, Paromalus, Carcinops, Pachycraerus, etc.
The members of the second series of dendrobiotic Histeridae have more or less cylindrical body and get the prey in the galleries of xylophagous insects. The subfamilies Trypeticinae, Niponiinae and strictly Trypanaeinae attained perfection in this direction. Genera Plegaderus, Teretrius, subgenus Cylister, etc. are less specialized. Many taxa of this series are highly specialized to a particular type of prey. E.g., most of Teretrius and Teretriosoma species prey predominantly on bostrichids and lyctids larvae.
The most numerous morphoecological group of the Histeridae (nearly 40% of all species) is the subclass geobiotes. They have oval or elliptic (sometimes nearly round) body, glabrous or sculptured integuments, strong legs with digging fore tibiae. They prey mainly on maggots, sometimes on catepillars and coleopterous larvae. This subclass includes in the great majority of the Saprininae and main part of the Histerinae. It is divided into five series: 1) saprobites (with group of coprobites, necrobites, phytosaprobites, vagabond hunters); 2) inhabitant of decayed roots in the soil; 3) psammobites (with groups of coastal and desert psammobites); 4) pholeobites (many amnog them are specialized for life in burrows or nests of particular host.
Saprobites. Many muscoid fly larvae develop on the dung of large mammals and carrion. These are selectively preyed on by numerous species of Hister, Margarinotus, Atholus, Saprinus, etc. The volatile and odiferous byproducts of microbiological degradation enable both flies and beetles to locate carrion and dung via olfaction. In addition to dung, there are numerous records of species associated with various type of vertebrate carrion.
A second guild of predatory histerids are specialists on dipteran larvae and eggs associated with rotting vegetation. Yet another group of histerid beetles preys on Diptera that develop on rotting fungi. These principially include species of Hister and Margarinotus, and, particularly, Notodoma. The Notodoma-species live in fungi (Formitopsis pinicola, Bjerkandera fumosa, Polysticus versicolor), mainly on trees, in wet forest and rainforest, feeding on insect larvae living in the early stages of rotting fungi. A fair number of histerid species inhabit hollow living trees. Some, including species of Gnathoncus and Dendrophilus, apparently prefer relatively dry tree cavities in which a mammal or bird is nesting. Abundant prey items in this microhabitat include Diptera, Coleoptera and Siphonaptera larvae. Some species are predaceous on certain speciefic hosts, such as the larvae of certain Chrysomelidae or the catepillars of certain moths. They can be found only by beating or sweeping the vegetation on which the larvae live (e.g. Hister helluo, Saprinus virescens). The flowers of Arum produce a carrion-smell and attract many species of Histeridae that subsequently entrapped in the calyxes.
Often there is a preference of various Histeridae for certain kinds of dung or carrion. Also the soil-type and the stage of exsiccation are of importance.
Inhabitants of decayed roots. The larvae of some Hypocaccus and Chalcionellus ssp. live in sand around roots of dune grasses and probably feed on larvae of weevils and possibly Diptera. They can also be found at the decaying root-bulbs of broom-rapes. They are often deeply dug in, at depths up to 20 cm where the sand becomes to be a little moist.
Psammobites. Fimbration of the body margin is apparently an adaptation to psammophily and is a common feature among sand-dwelling beetles, including histerids, tenebrionids, scarabs, etc. The beetles burrow often deep in sand and can be collected only by means of special and rather laborious methods. Consequently, these psammophilous species are the rearest repre- sentatives of the subfamily Saprininae. The most advanced genera, Philothis, Ctenophilothis, Philoxenus, Xenonychus, etc., are, especially by the form of body and legs, perfectly adapted to burrowing in sand. They occur mostly in sand dunes, often near roots of various desert plants or near plants or shrubs recently entombed by drifting sand and still having faded leaves buried in the sand. The beetles are capable of burrowing rapidly to a depth of 10 - 20 cm, and often even down to 50 cm, depending on the season, moisture and temperature of the sand. Like other Histeridae, they are predators of larvae and adults of other psammophilous insects, e.g. Diptera and Coleoptera (Scarabaeidae, Tenebrionidae). The fact that specimens of one species from a large area are sometimes concentrated at one spot otherwise uninhabited envi- ronment may suggests certain feeding preference in some species. Many species, including some Spilodiscus, Hypocaccus, Monachister, etc. are partial to sand dunes and beaches where shifting sand covers and kills the grasses and forbs growing here. This decaying plant material is fed upon by fly and scarab larvae which in turn are prey for these histerids. A few histerids are coastal wrack inhabitans. They include Neopachylopus, Eopachylopus and species of Baeckmanniolus and Halacritus. Numerous dipteran taxa are known to breed in wrack and they likely serve as food for the genera mentioned.
Phoelobites. There are many histerids which are obligate inhabitans of the burrows of reptilies, mammals, and even birds. A small amount of the histerids is also known as cave inhabitans. Adults of most burrow-dwelling histerids feed on fly eggs and larvae which develop on dung deposits in the burrow. Adults of Onthophilus ssp. consume fly eggs (but not larvae) and also filter feed on the liquid coating on fresh dung. Certain morphological features such elongate legs and antennae are common to many od these burrow-inhabiting taxa.
The inhabitans of bird and animal nests are a small group which does not show any obvious morphological modifications for their mode except perhaps in a slight lengthening of the an- tennae and hind tarsi and a slight enlargement of the eyes. Birdsﾕs nests in the open air, out- side tree holes or nesting boxes produce seldom Histeridae, but old nests of weaver-birds har- bour peculiar species. Accumulations of pigeonﾕs dung (or dung of other birds) may harbour Gnathoncus-species.
Little is known from literature about the histerids associated with caves. The existing informations refers to troglophyle or trogloxene species, in some cases determined only at generic level. Regarding the histerids, in which troglobitic species are generally rare (the few described so far belong mostly to the Palearctic fauna), it seems that Mexico is, in fact, one of the regions in which this family has most easily colonised the cave environment. The temeperate caves of the Palearctic region, though known fairly well as regards the animal population, have provided discoveries of very few specimens of troglobiotic Histeridae. Deep in the caves, in total darkness, blind microhisterids can be found in the algae and mould thriving on the moist walls where they prey on mites or eat fungal conidia. An American species, Geocolus caecus, is blind and flightless and inhabits soil and leaf litter at cave entrances. Anapleus wenzeli from Mexico is poorly specialized to the cave life and must be considered a troglophyle living on rotting wood fungi where it preys on tiny arthropods.
Third subclass is the microhisterids which consists of small beetles (length 0.6 -2.0) characterized by thin legs with slightly dilated tibiae and rounded body. They live as a rule at the plant detrite, litter, etc. and prey on small invertebrates (mites, minute insects, probably nematodes) scattered among such substrates. Nearly all members of this subclass are repre- sentatives of three tribes: Abraeini, Acritini and Bacaniini. Some members of this subclass are specialized to the life in deep litter (endogeans) or in caves (troglobites); they lose their eyes and wings. Species of Bacanius are known to inhabit relatively dry organic debris within tree cavities. Acritus and Aeletes commonly occur in leaf litter or under bark. These tiny histerids are thought to prey on mites. Some of the minute species of Acritus feed on Collembola and other small insects and their larvae under bark. Four species of Acritus have been recorded associated exclusively with Atta species, but only two of them are named to species. Based on their geographical distribution and on ant behavior (leaving external debris), the host of A. ignobilis is suspected to be Atta colombiana. On the other hand, A. attaphilus was collected in the fungus chamber. The Hawaiian Aeletes ssp. (both immature and adults) live in decaying wet wood and in trees. The species of the genus Halacritus are maritime and without a known exception are found in and under decaying seaweed. Specfic information regarding their food habits is lacking, but ir is possible that they might be scavangers rather than predators.
Fourth subclass (inquilines) is formed with the inhabitans of the nests of social insects, mainly ants, about 400 species, mostly from two highly specialized subfamilies: Hetaeriinae and Chlamydopsinae. Although sharing a common predaceous habit, members of the family Histeridae are ecologically diverse, with body forms that are well tailored to their preferred habitats. Among these, none exhibit more strikingly specialized morphological features than those that live in the colonies of social insects. In a broad sence, myrmecophiles are beetles that have some kind of association with ants. As defined, they live in or near ant nests. How- ever, this notation implies simply that the beetles are associated to, or with ants, without any explanation or description about their relationships.
The species of the subfamily Hetaeriinae are associated predominantly with army ants of the subfamily Ecitoninae, although some are associated with non-ecitoninae ants belonging to such genera as Formica, Lasius, Pheidole, Atta and Solenopsis or with termites of the subfamily Nasutitermitinae. Most of the species of the Chlamydopsinae exhibit remarkable excretory structures on the elytral humeri, referred to in the chlamydopsine literature as 綟pau- lettesﾓ due to their position. These structures, more generally known as trichomes, are the hallmarks of myrmecophilous beetles, and probably serve to disseminate appeasement or recognition substances, although it should be emphasized that the actual interactions of myrmecophiles with their hosts have been studied in few beetles.
Many species in the genus Paratropus can be found in antsﾕ nests, especially in the nests of the genus Annoma (Dorylinae), or walking in the raiding columns of these ants. The exact nature of the host-guest relationship between the ants and the Paratropus-species is not clear. Probably the latter are synoeketes. Some species of the genus are apparently committed to dead termitaria. As they are never found in living termitaria, they are not termitophilous in the strict sense, they are rather predators which belong to the first population that is established in a termitarium after its death. Occasionally, also other histerids hide in rubbish heaps outside the exits of subterraneous termite-galleries in wait for termites when appearing from their galleries (Monoplius sp.).
Many histerids associated with ants and termites have retained a generalized appearance and differ little from their free-living relatives, while others are very highly derived. There has been no trend towards any development of a typical myrmecoid body with its petiolate abdo- men. There are trends towards a reddishbrown coloration and dense, close surface punctation. It is unclear whether these characteristics are important to the integration of the guests into host colonies or whether there are cryptic adaptations that allow the guests to evade outside predators. Regressive specialization such as the reduction of eyes or ability to fly, have not occured in histerids. The folllowing morphological specialization among the histerids either increase their ability to survive defensive reactions by their hosts or are implicated in their integration into host colonies. The antennal scape is greatly enlarged to protect the antennal club when it is withdrawn into the antennal cavity. At least part of the antennal club is strongly sclerotized. The body surface may have prominent tufts of hairs, or prominent protu- berances, or deep pits or well-dveloped longitudinal costae or carinae. The body form of some genera is dorso-ventrally flattened or cylindrical. The legs either are usually long or have broadly expanded tibiae and sometimes femora. The tibiae may have very deep tarsal grooves in to which the tarsi may be withdrawn for protection or have prominent combs of hairs, or may have both of these character states.

酒井・小島・Mazur・菅谷・片倉