Comparative　morphology　of Day\u27s　organ of the family Pompilidae

昆虫綱ハチ目クモバチ科は今までに全世界から約4,200種が記録され、日本でも約40属110種あまりが確認されている。熱帯・亜熱帯域に多く分布し、体長は数㎜程度の小型種から5cmを超える大型種までが知られている。外部形態の特徴として、長い脚（後腿節はたいてい腹部の末端を超える）や中胸側板を斜走する溝があげられる。特にこの長い脚は、獲物となるクモの体を押さえつけるために有用であると考えられる。単独性のカリバチであり、労働寄生性の種を除く全ての種が幼虫のためにクモを狩る。巣は既存坑を利用するタイプ、土中に巣坑を掘るタイプ、地上に泥の巣をつくるタイプなどがある。デイ器官（Day’s organ）とはM. C. Day氏が発見した器官系の名称で、クモバチ科オスの腹部背板の節間膜に存在する小孔、突起および付属腺から成る。その腺細胞で産生したフェロモンを発散させる構造と推定され、フェロモンは交尾の際に種の認知に機能すると考えられている（Day 1984, 1988; Menke 1982）。しかし、その微細構造については未発表であり、機能はもちろん形態的な研究もほとんどなされていない。そこで本研究では、クモバチ科各分類群のデイ器官の形態と分布の特徴を明らかにし、内部構造を組織・細胞レベルで精査し、機能の推定を行うことを目的としている。 実験に用いた材料は、クモバチ科の4亜科、ヌスミグモバチ亜科、ムカシクモバチ亜科、クシヒゲクモバチ亜科、ナミグモバチ亜科から31属56種を選んだ。また外洋としては、Brady et al.（2009）の系統仮説に基づき、クモバチ科にもっとも近縁と考えられるアリバチ科2属2種、コツチバチ科2属2種を用いた。 外部形態については、乾燥標本・液浸標本から腹部第1～4節間膜を取り出し、走査型電子顕微鏡（SEM）を用いて観察した。内部構造については、野外で採集した個体の腹部を樹脂包埋し、準超薄切片および超薄切片を作成して、光学顕微鏡と透過型電子顕微鏡（TEM）で観察を行った。 SEMを用いた外部形態の観察から、以下の結果が得られた。（1）節間膜上にはデイ器官と思われる多様な「孔」や「突起」をもつ表面構造が、一部の種を除き、主に第2節間膜に存在する。（2）孔と突起の形状や密度は種間で様々である。（3）孔と突起の形状は一部の属（Dipogon属やPriocnemis加属）の亜属内で類似する。（4）孔と突起が存在する節間膜は一部例外を除き、属ごとに等しい。（5）孔と突起の有無については亜科ごとに一致しない。（6）同種の個体であれば、標本の状態が異なっていても同様な構造が確認できる（種内変異はほとんどない）。また、デイ器官の内部構造の光学顕微鏡観察により、節間膜の内側に、筋肉や脂肪細胞とは異なる1細胞が接していることが確認できた。TEM観察により、その細胞は腺細胞の特徴である巨大な核と多数のミトコンドリアやゴルジ体を持つことが確認できた。 以上よりデイ器官の孔や突起は、種特有の外部形態的特徴を持ち、同亜属内で類似する可能性のあることが考えられた。また、付属腺の細胞レベルの特徴から、それが分泌機能を持つことが推定された。The family Pompilidae (spider hunting wasps) comprises a large group in the order Hymenoptera, and is distributed in all zoogeographical regions. Approximately 4,200 species have been described around the world, and 110 species belonging to 40 genera have been recorded from Japan. Wasps of this family vary in body length from 5mm to more than 55mm, the latter being the largest in the Hymenoptera. These wasps are morphologically characterized by the mesopleuron（lateral plate of mesothorax）with a transverse suture and long-legged adult habitus（the hind femur usually extends to the apex of the matasoma (gaster)）. All pompilids except brood parasitic species hunt spiders and lay their egg on the spider’s abdomen（hence their common name）. The female constructs various types of nests, e.g., utilizes pre-existing narrow spaces in the ground or wood for her cells, digs burrows in the ground or wood, or builds barrel-shaped mud cells above the ground. Systematic studies on the higher taxa of the Pompilidae have been made by several taxonomists. As they attached significance to different characters, they proposed different classifications. Day (1988) presented a good review of the morphology of the Pompilidae and phylogenetic relationships between the family and other families of the Aculeata (stinging wasps and bees). Shimizu (1994) made the first cladistic analysis of the Pompilidae, based on morphological characters. Pitts (2006) revised Shimizu’s (1994) work, adding further morphological characters, and presented the most parsimonious tree for the species representative of all subfamilies. Because molecular phylogenetic analyses on the family have not been published, there are few reliable phylogenetic hypotheses useful for the inference of character evolution. In the present work, detailed morphology of the Pompilidae was investigated in order to detect new characters significant to taxonomic and phylogenetic analyses. Among the preceding works, Day’s (1988) is important. He stated, “the male abdomen of a large proportion of species contains a peculiar exocrine structure, which is located in the intersegmental membrane between the second and third terga, but is occasionally found anteriorly in tergum 3, or infrequently both intersegmentally and in the tergal surface between multiple segments. A great diversity of form suggests an evaporative surface fed by micropores from gland cells which are expected to produce some kind of pheromone.” This structure has been celled ‘Day’s organ’ (Menke1982). Subsequent investigations on the structure and function of this organ have not been published. The present study was thus intended to (1) elucidate morphological characteristics of Day’s organ for each species, genus and subfamily, using scanning electron microscopy (SEM); (2) examine the internal structure of this organ, using both optical microscopy and transparent electron microscopy (TEM); and (3) discuss the function of the organ based on its ultrastructure and mating behavior of some species. Although Day (1988) did not mention variously shaped projections on the intersegmental membranes often found near the micropores, these structures were regarded as part of Day’s organ and were also investigated in this study. For the ingroup material, 56 species belonging to 31 genera of Pompilidae were examined. The classification of the material follows Pitts (2006), in which four subfamilies are recognized and their Phylogenetic relationships are discussed. Species of Mutillidae and Tiphiidae were selected for the outgroup material, which families are considered to be closely related to the Pompilidae based on molecular information (Brady et al. 2009). As a result of the SEM observation, the following is clarified: (1) micropores and/or projections are found mainly on the second intersegmental membrane in the majority of the species (about 70％ of all); (2) the structure and density of the micropores and projections exhibit very few variation within a species, even when specimens preserved in different ways (dried specimens in the air or specimens in alcohol) were examined. This means that there are few infraspecific variations；（3）the structure and density of the micropores and projections were diverse among species；（4）the structure of the micropores and projections may be similar within a genus or subgenus, e. g., Dipogon, Priocnemis (Umbripennis); (5) the structure and density of the micropores and projections may be different among very closely related species; (6) the structure and density of the micropores and projections are not stable within a subfamily; (7) variously shaped micropores were found on the metasomal terga in a few species; (8) inthe outgroups, two species of Tiphiidae possess the intersegmental membranes with micropores and projections, which seem to be Day’s organs. The observation of the internal structure of Day’s organ using optical microscopy revealed that there is a large cell attached to the inside of the intersegmental membrane. The TEM observation also revealed that the cell has not only a very large nucleus but also numerous mitochondria and ribosomes in its cytoplasm, which are defining characteristics of gland cells, such as Dufour’s gland of hymenopterous insects. It is therefore presumed with certainty that Day’s organ is an exocrine gland system, as Day (1988) inferred. Probably the cell attached to the intersegmental membrane produces a sex pheromone, which is evaporated through the micropores and projections of the membrane and ‘plays a role in species recognition during mating’ (Day 1988). This is supported by the mating behavior of a pompilid, Auplopus carbonarius Scopoli, in which the male approaches the female, raising the metasoma, elevating the second metasomal tergum to expose the second intersegmental membrane, and vibrating the wings. With regard to the external morphology of Day’s organ, it is summarized that the structure is invariable within a species and may be stable within a genus or subgenus, although it may be different among very closely related species. In order to clarify the distribution of Day’s organ outside the Pompilidae, more data and taxa are needed.首都大学東京, 2013-03-25, 修士(理学