素人ヤドリバエ屋の庵室
a hermitage of an amateur Tachinidist
１.What's Tachinidae?
Tachinidae is, more precisely,
Family Tachinidae, Superfamily Oestroidea, Subsection Calyptratae, Section Schizophora, Clade Eremoneura, Infraorder Muscomorpha, Order Diptera, Insecta.
It is one of the largest families in Diptera (flies), with no fewer than 8,592 species known worldwide as of 2020 (O’Hara et al., 2020).
Most Tachinid species are parasitoids...
they invade their host's body, and make it their home or refuge.
They are notoriously well-known among caterpillar lovers. When you raise a caterpillar, it may suddenly die, and instead, something that looks like a red bean is lying nearby. After a while, flies emerge from it. This must be a nightmare for caterpillar breeders. And that fly often belongs to Tachinidae.
During their larval stage, they feed on their host's body. Although you may think that tachinid flies usually parasitize caterpillars, there are also species - such as Meigenia, Istocheta, and Anthomyiopsis - whose hosts are Coleoptera, and others - such as Phasia and Ectophasia - whose hosts are Hemiptera. Some genera, such as Phorocerosoma and Exorista, include species whose hosts are Orthoptera or Mantodea.
Generalists also exist. For instance, Bessa parallela, which is common in Tokyo, can parasitize Coleoptera, Lepidoptera, and Hymenoptera. This is quite an unusual case. "The ability to parasitize" means "the ability to tactically evade its host's immune system." It is fascinating how Bessa parallela evade all the immune system of Coleoptera, Lepidoptera, and Hymenoptera.
* Please note that not only tachinidae species are parasitoid among Diptera. Some parasitoid species also exist in other families, such as Sarcophagidae and Phoridae...
Such tachinid flies are relatvely similar in appearance to large flies, such as Muscidae and Sarcophagidae, which often emerge inside our house. However, many tachinids are characterized by their sharp and robust bristles on the abdomen.
※There are also species with weak or no bristles on the abdomen, especially in the subfamily Phasiinae.
The body length is often large compared to other flies, but some can be small, around 2-3 millimeters. The body colour also varies greatly among species from grey to metallic green. Sarcophagids are often mistaken for Tachinids, but they typically have three distinct black longitudinal lines on their scutum. Additionally, their abdomens tend to have a tessellated pattern, making them distinguishable from tachinids once you become familiar with them.
Such tachinid flies are relatvely similar in appearance to large flies, such as Muscidae and Sarcophagidae, which often emerge inside our house. However, many tachinids are characterized by their sharp and robust bristles on the abdomen.
※There are also species with weak or no bristles on the abdomen, especially in the subfamily Phasiinae.
The body length is often large compared to other flies, but some can be small, around 2-3 millimeters. The body colour also varies greatly among species from grey to metallic green. Sarcophagids are often mistaken for Tachinids, but they typically have three distinct black longitudinal lines on their scutum. Additionally, their abdomens tend to have a tessellated pattern, making them distinguishable from tachinids once you become familiar with them.
↑ Tachina amurensis
2. Identifying Tachinidae
(not translated yet, sorry)
Taxonomic research on Tachinidae is lagging behind compared to that of many other insect groups. Here are four possible reasons below.
①Only a handful of people are interested in Tachinidae.
②Identifying them requires advanced, highly specialized knowledge.
③The process of identification takes much time and also demanding.
④There's too little previous study.
Starting with point ① - the biggest issue is that Tachinidae-lovers are extremely niche. In the first place，Diptera-lovers in general are also very rare.
Among insects, there are five major families, known for having particularly big number of species:
a) Coleoptera
b) Lepidoptera
c) Hymenoptera
d) Hemiptera
e) Diptera
Needless to say that Diptera is probably the most unpopular of these five. And among dipterists, those who specialize in Tachinidae are an even smaller minority. This would be the biggest reasons why Tachinidae research is still so limited.
Next, let's move onto points ② and ③. When you identify a butterfly, a large beetle, or a dragonfly, you can often tell the species easily by just looking at its color pattern, body shape, or size. However, this is not the case with Tachinidae. It is so risky—or often impossible—to identify by just examining such traits —because so many species look almost identical to each other. On top of that, some species show an abundant intraspecific variation.
So, what traits do we use to identify tachinids? The answer is... the number, position, length, and even the angle of "bristles" on their bodies —plus the shape of their genitalia. Actually, you might be surprised to hear this, but each one of the "bristles" on the body of a tachinid is classified and named!
↑Shown above is only the morphological terms of head in lateral view. These are only the handful of terms among numerous.
You need to be familiar with all of these morphological terms if identify. This is why the identification of Tachinidae is difficult. And these terms can often be replaced with other terms according to the age when the paper was written, and sometimes be written in abbreviation and that abbr. has a little variation among each papers. Reading taxonomical paper of Tachinidae is quite a hard work.
When identifying, you need to handle a binocular stereo microscope and observe such "bristles" in detail one by one. But sometimes, bristles can often be fallen off or broken, or in rare cases, even its number can change according to individual mutations. These cases sometimes make its identification drastically difficult.
Examining the bristle arrangement is fundamental, but sometimes tricky in cases explained above. Also, there's a possibility that it is an undescribed or encryptic species whose appearances are almost the same with that of known ones. Therefore, we also refer to the shape of its genitalia.
Genitalia differs according to species. Basically, there are distinctive characteristics in male genitalia, so we usually identify by its male genitalia. Male genitalia consists of cercus, surstylus, phallus and so on. On the other hand, female genitalia can also sometimes be the key for identification, with slight differences among species. But the characteristics of female genitalia are usually not described in papers, and the interspecies differences are often very faint. Female genitalia means ovipositor, and so on.
When observing its male terminalia, we soak it to KOH (or lactic acid as substitute), and then dissect it in glycerine. That's a quite hard process. Even though handling them prudently, me, like an inexpert person, sometimes damage them. I need to practice more, anyway.
↑Eumea linearicornis (Zetterstedt)の雄交尾器スケッチ
↑ノコギリハリバエ Compsilura concinnata (Meigen) の雄交尾器
これを聞くと，雄交尾器さえ見ることができれば簡単に同定できるということ？と思う方もいるかもしれませんが，全くそんなことはありません．雄交尾器を見ればたしかに，より厳密な同定ができます．しかし，それが正しいのかどうかなんて誰にもわからないのです（DNA解析とかしない限り）．
交尾器を見て文献の図と比較しても結局わからないことも多々．なぜなら，別種でも交尾器の図がまるでそっくりなことも多いのです．こればかりは実物を見て比較しないとわからない，というか見てもわからないものも多いだろう．それに加え，交尾器はその保管状態・見る角度などにより見た目が大きく変わり得ます．それも考慮しないといけませんので，極めて難しいのです．
卵の大きさも分類の一つの鍵となります．例えば，CarceliaとCarceliellaという二つの属は外見上非常によく見た目をしています．どちらも複眼に毛が密生し，頬がとても狭い．しかし，Carceliaの卵は通常の大きさな一方で，Carceliellaは微小卵型です．この時点で，この二つの属は全く別の族に分類されます．
難しい属にもなると，外部形態での識別がほぼ不可能になる場合もあります．見た目はそっくりで，寄生先hostが違う，DNAが違うだけという場合があります．この辺りは，まだ学生の私には手が出せません．DNA解析のための機器なんて持っているはずがありませんから．とりあえず，片側の脚をもぎ取って，無水エタノール内に入れて，DNA用サンプルとして保管しています．
・・・
ヤドリバエは種数が非常に多いですから，まず属を判別するだけでも，549個のcoupletを持つ検索表を用いて同定を行います．慣れていない人が行うと，それだけで何時間も要することになるでしょう(汗)．検索表だって間違っている情報が含まれている場合も多いですし，それも悩みの種です．
加えて, 日本語で書かれた図鑑や文献はほぼ皆無なので, ほとんどは英語, ときどき中国語，ロシア語やドイツ語の学術論文や書籍を読み漁ることになります．イタリア語やフランス語の文献を参考にすることもありますね．今の時代，翻訳ツールやChat GPTに翻訳してもらうこともできるので随分便利です．が，専門用語等はそれらでもうまく翻訳されません．英語はもちろん普通に読めるべきですし，それとは別に，ロシア語・ドイツ語も少々知識として持っているといいでしょう．（私は大学の第二外国語でロシア語を選んだので，ロシア語の知識は手に入れました．今，ドイツ語もすこし勉強しています．）
↑中国語の文献
↑ドイツ語の文献
↑ロシア語の文献
↑The male terminalia of Compsilura concinnata (Meigen)
これを聞くと，雄交尾器を見さえすれば簡単に同定できるということ？と思う方もいるかもしれませんが，全くそんなことはありません．雄交尾器を見ればたしかに，より厳密な同定ができます．しかし，それが正しいのかどうかなんて誰にもわからないのです（DNA解析とかしない限り）．
交尾器を見て文献の図と比較しても結局わからないことも多々．なぜなら，別種でも交尾器の図がまるでそっくりなことも多いのです．こればかりは実物を見て比較しないとわからない，というか見てもわからないのかもしれない．それに加え，交尾器はその保管状態・見る角度などにより見た目が大きく変わり得ます．それも考慮しないといけませんので，極めて難しいのです．
ところで，ヤドリバエの同定は非常に厳密です．それもそのはず，よく似た種類ばかりですから，厳密にしないと区別できないのです．例えば, 「〇〇剛毛の生えている角度が水平に対して30°〜60°の間であるかどうか」とか「頬の高さが複眼の高さの0.25-0.27倍かどうか」とか．20世紀以降の論文であれば，１つの種の記載文だけで大量の数値が出てきます．それを一つ一つ計測して同定を確認していくのは極めて地道です． その数値も，その人の計測の仕方によって若干変わってくる場合があるので，これまた困りますね．
さらに，ヤドリバエは種数が非常に多いですから，まず属を判別するだけでも，549個のcoupletを持つ検索表を用いて同定を行います．慣れていない人がこれを行うと，それだけで何時間も要することになるでしょう(汗)．検索表だって万能で完璧に正確でない場合も多いですし，それも悩みの種です．
加えて, 日本語で書かれた図鑑や文献はほぼ皆無なので, ほとんどは英語, ときどき中国語，ロシア語やドイツ語の学術論文や書籍を読み漁ることになります．イタリア語やフランス語の文献を参考にすることもありますね．今の時代，翻訳ツールやChat GPTに翻訳してもらうこともできるので随分便利です．が，専門用語等はそれらでもうまく翻訳されません．英語はもちろん普通に読めるべきですし，それとは別に，ロシア語・ドイツ語も少々知識として持っているといいでしょう．（私は大学の第二外国語でロシア語を選んだので，ロシア語の知識は手に入れました．今，ドイツ語もすこし勉強しています．）