What's Tachinidae?

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.

↑ 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.

交尾器を観察する際は，生殖節をKOH（代用として乳酸を使用可）で溶かし，その後グリセリン内で解剖します．かなり大変な作業です．慎重にやっても，自分のような素人は，すぐ傷をつけてしまったり破損してしまったりします．まだまだ鍛錬が必要なようです．

↑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．）

↑中国語の文献

↑ドイツ語の文献

↑ロシア語の文献

そのため，ヤドリバエの分類を行うには，なにより地道な作業を厭わず続ける忍耐力が必要です．相当“ヤドリバエ愛”が強くなければ続けられないことです．

最後に，④について．先行研究が少ない中で，同定を行うことは，まさに暗中模索．まだまだ未記載種が膨大にいることが予想されるため，自分の手元にある標本が既知種であるという保証はありません．その個体が既知種と同一と判断して良いものなのか，それともあらゆる既知種とも異なるのかを判断するのは極めて難しいことです．究極的には，100%正確な同定というのは無理なのかもしれません．いくら正しい同定をしたと思っていても，後々の研究で，既知種とものすごく似た新種だったと明かされる可能性は捨てきれない．

これらの理由から，ヤドリバエの同定ができる人は，専門家とアマチュアを合わせて日本国内では，せいぜい30人くらいでしょうか（勝手な推測ですみません）．専門家としてヤドリバエの分類を研究しておられるのは，国内では嶌洪先生と舘卓司先生のたった２名です．（私のような素人がこんなサイト作ってつくづく恐縮なところです…）

全世界を見ても，研究者は比較的数が少ないでしょう．ただ，ヤドリバエの研究が遅れているという現状は逆に，ヤドリバエ研究の魅力と読み替えられます．研究が進んでいないため，上にも記した通り，未記載種が山ほどいる！と予想されるのです．自分で新種を記載するのも全然夢ではありません．実際，自分も，未記載種と思われる個体の標本を手元に複数持っています．

ところで，ヤドリバエの分類なんて調べる意義は有るのか？と言われそうです．正直，私がヤドリバエの研究をしたいのは，自分がヤドリバエ好きで，意義の有無以前に純粋にヤドリバエの分類が楽しいからというのが本音です．

ただ，この研究に大いなる意義があるのは事実でしょう．嶌・篠永(2014)から引用すると，

『ヤドリバエは，一部をのぞきそのほとんどが幼虫時代を他の昆虫類の内部寄生者として成長する捕食寄生者であり，地域生物群集の調節に大きな役割をはたしているものと考えられる．またそのために・ヤドリバエ科は，寄主となる地域昆虫群集の多様度を知るうえでの大きな目安ともなりうる．』

ということです．つまり，ヤドリバエはある地域の生物多様性の指標生物と言えるのです．

ヤドリバエ（Tachinidae: 愛称たきにぃ）...

とっても多様性が豊富な昆虫...とっても同定が難しい昆虫...

一緒にヤドリバエ沼にハマってくれる人を探しています．

ぜひ，そこのあなたも！　ヤドリバエやりませんか？

参考文献)

嶌　洪・篠永哲, 2014. 皇居のヤドリバエ相. 国立科博専報(50): 447-457

O’Hara JE, Henderson SJ, Wood DM. 2020. Preliminary checklist of the Tachinidae (Diptera). Version. 2.1 [PDF document].