先日，家から少し遠出した時に見かけて写真に撮っていた。
体長は10ミリぐらい。
最近は、昆虫の観察に時間を割いていなかったので、一瞬、何物と思ったが、触角の形状と左右に張り出した複眼の形状から、ジョウカイボンの仲間を予測出来た。
ジョウカイボンの仲間を詳しく当たってみると，クビアカジョウカイという種が浮上して来た。かなり似た種にムネアカクロジョウカイという種もいたが、サイズはクビアカジョウカイに近く，ゆえに、上の写真の個体はクビアカジョウカイだと思われる。
習性に関する情報は、インターネット上に少なかったが、アブラムシのような極小の昆虫を捕食している情報が散見出来た。また、花に集まるという情報も目にしたが、まさにタンポポの花に来ていた。私が知っている近所のジョウカイボン達が花に集まっている気はしないので、花に集まるジョウカイボンも居るんだというのが、私の正直な感想であった。
生息分布に関しては、情報が錯綜というか混乱しており，北海道から九州までというものもあれば、元々、西日本に居る種だとの情報もあった。この生息分布の情報の錯綜は、本種とムネアカクロジョウカイとを、そもそも区別出来ていない方達が沢山いる可能性から派生しているのではと思う。かく言う私も，クビアカジョウカイとムネアカクロジョウカイを区別出来るのか、現段階では疑わしい。ただ、ムネアカクロジョウカイの方が少し大きいという情報から、上の写真も個体をクビアカジョウカイと思っている感は否めない。
ここで、一つ目を向けたくなるのは、少しづつ違う近似種は、ある程度の地理的分断を経験した結果と考えたい自分がいる。すると，やはり、1600万年ぐらい前にフォッサマグナの時代に日本列島が海溝で分断されていて、再度陸続きに戻るまでの何千万年かの間に、それぞれ少しづつ遺伝的要因に変化を起こしたのかと思いたい。ゆえに、両種の生息分布にも規則性があるはずだと私は思いたがっている。
ちなみに、海外の生息分布は、上の学名だと，ヒットしてこなかった。また、ムネアカクロジョウカイの学名でも，ヒットしてこなかった。
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happened to see this and took a photo when I went on a short trip away from home the other day.
The body length was about 10 millimeters.
Since I had not spent much time observing insects recently, I briefly wondered what family it might belong to at first glance. However, based on the shape of the antennae and the laterally protruding compound eyes, I was able to infer that it belonged to the soldier beetle group (Cantharidae).
After looking more closely into the soldier beetles, one species that came up was Kubiaka-jōkaibōn (a species of soldier beetle). And also there is a very similar species called Muneaka-kuro-jōkaibōn,but the size of the individual in the photo is closer to that of Kubiaka-jōkaibōn. For this reason, I believe that the specimen in the photo is Kubiaka-jōkaibōn.
There was not much information available online regarding its behavior, but I did find scattered reports indicating that it preys on rather smaller insects such as aphids. I also came across information stating that it visits flowers, and indeed, this individual was found on a dandelion. Personally, I do not have the impression that the soldier beetles I am familiar with in my neighborhood gather on flowers, so my honest reaction was a sense of surprise—realizing that there are soldier beetles that do visit flowers.
As for its distribution, the information seems inconsistent or confused. Some sources state that it is found from Hokkaido to Kyushu, while others claim that it is originally a species native to western Japan. I suspect that this confusion may stem from the fact that many people are unable to clearly distinguish this species from Muneaka-kuro-jōkaibōn. To be fair, I myself am not entirely confident that I can reliably tell these two species apart at this stage. Still, based on the information that Muneaka-kuro-jōkaibōn is slightly larger, I cannot deny that my identification of the photographed individual as Kubiaka-jōkaibōn relies somewhat on size alone.
One thought that naturally comes to mind is that closely related species with subtle differences may be the result of having experienced a certain degree of geographic isolation. If that is the case, one might imagine that when the main Japanese island was divided by ocean trenches during the era of the Fossa Magna around 16 million years ago, and over the tens of millions of years until the land reconnected, each underwent gradual genetic changes. If so, I feel inclined to believe that there should be some underlying pattern in the respective distribution ranges of these two species.
Incidentally, I was unable to find any information on its overseas distribution when searching using the scientific name mentioned above. The same was true when I searched using the scientific name of Muneaka-kuro-jōkaibō—no relevant results turned up.
