河畔のエノキにX字の文様が見えます。何だろうと近づくとコガタコガネグモの巣でした。コガタコガネグモは、巣の真ん中にこのような帯を作ります。「かくれ帯」あるいは「白帯」と呼ばれます。これを作る目的ははっきりとはわかりませんが、「天敵に対する防衛」と「餌の誘引」という説が有力なようです。前者は網を目立たせて鳥などが網を破るのを防ぐ、後者は白帯によって紫外線が反射され、昆虫類を誘引することができる、というものです。このクモはオニグモのように樹と樹の間に大きな巣を張るのではなく、林縁や切り通しなどの、奥が閉ざされた場所に巣をかけることが多く、人家では窓枠にガラスと並行に巣をかけて嫌われることも多いです。そのような習性からこのような幹の凹凸を巧みに利用しているのでしょう。面白い造形だと思いました。
I saw an X-shaped pattern on an Enoki tree by the river. Wondering what it was, I found it was the nest of the Japanese Kogatakoganegumo, literally a small golden spider (Argiope minuta) . They make a band like this in the middle of their web. It is called a "hidden band" or "white band". The purpose of making such a band is not clear, but the two most popular theories are "defense against natural enemies" and "attraction of food. The former is to make the web more conspicuous and prevent birds from breaking it, while the latter is to attract insects by reflecting ultraviolet rays from the white band. These spiders do not build large nests between trees like the Onigumo, or an ogre spider (Araneus ventricosus), but often build their nests in closed-off areas such as forest edges and cuttings, and are often disliked by people who find their nests built parallel to the glass in window frames. It is probably because of this habit that this particular spider has made such clever use of the unevenness of the trunk. I thought it was a quite interesting formation.
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Photos 2021-12-05
