Sunday the 15th October was the date of the Amateur Entomologists’ Society field meeting at Rye Harbour Sussex Wildlife Trust reserve. It’s not often that I do a focused invertebrate hunt at this time of year, so I was looking forward to seeing what we found.
The field meeting was unexpectedly good on the arachnid side of things. I had no idea that the reserve was so rich in numbers and species of spider. We spent most of our time in a compartment of saltmarsh and shingle, where the shingle was really shallow. The layer of shingle was about two stones thick, and the soil beneath it was compact. This meant that the spiders could not escape deeper into the shingle as they would at most shingle sites.
One of the highlights of the field meeting spider-wise was the fantastic Myrmarachne formicaria. As the scientific name suggests, this species of spider is an ant-mimic (the prefix ‘myrm’ means ‘ant’ and a formicarium is an ant farm). The mimicry of ants in the animal kingdom is known as ‘myrmecomorphy’ and is quite common across a number of invertebrate orders. Invertebrates that are known to be ant mimics include young grasshoppers, true bugs (Hemiptera), flies, beetles and of course spiders.
But why do so many invertebrates mimic ants? Ants are known among the predators of invertebrates to be aggressive or distasteful, so they are avoided. And the predators will also avoid any insects that look like them, but lack any means of defence such as the young grasshoppers. This is known as Batesian mimicry, as it was first described by H W Bates. Some ant mimics have even gone as far as to mimic the ants chemically as well, by emitting ant-like pheromones, which is referred to as Wasmannian mimicry.
However, Myrmarachne formicaria may mimic ants for a different reason. It is thought that some spiders mimic ants not only for protection against predators but also so that they can hunt the ants themselves. Ants will overlook the spiders as one of their own colony, giving the spider the perfect opportunity for a meal. This type of mimicry is aggressive mimicry. As you can see from the photo below, the spider has a long abdomen, which helps it to resemble an ant.
Myrmarachne formicaria really was a fantastic sight, and I hadn’t personally seen anything like it before. It was fascinating to learn how and why so many different invertebrates mimic ants. The sheer number of ant mimics must indicate that ants are one of the most successful of all invertebrates.