Myrmecomorphy in action!

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 male by Evan Jones

Myrmarachne formicaria, photo by Evan Jones, one of the field meeting attendees.

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.

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A Quick Walk down Mill Lane

 

This morning I walked along Mill Lane which runs along the east side of Hedgecourt Lake. What struck me first was the height that the Water Dock had grown to compared to the last time I visited the lake. It was at least the height of me if not taller. This is not unusual though, some plants can grow to more than 2 metres. This plant can grow in very tough conditions, these ones grow on the concrete wall of the lake against the hard wind and the severe waves that sometimes form. Equally large were the leaf mines that covered almost half of each leaf. Some leaf-mined leaves were about 75 centimetres long, so the leaf mines were huge! Whose larvae are growing up inside those leaf mines?

To identify the culprit, I visited a site I use regularly: http://www.ukflymines.co.uk
I went to the Rumex (Dock) section of the site and I saw that luckily there are not that many leaf mines on Dock in Britain. From the photos on that page I think that my species is Pegomya solennis. Pegomya solennis is a species of fly, unfortunately not new for my list although very impressive. This species is not a large fly, but the key to its very big mines lies in teamwork. In each mine there are two or more larvae that at first work together making a wide corridor. They then separate and each form a large blotch which all fuse together making one large blotch from which they feed. Sometimes the blotch size is increased even further when the blotch joins the blotch from a different leaf mine.

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The larvae feed on the leaf from inside these blotches.

Along the grassy verge between the lane and the lake there are several large patches of Bird’s-foot Trefoil, Ribwort Plantain and Black Medick. I thought these large patches might hold some specialist species but alas not much was swept from them using my net. The only species of interest was a single Platycheirus peltatus, a species of hoverfly and my 94th fly species on my Pan-species List.

Moving on, it was still windy and not too warm so most of the wildlife was hiding away. I stepped down onto one of the fishing jetties to get a closer view of the lake wall without the risk of falling in to the nippy water. On top of the concrete wall was a hole, which was occupied…

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The white spherical object in the centre of the hole is an egg sac, which belongs to a very large spider. I tried to coax it out from the hole, but it wouldn’t come. I’m not certain what it is, but one possibility is the mouse spider, Scotophaeus blackwalli. However it is usually found in sheltered places indoors so I’m not sure what it is doing on this exposed wall if I am correct.

On some more Water Dock further along I noticed a colony of aphids. It is often not hard to identify aphids when you see a colony on a particular species of plant. For this one I just searched with Google ‘aphids on dock’ and I was presented with two options: Aphis rumicis and Aphis fabae. Aphis rumicis is more plant-specific, being found on mainly dock and sometimes on rhubarb. Aphis fabae (the Black Bean Aphid), is much less so being found on a wide range of vegetables. Unfortunately the two are quite similar, although I am leaning towards A. fabae due to the paler legs shown in many of the photos I have seen on this species. My 50th hemipteroid (bug) for my Pan-species List! In my (not great) photo, the colony appears to be being attended by a Lasius ant.

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Suddenly the sun emerged from behind the clouds and everything seemed to suddenly wake up. There were loads of umbellifers on the lake shore which were great for invertebrates although I had little time to examine them closely. What I did find, however, is a mini-miner. These are very tiny bees in the genus Andrena, which I don’t come across too often. Their larger relatives in the same genus I more often come across. There are 10 mini-miner species compared to 57 other Andrena species, although I find them much harder to identify. Also, many of them have very restricted distributions.

I potted this tiny bee and when I arrived back home I took a few hasty shots though the gap between the lid and the pot. The long, very white hair caught my eye and helped me when I attempted to identify it using Steven Falk’s Field Guide to the Bees of Great Britain and Ireland. Currently I think the most likely species is Andrena niveata, the Long-fringed Mini-miner. Modern records are restricted to South-east England and it is not very common, therefore I am tentative with my identification and I will hopefully get it checked under the microscope or with an expert. In the book it says: ‘the body hairs are much whiter than in other mini-miners…the overall effect is thus of a very silvery, strongly marked mini-miner’. This definitely fits my bee.

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I was very pleased with the number of interesting and new species that I found this morning, given the short amount of time and the unfavourable weather conditions. I can’t wait until the summer holidays when there will be more time to explore!