Tight but nice
When I was living in Canberra (the capital of Australia) we had a pomegranate plant (it was more a bush than a tree) that produced the odd fruit and had beautiful shiny leaves. But some of the leaves contained whitish lines, a kind of roadmap seemingly scribbled onto (or perhaps into) them. And -not to be misunderstood- it is about the upper and lower leaf sheets and what’s between them I want to write.
Generally speaking plant leaves are thin and very few people other than biologists might give it a thought what’s inside a leaf. Well, after I had noticed these whitish scribbly lines on our pomegranate leaves I investigated and found out that between the two sheets of upper and lower epidermis with their shiny and waxy cuticle some larval insects, collectively referred to as “leaf miners”, have their homes. Obviously these larvae cannot be very large and have to be flat and thin to fit between the sheets. Who are they and why have they selected such confined spaces for their development?
Actually the leaf miners are mostly larvae of very tiny moths, some flies, and a special group of leaf wasps. A group of minute beetles has also found the niche between the sheets a comfort zone. And so the question arises what’s so wonderful about the space between the upper and lower leaf sides. Well, there is of course the so-called palisade layer with its densely apposed chlorophyll-rich cells, the material bigger caterpillars, which attack the leaves from the outside and grow fat with, munch on. Further inside a leaf towards its undersurface the spongy layer provides a loose aggregation of cells (also containing chlorophyll) and close access to the stomata, the “breathing pores” of a leaf. It’s easy to imagine that living inside a leaf you are protected from attacks by spiders and even birds; at the same time you don’t have to search for food, because you are surrounded by it and being close to the leaf’s breathing pores your home is nicely ventilated. It’s a bit crammed, but what is perhaps surprising is that not more insects have chosen this habitat to live in. However, the drawback is that space is limited, that moving around inside the leaf is difficult and you can’t be too big. The white and scribbly lines visible on the leaf then are actually the feeding trails devoid of the consumed green cells, the “tunnels” so-to-speak, which the leaf miners have excavated inside the leaf between the outer and inner epidermal sheets.
After the leaf miner has turned into a pupa and the leaf has wilted and been shed, an adult insect will emerge. In the case of moths, these minute adult moths of 2-3 mm size, are as fascinating as their offspring and I had given two of my students the task to study the eyes of these tiny leaf miner moths. The research turned out more exciting than we could have imagined: through Anna Honkanen’s work on Ectoedemia argyropeza (a parthenogenetic leaf miner with a population of only females) and Stefan Fischer’s work on several other tiny species of leaf miner moths, we determined not only limits of eye miniaturization, but how an eye originally designed to work in the dark (like that of moths generally), could be modified to function under brighter conditions, for example daylight. Since at night not enough light would get into their tiny eyes, leaf miner moths had turned into largely daytime-active species. I guess the logical progression from this would be an examination of the eyes of leaf-miner flies, wasps, or beetles, but for that I’d need some new students willing to devote some years of their life to work with these tiny but fascinating insects.
© Dr V.B. Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com, 2017.
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