The tiniest of moths

The family Nepticulidae hold some of the smallest moths known, ranging from 3-8mm wing-tip to wing-tip.  For a comparison I have imaged two moths above: the largest known – Coscinocera hercules that tips the scales at nearly 9 inches, and one of the smallest (yes that tiny little speck below the Hercules moth) – Ectoedemia rubifoliella, also imaged below.  The Nepticulidae are surprisingly diverse, with over 800 species described that likely represent only 10% of the actual diversity (Powell, 2009).  In the United States we have only 80 species, of which 25 are known from the west.  When you compare that diversity to the 100 or so species known from Great Britain, it’s clear that the US knowledge is vastly lacking.  Actually, over 80% of all nepticulid diversity is known from Europe alone.  A strange inversion when you consider that the neotropics are the world’s most diverse ecosystems yet have only 74 known Nepticulidae species!  (Puplesis, 2000).  Why is this so?

Ectoedemia rubifoliella 3.3mm

Stigmella ostryaefoliella 3.1mm

The European diversity can easily be explained away due to a high concentration of bored Lepidopterists.  The Holarctic fauna is not the most diverse and it therefore has become the best understood on the planet, not to mention they have had a long history of gentleman entomologists dating back hundreds of years.  But the rest of the Nepticulidae diversity remains a mystery because they are really, really small, hard to spread, and difficult to identify as adults!  I have actually had little practice or success with mounting Nepticulidae, and the above specimens should be credited to Dr. Dave Wagner.  The very few that I do have in my collection are simply pinned and un-spread; and even the pinning proves hard enough when a slip of the hand can obliterate the entire specimen.  Apparently the best method for mounting is to knock them down in the freezer and pin them while they are still alive.  Not the most humane, but the only way to keep the moth from drying before your eyes and becoming impossible to manipulate.  As hard as the adults are to manage, the larvae are rather characteristic in that most are leaf miners – they feed on the material between the leaf epidermises.  This lends to the common name of “leaf blotch miners” because you can see the translucent patches the moths have ‘mined’ out from inside the leaf.  Not only is each species rather host-specific, but they tend to form very characteristic mine patterns within the leaf.  So if you find a leaf mine and you know the species of plant, chances are you can find out the species of Nepticulid within it (however not all leaf mines are nepticulids, there are lots of other insects that do this as well).  Rearing these moths are also rather simple, all you have to do is pop the leaf in a bag and wait for the moth to finish feeding.  One caterpillar only needs one leaf (or tiny section of leaf) – but care has to be taken to keep the leaf green while the caterpillar feeds.  If the leaf dies, so will the caterpillar.  Because of this paradoxical ability to identify the mines and not the adults there is a surprising amount of ecological research done on them, especially since a few pose threats to commercial crops.  The first image below clearly illustrates the caterpillar feeding within the leaf – and the trail of frass it has left behind.

Stigmella aceris (link to image credit)

Stigmella paradoxa (link to image credit)

If you look at the above images of mines it’s not all that difficult to imagine structures like this fossilizing.  And amazingly, they have!  The first image below (Labandeira et al., 1994) shows a variety of leaf mining Nepticulidae mines (and a Gracillariidae) from the mid-Cretaceous (97 million years ago).  The spectacular thing about leaf mines is that you can get down to genus level and sometimes even species.  The authors were able to differentiate between the nepticulid genera Stigmella and Ectoedemia based on the patterns preserved in the fossils; patterns we still use to help separate genera today.  The bottom illustration is from a mine discovered in Japan that is only around 8 million years old (Kuroko, 1987).

(Labanderia, 1994)

(Kuroko, 1987)

References

Kuroko, H. (1987). A Fossil Leaf Mine of Nepticulidae (Lepidoptera) from Japan. Bulletin Sugadaira Montane Res. Cen., No.8, 119-121.

Labandeira, C. (1994). Ninety-Seven Million Years of Angiosperm-Insect Association: Paleobiological Insights into the Meaning of Coevolution Proceedings of the National Academy of Sciences, 91 (25), 12278-12282 DOI: 10.1073/pnas.91.25.12278

PUPLESIS, R., DIŠKUS, A., ROBINSON, G., & ONORE, G. (2002). A review and checklist of the Neotropical Nepticulidae (Lepidoptera) Bulletin of The Natural History Museum. Entomology Series, 71 (01) DOI: 10.1017/S0968045402000032

Powell, J.A., Opler, P.A. (2010). Moths of Western North America – by J. A. Powell and P. A. Opler Systematic Entomology, 35 (2), 347-347 DOI: 10.1111/j.1365-3113.2010.00525.x