Sunday, August 1, 2010

Sunday Sawfly: The Saw in Sawfly

Last Winter seems to have knocked back several of the sawfly denizens of the Home Bug Garden. The Imported Currant worm that usually devastates my red currant has been nowhere to be seen and the Willow Gall Sawfly is much reduced over previous years. I intend to post on these critters on future Sundays (using pictures from previous years), but today I thought I should explain why these relatives of bees, ants, and wasps with a caterpillar-like larval stage are called sawflies.

Unlike many ‘common names’, ‘sawfly’ seems pretty straightforward: a fly with a saw or a fly that saws. ‘Saw’ has a good Old Norse-Old English origin, and has been used for saw-like structures on animals at least since the mid-1700s according to the OED (1993 New Shorter version). ‘Fly’ is another good Old English word for any insect that flies. So, presumably some old English person put two and two together and got sawfly. Calling anything but a two-winged dipterous fly a fly really bugs entomologists, but Entomology itself is rather new to English, arriving from France in the mid-1800s according to the OED, long after fly was in general and indiscriminate use.

So, maybe ‘sawfly’ was an old English common name for sawflies, but I’m skeptical. The saw in a sawfly is the ovipositor (‘egg’ + ‘placer’), i.e. the egg-laying organ of an insect, in this case one with two pairs of membranous wings (Hymenoptera). We are most familiar with the hymenopteran ovipositor in the form it often takes in the Aculeata (bees, wasps, ants), a sting. However, the ovipositor may be very conspicuous in other hymenopterons, e.g. the ichneumonid fly, I mean wasp, that Adrian at The Bug Whisperer so beautifully captured a few months ago. One wonders why these are not called drillflies (or share the rude common name that foresters often apply to horntails [Siricidae] that confuses female oviposition into a stump with male sperm transfer).

In any case, sawflies have an ovipositor with a pair of blades that usually have a serrate lower edge and are used to saw a slit and deposit an egg into plant tissue. However, when not in use, sawflies withdraw their saws into the body, the saws are not very large to begin with, and seeing those saw teeth (which one may have to count for a species identification) is easy only under high magnification. So, my hypothesis is that some entomologist dreamed up ‘sawfly’ as a common name. Alas, my New Shorter OED is only the two giant volume edition that I could afford as a young professor and it has no information on the first usage of ‘sawfly’ (nor does the online OED). If I can hold out until 2037, the estimated date for the completion of the 3rd Edition of the full OED, then perhaps I can totter into some library and harass the librarian (presumably a robot by this time) into checking for me.

Just as an aside, not all the HBG sawflies had a hard winter and the loosestrife sawfly is again eating all my Creeping Jenny. On 16 July this year I caught an attractive black and orange adult sawfly in the back yard and induced it to make the supreme sacrifice for science - drown in alcohol. This is the very first sawfly that I've run through all the keys to a species identification: Monostegia abdominalis, the ravager of my one remaining loosestrife, Creeping Jenny. A picture of its saw is above. Unfortunately, we don't have any pictures of the entire adult (they tend to shrivel-up when removed from the alcohol, as do many entomologists), but Molly Jacobson caught a good snap of one in New Hampshire this May and posted it on BugGuide. My adult was crammed with eggs. So, it may be that this species is bivoltine in Alberta, as in Quebec - see previous post for more details. However, given how late the Spring was (snow at the end of May), perhaps she just woke up late from her winter hibernation.


  1. Do all sawflies insert eggs within the plant tissue? I once found mating pairs of pine sawfly on mugho pine, and later found eggs in neat rows along the outside of the needles.

  2. Hi Adrian,

    That's an interesting question. It never pays to generalize too broadly about what insects are supposed to do - there's always likely to be one or more that do things differently. Also, 'sawfly' is a very general term that encompasses a number of families with differing habits.

    I did find a chapter by Peter Price in a book on adaptations in herbivorous insects that stated that tenthridinid sawflies invariably slit the plant cuticle to bath the newly deposited eggs in water. I suppose this doesn't necessarily mean the eggs are deposited too deeply, so they could still be seen as bumps on the leaves. I have a posting coming up on the imported currant worm where rows of eggs are readily visible along the veins of the leaves.

    Your pine sawflies were probably in the family Diprionidae, but all the papers I could find on dirpionids also claimed they inserted their eggs in host tissues. None said how deeply, but it would be interesting if some laid them on the surface of a plant without making any incision. They'd probably need to make some kind of glue in that case or hide the eggs among bud scales or the like. I'll see if I can find out more from a real sawfly specialist.



  3. Thanks. I'll see if I can dig up the scans from the old slides and then post them on AlbertaBugs.

  4. How did you get that ovipositor photo? Awesome!